The Wonders of the Human Body: An Extensive Overview

Can we count blessings of God? 

Contained herein is a testament to the profound complexity and miraculous design of the human form. Each named part, each intricate function, from the grand systems to the smallest molecular interactions, speaks to an awe-inspiring level of detail and purpose. Please keep in mind that even in this detail level it is possible to go way deeper and even write books about just one item in the list for example beta cells. It is hoped that by exploring this vast list, one may gain a deeper appreciation for the countless gifts embodied within us, fostering a profound sense of gratitude for these marvels. Did you ever offered gratitude to our Creator for the following?

PART 1: ANATOMICAL PARTS & ARTEFACTS

I. Skeletal System (Bones, Cartilages, Joints)

A. Bones (Osseous Tissue Structures)

  • Skull (Cranium & Facial Skeleton):
    • Cranial Bones (Neurocranium):
      • Frontal Bone: Squamous part, Orbital part, Nasal part, Supraorbital margins, Supraorbital foramen/notch, Glabella, Frontal sinuses, Zygomatic process.
      • Parietal Bones (2): Superior/Inferior temporal lines, Parietal eminence, Parietal foramen.
      • Temporal Bones (2): Squamous part (Zygomatic process, Mandibular fossa), Tympanic part (External acoustic meatus, Styloid process), Petrous part (Mastoid process, Mastoid air cells, Internal acoustic meatus, Carotid canal, Jugular fossa, Stylomastoid foramen), Auditory Ossicles within (Malleus, Incus, Stapes).
      • Occipital Bone: Squamous part (External occipital protuberance, Superior/Inferior nuchal lines), Basilar part, Lateral parts (Occipital condyles, Hypoglossal canal, Jugular notch), Foramen magnum.
      • Sphenoid Bone: Body (Sella turcica, Sphenoidal sinuses), Lesser Wings (Optic canal, Anterior clinoid processes), Greater Wings (Foramen rotundum, Foramen ovale, Foramen spinosum, Superior orbital fissure), Pterygoid Processes (Medial/Lateral pterygoid plates, Pterygoid hamulus).
      • Ethmoid Bone: Cribriform plate (Olfactory foramina), Crista galli, Perpendicular plate, Ethmoidal labyrinths (Superior/Middle nasal conchae, Ethmoidal air cells/sinuses).
    • Facial Bones (Viscerocranium):
      • Mandible: Body (Mental protuberance, Mental foramen, Alveolar process), Ramus (Coronoid process, Condylar process/Head, Mandibular notch, Angle, Mandibular foramen, Lingula).
      • Maxillae (2): Body (Maxillary sinus, Infraorbital foramen, Alveolar process, Anterior nasal spine), Frontal process, Zygomatic process, Palatine process, Incisive canal/foramen.
      • Zygomatic Bones (2): Frontal process, Temporal process (forms Zygomatic arch with temporal bone), Maxillary process, Orbital surface.
      • Nasal Bones (2).
      • Lacrimal Bones (2): Lacrimal groove, Fossa for lacrimal sac.
      • Palatine Bones (2): Horizontal plate (forms posterior hard palate), Perpendicular plate.
      • Inferior Nasal Conchae (2).
      • Vomer.
    • Sutures: Coronal, Sagittal, Lambdoid, Squamous, Sphenofrontal, Sphenoparietal, Sphenosquamosal, Occipitomastoid, Parietomastoid, Sphenoethmoidal, Frontozygomatic, etc. (many more specific sutures).
    • Fontanelles (in infants): Anterior, Posterior, Anterolateral (Sphenoidal), Posterolateral (Mastoid).
    • Wormian (Sutural) Bones: Variable, small bones within sutures.
  • Hyoid Bone: Body, Greater Cornua, Lesser Cornua.
  • Vertebral Column:
    • Cervical Vertebrae (C1-C7):
      • Atlas (C1): Anterior/Posterior arches, Lateral masses, Transverse foramina.
      • Axis (C2): Dens (Odontoid process), Body, Transverse foramina.
      • Typical (C3-C6): Body, Vertebral arch (Pedicles, Laminae), Vertebral foramen, Transverse processes (with transverse foramina), Spinous process (bifid).
      • Vertebra Prominens (C7): Long, non-bifid spinous process.
    • Thoracic Vertebrae (T1-T12): Body (with costal facets), Vertebral arch, Vertebral foramen, Transverse processes (with costal facets for T1-T10), Spinous process (long, points inferiorly).
    • Lumbar Vertebrae (L1-L5): Large body, Vertebral arch, Vertebral foramen, Short/blunt spinous process, Transverse processes, Mammillary processes, Accessory processes.
    • Sacrum (5 fused vertebrae S1-S5): Base, Apex, Auricular surfaces (for SI joint), Median sacral crest, Intermediate sacral crests, Lateral sacral crests, Anterior/Posterior sacral foramina, Sacral canal, Sacral hiatus, Sacral cornua.
    • Coccyx (3-5 fused vertebrae Co1-Co4/5): Base, Apex, Coccygeal cornua.
    • Intervertebral Discs: Annulus Fibrosus (outer fibrocartilage), Nucleus Pulposus (inner gelatinous).
  • Thoracic Cage:
    • Sternum: Manubrium (Jugular/Suprasternal notch, Clavicular notches), Body (Sternal angle/Angle of Louis), Xiphoid Process.
    • Ribs (12 pairs):
      • True Ribs (1-7): Attach directly to sternum via costal cartilages. (Head, Neck, Tubercle, Shaft/Body, Angle, Costal groove).
      • False Ribs (8-12):
        • Vertebrochondral (8-10): Attach to costal cartilage of rib above.
        • Floating Ribs (11-12): No anterior attachment.
    • Costal Cartilages (Hyaline cartilage).
  • Pectoral (Shoulder) Girdle:
    • Clavicle (2): Sternal end, Acromial end, Shaft.
    • Scapula (2): Spine, Acromion, Coracoid process, Glenoid cavity/fossa, Supraspinous fossa, Infraspinous fossa, Subscapular fossa, Superior/Medial/Lateral borders, Superior/Inferior angles.
  • Upper Limb:
    • Humerus (2): Head, Anatomical neck, Surgical neck, Greater/Lesser tubercles, Intertubercular sulcus, Deltoid tuberosity, Shaft, Radial groove, Medial/Lateral epicondyles, Capitulum, Trochlea, Olecranon fossa, Coronoid fossa, Radial fossa.
    • Radius (2): Head, Neck, Radial tuberosity, Shaft, Ulnar notch, Styloid process.
    • Ulna (2): Olecranon process, Coronoid process, Trochlear notch, Radial notch, Head, Styloid process.
    • Carpals (8 per hand x 2, Proximal row lateral to medial, Distal row lateral to medial):
      • Scaphoid, Lunate, Triquetrum, Pisiform.
      • Trapezium, Trapezoid, Capitate, Hamate (with hook).
    • Metacarpals (I-V, 5 per hand x 2): Base, Shaft, Head.
    • Phalanges (14 per hand x 2): Proximal, Middle (not in thumb), Distal. (Base, Shaft, Head).
  • Pelvic Girdle (Os Coxae / Innominate Bone x 2, each from fused Ilium, Ischium, Pubis):
    • Ilium: Iliac crest, Anterior/Posterior Superior Iliac Spines (ASIS, PSIS), Anterior/Posterior Inferior Iliac Spines (AIIS, PIIS), Iliac fossa, Gluteal lines (Posterior, Anterior, Inferior), Auricular surface, Greater sciatic notch.
    • Ischium: Ischial tuberosity, Ischial spine, Lesser sciatic notch, Ramus of ischium.
    • Pubis: Superior pubic ramus, Inferior pubic ramus, Body of pubis, Pubic crest, Pubic tubercle, Obturator foramen (formed with ischium).
    • Acetabulum (socket for femur head, formed by all three bones).
    • Pelvic Brim, True Pelvis, False Pelvis.
  • Lower Limb:
    • Femur (2): Head (Fovea capitis), Neck, Greater/Lesser Trochanters, Intertrochanteric line/crest, Shaft, Linea aspera, Gluteal tuberosity, Medial/Lateral Condyles, Medial/Lateral Epicondyles, Intercondylar fossa, Patellar surface.
    • Patella (2) (Sesamoid bone): Apex, Base, Articular facets.
    • Tibia (2): Medial/Lateral Condyles (Intercondylar eminence), Tibial tuberosity, Anterior border (shin), Medial malleolus, Fibular notch, Soleal line.
    • Fibula (2): Head, Neck, Shaft, Lateral malleolus.
    • Tarsals (7 per foot x 2):
      • Talus (articulates with tibia/fibula).
      • Calcaneus (heel bone, Calcaneal tuberosity, Sustentaculum tali).
      • Navicular.
      • Medial Cuneiform, Intermediate Cuneiform, Lateral Cuneiform.
      • Cuboid.
    • Metatarsals (I-V, 5 per foot x 2): Base, Shaft, Head. (Tuberosity of 5th metatarsal).
    • Phalanges (14 per foot x 2): Proximal, Middle (not in great toe), Distal. (Base, Shaft, Head).
  • Sesamoid Bones: Patella is largest. Others common in tendons of thumb (adductor pollicis, flexor pollicis brevis), great toe (flexor hallucis brevis), occasionally elsewhere. Fabella (in lateral head of gastrocnemius).

B. Cartilages (not part of bones, but skeletal system components)

  • Articular Cartilage (Hyaline, covers joint surfaces).
  • Costal Cartilages (Hyaline, connect ribs to sternum).
  • Intervertebral Discs (Fibrocartilage).
  • Menisci (Fibrocartilage, e.g., in knee - Medial/Lateral).
  • Articular Discs (Fibrocartilage, e.g., temporomandibular joint, sternoclavicular joint).
  • Laryngeal Cartilages (Thyroid, Cricoid, Arytenoids, etc. - see Larynx).
  • Tracheal & Bronchial Rings (Hyaline).
  • Nasal Cartilages (Septal, Lateral, Alar).
  • Auricular Cartilage (Elastic, in external ear).
  • Epiglottis (Elastic).

C. Joints (Articulations) - Classified by Structure & Function

  • Fibrous Joints (Synarthroses - immovable, or Amphiarthroses - slightly movable):
    • Sutures (e.g., skull sutures - synostosis when fused).
    • Syndesmoses (e.g., distal tibiofibular joint, interosseous membranes between radius/ulna, tibia/fibula).
    • Gomphoses (e.g., teeth in alveolar sockets).
  • Cartilaginous Joints (Amphiarthroses - slightly movable):
    • Synchondroses (Hyaline cartilage, usually temporary, e.g., epiphyseal plates, first sternocostal joint).
    • Symphyses (Fibrocartilage, e.g., pubic symphysis, intervertebral discs).
  • Synovial Joints (Diarthroses - freely movable):
    • General Structure: Articular capsule (Fibrous layer, Synovial membrane), Synovial fluid, Articular cartilage, Joint cavity.
    • Accessory Structures: Ligaments (Extracapsular, Intracapsular), Articular discs/Menisci, Bursae (e.g., subacromial, olecranon, prepatellar), Tendon sheaths.
    • Types by Shape of Articular Surfaces:
      • Plane (Gliding) Joints (e.g., intercarpal, intertarsal, acromioclavicular).
      • Hinge Joints (e.g., elbow, interphalangeal, knee - modified hinge).
      • Pivot Joints (e.g., proximal radioulnar, atlantoaxial).
      • Condyloid (Ellipsoidal) Joints (e.g., radiocarpal, metacarpophalangeal).
      • Saddle Joints (e.g., carpometacarpal joint of thumb).
      • Ball-and-Socket Joints (e.g., shoulder/glenohumeral, hip/coxal).
  • Ligaments (Examples - thousands exist):
    • Spine: Anterior/Posterior Longitudinal, Ligamentum Flavum, Interspinous, Supraspinous, Nuchal ligament.
    • Shoulder: Coracohumeral, Glenohumeral (Superior, Middle, Inferior), Coracoacromial, Acromioclavicular.
    • Elbow: Ulnar Collateral, Radial Collateral, Annular ligament.
    • Wrist/Hand: Numerous carpal ligaments, collateral ligaments of MCP/IP joints.
    • Hip: Iliofemoral, Pubofemoral, Ischiofemoral, Ligamentum teres femoris.
    • Knee: Medial (Tibial) Collateral (MCL), Lateral (Fibular) Collateral (LCL), Anterior Cruciate (ACL), Posterior Cruciate (PCL), Patellar ligament, Transverse ligament.
    • Ankle/Foot: Deltoid ligament (medial), Anterior Talofibular, Calcaneofibular, Posterior Talofibular (lateral), Spring ligament.

II. Muscular System

A. Muscles of Head & Neck

  • Muscles of Facial Expression:
    • Scalp: Occipitofrontalis (Frontal belly, Occipital belly), Epicranial aponeurosis (Galea aponeurotica).
    • Orbital Group: Orbicularis Oculi (Orbital, Palpebral, Lacrimal parts), Corrugator Supercilii, Levator Palpebrae Superioris (skeletal part).
    • Nasal Group: Nasalis (Transverse/Compressor, Alar/Dilator parts), Procerus, Depressor Septi Nasi.
    • Oral Group: Orbicularis Oris, Buccinator, Risorius, Levator Labii Superioris, Levator Labii Superioris Alaeque Nasi, Zygomaticus Major, Zygomaticus Minor, Levator Anguli Oris, Depressor Anguli Oris, Depressor Labii Inferioris, Mentalis, Platysma.
    • Auricular Muscles (Anterior, Superior, Posterior - often poorly developed).
  • Muscles of Mastication: Masseter (Superficial, Deep parts), Temporalis, Medial Pterygoid, Lateral Pterygoid (Upper, Lower heads).
  • Extrinsic Muscles of the Eye (see Sensory Organs)
  • Muscles of the Tongue:
    • Extrinsic: Genioglossus, Hyoglossus, Styloglossus, Palatoglossus.
    • Intrinsic: Superior Longitudinal, Inferior Longitudinal, Transverse, Vertical.
  • Muscles of the Pharynx (see Digestive System)
  • Muscles of the Larynx (see Respiratory System)
  • Muscles of the Soft Palate: Tensor Veli Palatini, Levator Veli Palatini, Palatoglossus, Palatopharyngeus, Musculus Uvulae.
  • Muscles of the Neck (Anterior & Lateral):
    • Superficial: Platysma, Sternocleidomastoid (SCM).
    • Suprahyoid Group: Digastric (Anterior, Posterior bellies), Stylohyoid, Mylohyoid, Geniohyoid.
    • Infrahyoid (Strap) Group: Sternohyoid, Sternothyroid, Thyrohyoid, Omohyoid (Superior, Inferior bellies).
    • Scalene Group (Lateral Vertebral): Scalenus Anterior, Scalenus Medius, Scalenus Posterior.
    • Prevertebral Group: Longus Colli, Longus Capitis, Rectus Capitis Anterior, Rectus Capitis Lateralis.

B. Muscles of the Trunk

  • Deep Muscles of the Back (Erector Spinae & Transversospinalis):
    • Erector Spinae (Sacrospinalis) - superficial deep group:
      • Iliocostalis (Lumborum, Thoracis, Cervicis).
      • Longissimus (Thoracis, Cervicis, Capitis).
      • Spinalis (Thoracis, Cervicis, Capitis - often blended).
    • Transversospinalis Group - deep to Erector Spinae:
      • Semispinalis (Thoracis, Cervicis, Capitis).
      • Multifidus.
      • Rotatores (Longus, Brevis).
    • Minor Deep Back Muscles: Interspinales, Intertransversarii, Levatores Costarum.
  • Suboccipital Muscles: Rectus Capitis Posterior Major, Rectus Capitis Posterior Minor, Obliquus Capitis Superior, Obliquus Capitis Inferior.
  • Superficial & Intermediate Back Muscles (belong to upper limb functionally):
    • Superficial: Trapezius (Upper, Middle, Lower fibers), Latissimus Dorsi.
    • Deeper (Superficial group): Levator Scapulae, Rhomboid Major, Rhomboid Minor.
    • Intermediate (Respiratory): Serratus Posterior Superior, Serratus Posterior Inferior.
  • Muscles of the Thorax:
    • Pectoralis Major (Clavicular, Sternal heads), Pectoralis Minor, Subclavius, Serratus Anterior.
    • Intercostals: External Intercostals, Internal Intercostals, Innermost Intercostals.
    • Transversus Thoracis (Sternocostalis).
    • Diaphragm (Sternal, Costal, Lumbar parts; Central tendon; Openings: Aortic hiatus, Esophageal hiatus, Caval opening).
  • Muscles of the Abdominal Wall:
    • Anterolateral: Rectus Abdominis (Linea alba, Tendinous intersections), External Oblique (Aponeurosis, Inguinal ligament), Internal Oblique (Aponeurosis), Transversus Abdominis (Aponeurosis, Conjoint tendon). Pyramidalis (variable).
    • Posterior: Quadratus Lumborum, Psoas Major (often grouped with Iliacus as Iliopsoas - see Lower Limb), Psoas Minor (variable).
  • Muscles of the Pelvic Floor & Perineum:
    • Pelvic Diaphragm: Levator Ani (Puborectalis, Pubococcygeus, Iliococcygeus), Coccygeus (Ischiococcygeus).
    • Urogenital Diaphragm (Deep Perineal Pouch): Sphincter Urethrae, Deep Transverse Perineal. (Term less used, now part of Perineal Membrane structures).
    • Superficial Perineal Pouch:
      • Male: Bulbospongiosus, Ischiocavernosus, Superficial Transverse Perineal.
      • Female: Bulbospongiosus, Ischiocavernosus, Superficial Transverse Perineal, Sphincter Urethrovaginalis, Compressor Urethrae.
    • Anal Triangle: External Anal Sphincter.

C. Muscles of the Upper Limb

  • Muscles Connecting Upper Limb to Vertebral Column (see Superficial Back)
  • Muscles Connecting Upper Limb to Thoracic Wall (see Thorax)
  • Muscles of the Shoulder (Intrinsic): Deltoid (Anterior, Middle, Posterior fibers), Rotator Cuff (Supraspinatus, Infraspinatus, Teres Minor, Subscapularis), Teres Major.
  • Muscles of the Arm (Brachium):
    • Anterior Compartment (Flexors): Biceps Brachii (Long head, Short head, Bicipital aponeurosis), Brachialis, Coracobrachialis.
    • Posterior Compartment (Extensors): Triceps Brachii (Long head, Lateral head, Medial head), Anconeus.
  • Muscles of the Forearm (Antebrachium):
    • Anterior Compartment (Flexors & Pronators):
      • Superficial Layer: Pronator Teres, Flexor Carpi Radialis, Palmaris Longus, Flexor Carpi Ulnaris, Flexor Digitorum Superficialis (FDS - Humeroulnar and Radial heads, tendons split to insert on middle phalanges).
      • Deep Layer: Flexor Digitorum Profundus (FDP - tendons insert on distal phalanges), Flexor Pollicis Longus (FPL), Pronator Quadratus.
    • Posterior Compartment (Extensors & Supinators):
      • Superficial Layer: Brachioradialis (functionally a flexor at elbow), Extensor Carpi Radialis Longus (ECRL), Extensor Carpi Radialis Brevis (ECRB), Extensor Digitorum (EDC, or Extensor Digitorum Communis), Extensor Digiti Minimi (EDM), Extensor Carpi Ulnaris (ECU).
      • Deep Layer: Supinator, Abductor Pollicis Longus (APL), Extensor Pollicis Brevis (EPB), Extensor Pollicis Longus (EPL), Extensor Indicis (EI).
      • (APL, EPB, EPL form the "Anatomical Snuffbox" boundaries).
  • Muscles of the Hand (Intrinsic):
    • Thenar Muscles (thumb eminence): Abductor Pollicis Brevis, Flexor Pollicis Brevis (Superficial, Deep heads), Opponens Pollicis. (Adductor Pollicis - often grouped here but different innervation).
    • Hypothenar Muscles (little finger eminence): Abductor Digiti Minimi, Flexor Digiti Minimi Brevis, Opponens Digiti Minimi. Palmaris Brevis (superficial).
    • Intermediate Muscles (Midpalmar):
      • Lumbricals (4): Arise from FDP tendons, insert on extensor expansions.
      • Interossei:
        • Dorsal Interossei (4, DAB - Dorsal ABduct).
        • Palmar Interossei (3 or 4, PAD - Palmar ADduct).
      • Adductor Pollicis (Transverse head, Oblique head).
    • Extensor Hoods / Expansions: Complex fibrous expansions on dorsal aspects of digits where extensor tendons, lumbricals, and interossei attach.
  • Fascia of the Upper Limb:
    • Pectoral Fascia, Axillary Fascia, Deltoid Fascia.
    • Brachial Fascia (Medial and Lateral Intermuscular Septa).
    • Antebrachial Fascia.
    • Extensor Retinaculum, Flexor Retinaculum (Transverse Carpal Ligament - forms Carpal Tunnel). Palmar Aponeurosis.

D. Muscles of the Lower Limb

  • Muscles of the Gluteal Region (Hip Extensors, Abductors, External Rotators):
    • Superficial Abductors/Extensors: Gluteus Maximus, Gluteus Medius, Gluteus Minimus, Tensor Fasciae Latae (TFL - acts on Iliotibial Tract).
    • Deep External Rotators (Short Lateral Rotators): Piriformis, Obturator Internus (with Gemellus Superior & Gemellus Inferior), Quadratus Femoris. (Obturator Externus is also a rotator but situated more anteriorly/medially).
  • Muscles of the Thigh:
    • Anterior Compartment (Hip Flexors & Knee Extensors):
      • Sartorius.
      • Quadriceps Femoris: Rectus Femoris, Vastus Lateralis, Vastus Medialis (Vastus Medialis Obliquus - VMO part), Vastus Intermedius. (Common quadriceps tendon inserting on patella, then Patellar Ligament to tibial tuberosity).
      • Iliopsoas Group (often considered here for hip flexion): Psoas Major, Iliacus. (Psoas Minor if present). Articularis Genu (deep to Vastus Intermedius).
    • Medial Compartment (Adductors of Thigh):
      • Adductor Longus, Adductor Brevis, Adductor Magnus (Adductor part, Hamstring/Ischiocondylar part), Pectineus, Gracilis. Obturator Externus.
    • Posterior Compartment (Hamstrings - Hip Extensors & Knee Flexors):
      • Biceps Femoris (Long head, Short head).
      • Semitendinosus.
      • Semimembranosus.
      • (Pes Anserinus - common tendinous insertion of Sartorius, Gracilis, Semitendinosus on medial tibia).
  • Muscles of the Leg (Crural Muscles):
    • Anterior Compartment (Dorsiflexors & Toe Extensors):
      • Tibialis Anterior.
      • Extensor Digitorum Longus (EDL).
      • Extensor Hallucis Longus (EHL).
      • Fibularis (Peroneus) Tertius (variable).
    • Lateral Compartment (Evertors & Plantarflexors):
      • Fibularis (Peroneus) Longus.
      • Fibularis (Peroneus) Brevis.
    • Posterior Compartment (Plantarflexors & Toe Flexors):
      • Superficial Group:
        • Gastrocnemius (Medial head, Lateral head).
        • Soleus.
        • (Triceps Surae = Gastrocnemius + Soleus, inserting via Calcaneal/Achilles Tendon).
        • Plantaris (small muscle with long tendon).
      • Deep Group:
        • Popliteus.
        • Flexor Digitorum Longus (FDL).
        • Flexor Hallucis Longus (FHL).
        • Tibialis Posterior.
  • Muscles of the Foot (Intrinsic):
    • Dorsal Surface: Extensor Digitorum Brevis (often includes Extensor Hallucis Brevis as medial part).
    • Plantar Surface (Layers):
      • Layer 1 (Superficial): Abductor Hallucis, Flexor Digitorum Brevis, Abductor Digiti Minimi.
      • Layer 2: Quadratus Plantae (Flexor Accessorius), Lumbricals (4). (Tendons of FHL and FDL pass through this layer).
      • Layer 3: Flexor Hallucis Brevis (Medial, Lateral heads), Adductor Hallucis (Oblique, Transverse heads), Flexor Digiti Minimi Brevis.
      • Layer 4 (Deepest): Plantar Interossei (3, PAD), Dorsal Interossei (4, DAB - relative to 2nd toe axis).
  • Fascia of the Lower Limb:
    • Superficial Fascia (subcutaneous tissue).
    • Deep Fascia (Fascia Lata of thigh - Iliotibial tract; Crural Fascia of leg).
    • Intermuscular Septa (Anterior, Posterior in thigh; Anterior, Posterior, Transverse in leg).
    • Retinacula at Ankle: Superior/Inferior Extensor Retinacula, Flexor Retinaculum, Fibular (Peroneal) Retinacula.
    • Plantar Aponeurosis (Plantar Fascia).

E. Microscopic Muscles (associated with other systems)

  • Arrector Pili muscles (in skin, attached to hair follicles).
  • Intrinsic Muscles of the Eye: Ciliary Muscle (focusing), Sphincter Pupillae (constricts pupil), Dilator Pupillae (dilates pupil).
  • Dartos Muscle (smooth muscle in scrotal wall).
  • Cremaster Muscle (skeletal muscle covering testis and spermatic cord - derived from internal oblique).
  • Smooth muscle in walls of blood vessels (Tunica media), airways (trachea, bronchi), GI tract (Muscularis externa, Muscularis mucosae), urinary tract (Detrusor muscle of bladder), reproductive tract (Myometrium of uterus, Ductus deferens wall).

III. Lymphatic System & Immune Structures

  • A. Lymphatic Vessels:
    • Lymphatic Capillaries (Blind-ended, overlapping endothelial cells forming minivalves, anchoring filaments). Lacteals (specialized lymph capillaries in small intestinal villi).
    • Collecting Lymphatic Vessels (Have valves, thinner walls than veins, three tunics).
    • Lymphatic Trunks (formed by union of largest collecting vessels):
      • Lumbar Trunks (Paired, drain lower limbs, pelvic organs, kidneys, adrenal glands, abdominal wall).
      • Intestinal Trunk (Single, drains stomach, intestines, pancreas, spleen, part of liver).
      • Bronchomediastinal Trunks (Paired, drain thoracic wall, lungs, heart).
      • Subclavian Trunks (Paired, drain upper limbs, inferior neck, superior thoracic wall).
      • Jugular Trunks (Paired, drain head and neck).
    • Lymphatic Ducts:
      • Thoracic Duct (arises from Cisterna Chyli, ascends, drains into junction of left internal jugular and left subclavian veins; drains most of body).
      • Right Lymphatic Duct (drains right upper limb, right side of head/neck, right thorax; drains into junction of right internal jugular and right subclavian veins).
    • Cisterna Chyli (dilated sac at inferior end of thoracic duct, L1-L2 vertebrae).
  • B. Lymphoid Tissues & Organs:
    • Primary Lymphoid Organs (where lymphocytes mature):
      • Red Bone Marrow (site of B and T lymphocyte origin; B cell maturation).
      • Thymus Gland (site of T lymphocyte maturation):
        • Capsule, Trabeculae dividing it into lobules.
        • Cortex (densely packed developing T cells/thymocytes, epithelial reticular cells, macrophages, Hassall's corpuscles beginning).
        • Medulla (fewer lymphocytes, more mature T cells, prominent Hassall's/thymic corpuscles - concentric layers of epithelial cells, dendritic cells, macrophages). Blood-thymus barrier.
    • Secondary Lymphoid Organs & Tissues (where mature lymphocytes encounter antigens and are activated):
      • Lymph Nodes:
        • Structure: Bean-shaped, fibrous capsule, trabeculae. Afferent lymphatic vessels (enter convex surface), Efferent lymphatic vessels (exit at hilum).
        • Cortex: Outer cortex (B-cell rich follicles/lymphoid nodules, some with germinal centers - sites of B cell proliferation), Deep/Paracortex (T-cell rich zone, dendritic cells presenting antigens). High endothelial venules (HEVs) for lymphocyte entry from blood.
        • Medulla: Medullary cords (plasma cells, B cells, macrophages), Medullary sinuses (converge on efferent vessels).
        • Reticular connective tissue stroma.
        • Major Groups (more specific):
          • Head: Occipital, Mastoid (Retroauricular), Parotid (Preauricular), Submandibular, Submental, Facial.
          • Neck: Superficial Cervical (along EJV), Deep Cervical (along IJV - Jugulodigastric, Jugulo-omohyoid), Supraclavicular. Retropharyngeal.
          • Axillary: Pectoral (Anterior), Subscapular (Posterior), Humeral (Lateral), Central, Apical.
          • Thorax: Parasternal, Intercostal, Diaphragmatic, Mediastinal (Anterior, Posterior), Tracheobronchial (Pulmonary, Hilar, Carinal, Paratracheal).
          • Abdomen/Pelvis: Pre-aortic (Celiac, Superior Mesenteric, Inferior Mesenteric), Para-aortic (Lumbar), Common Iliac, External Iliac, Internal Iliac, Sacral, Inguinal (Superficial - horizontal/vertical groups, Deep), Popliteal.
          • Mesenteric nodes (numerous, along mesenteric vessels).
      • Spleen:
        • Capsule, Trabeculae. Hilum.
        • White Pulp (lymphoid tissue around central arteries): Periarteriolar Lymphoid Sheaths (PALS - T cells), Lymphoid follicles/nodules (B cells, may have germinal centers). Marginal zone.
        • Red Pulp (blood-filled venous sinuses and splenic cords/Cords of Billroth): Macrophages (remove old RBCs/platelets, pathogens), lymphocytes, plasma cells, RBCs.
      • Mucosa-Associated Lymphoid Tissue (MALT): Diffuse lymphoid tissue and aggregates in mucous membranes.
        • Tonsils (form Waldeyer's Ring):
          • Palatine Tonsils (stratified squamous epithelium, deep crypts).
          • Lingual Tonsil (at base of tongue, crypts).
          • Pharyngeal Tonsil (Adenoids - in nasopharynx, pseudostratified ciliated epithelium, folds not crypts).
          • Tubal Tonsils (around openings of pharyngotympanic tubes).
        • Peyer's Patches (aggregated lymphoid nodules in ileum of small intestine, M cells).
        • Appendix (contains lymphoid nodules in its wall).
        • Bronchus-Associated Lymphoid Tissue (BALT).
        • Gut-Associated Lymphoid Tissue (GALT - includes Peyer's patches, appendix, solitary lymphoid nodules throughout GI tract).
      • Solitary Lymphoid Nodules: Found in lamina propria of many mucous membranes.
  • C. Lymph (Fluid): Interstitial fluid that has entered lymphatic capillaries. Contains water, proteins, electrolytes, lymphocytes, pathogens, cell debris, absorbed fats (chyle from intestine).

IV. Cardiovascular System (Blood Vessels & Heart)

  • A. Heart:
    • Location: Mediastinum, enclosed in Pericardium.
    • Pericardium:
      • Fibrous Pericardium (outer tough layer).
      • Serous Pericardium: Parietal layer (lines fibrous pericardium), Visceral layer (Epicardium - covers heart surface). Pericardial Cavity (between layers, contains pericardial fluid).
    • Heart Wall (3 layers):
      • Epicardium (Visceral pericardium): Serous membrane, often with adipose tissue.
      • Myocardium: Cardiac muscle tissue, contractile layer. Fibrous skeleton of the heart (collagen/elastin framework).
      • Endocardium: Endothelium and underlying CT, lines chambers and covers valves. Continuous with endothelium of blood vessels.
    • Chambers:
      • Right Atrium: Receives deoxygenated blood from Superior Vena Cava (SVC), Inferior Vena Cava (IVC), Coronary Sinus. Pectinate muscles (in auricle and anterior wall), Crista terminalis, Fossa ovalis (remnant of foramen ovale), Opening of coronary sinus. Right Auricle (external appendage).
      • Right Ventricle: Receives blood from RA through Tricuspid Valve. Trabeculae carneae (muscular ridges), Papillary muscles (Anterior, Posterior, Septal), Chordae tendineae (attach papillary muscles to tricuspid valve cusps), Conus arteriosus (infundibulum - smooth outflow tract to pulmonary trunk), Septomarginal trabecula (Moderator band - contains part of right bundle branch).
      • Left Atrium: Receives oxygenated blood from 4 Pulmonary Veins. Smooth walls (mostly derived from embryonic pulmonary veins). Left Auricle (external appendage, contains pectinate muscles). Valve of foramen ovale (remnant).
      • Left Ventricle: Receives blood from LA through Mitral (Bicuspid) Valve. Thickest myocardium. Trabeculae carneae, Papillary muscles (Anterior, Posterior), Chordae tendineae (attach to mitral valve cusps). Aortic vestibule (smooth outflow tract to aorta).
    • Septa:
      • Interatrial Septum: Separates atria.
      • Interventricular Septum: Separates ventricles (Muscular part, Membranous part).
    • Valves (ensure unidirectional blood flow):
      • Atrioventricular (AV) Valves:
        • Tricuspid Valve (Right AV): 3 cusps (Anterior, Posterior, Septal).
        • Mitral (Bicuspid) Valve (Left AV): 2 cusps (Anterior, Posterior).
      • Semilunar (SL) Valves: Each has 3 pocket-like cusps.
        • Pulmonary Valve: Between RV and Pulmonary Trunk. (Anterior, Right, Left cusps).
        • Aortic Valve: Between LV and Aorta. (Posterior, Right, Left cusps - associated with origins of coronary arteries from aortic sinuses/sinuses of Valsalva).
    • Great Vessels (connected to heart):
      • Superior Vena Cava (SVC), Inferior Vena Cava (IVC), Coronary Sinus (enter RA).
      • Pulmonary Trunk (exits RV, bifurcates into Right & Left Pulmonary Arteries).
      • Pulmonary Veins (4, enter LA).
      • Aorta (Ascending Aorta exits LV).
    • Conduction System (coordinates heartbeat):
      • Sinoatrial (SA) Node (Pacemaker, in RA wall near SVC opening).
      • Internodal Pathways (Anterior, Middle, Posterior - conduct impulse from SA to AV node). Bachmann's bundle (to LA).
      • Atrioventricular (AV) Node (in interatrial septum, near coronary sinus opening).
      • Atrioventricular (AV) Bundle (Bundle of His - passes from AV node through fibrous skeleton to interventricular septum).
      • Right and Left Bundle Branches (travel down interventricular septum).
      • Purkinje Fibers (Subendocardial branches - spread throughout ventricular myocardium).
    • Coronary Circulation (blood supply to myocardium):
      • Right Coronary Artery (RCA - arises from right aortic sinus): Branches - Sinoatrial Nodal branch (often), Right Marginal artery, Posterior Interventricular artery (PDA - in 80-85%, "right dominant"), AV Nodal branch (often).
      • Left Coronary Artery (LCA - arises from left aortic sinus, short): Branches -
        • Anterior Interventricular artery (LAD - Left Anterior Descending artery - in anterior interventricular sulcus). Diagonal branches. Septal branches.
        • Circumflex artery (LCX - in coronary sulcus). Left Marginal artery. Posterior Interventricular artery (in 15-20%, "left dominant" or "codominant" if both RCA/LCX contribute).
      • Cardiac Veins:
        • Great Cardiac Vein (travels with LAD).
        • Middle Cardiac Vein (travels with PDA).
        • Small Cardiac Vein (travels with Right Marginal artery).
        • Anterior Cardiac Veins (drain directly into RA).
        • Thebesian Veins (Venae Cordis Minimae - smallest, drain directly into chambers).
      • Coronary Sinus (large vein in posterior coronary sulcus, empties into RA).
    • Fetal Heart Remnants:
      • Fossa Ovalis (remnant of Foramen Ovale in interatrial septum).
      • Ligamentum Arteriosum (remnant of Ductus Arteriosus, connects pulmonary trunk to aorta).
  • B. Arteries (carry blood away from heart)
    • General Structure: Tunica Intima (endothelium, subendothelial layer, internal elastic lamina), Tunica Media (smooth muscle, elastic fibers, external elastic lamina), Tunica Adventitia/Externa (collagen, elastin, vasa vasorum, nervi vasorum).
    • Elastic Arteries (Conducting): Large, e.g., Aorta and its major branches. High elastin content.
    • Muscular Arteries (Distributing): Medium-sized, e.g., brachial, femoral. Thick tunica media.
    • Arterioles (Resistance vessels): Smallest arteries, lead to capillary beds. Regulate blood flow.
    • Pulmonary Circulation Arteries:
      • Pulmonary Trunk -> Right & Left Pulmonary Arteries -> Lobar Arteries -> Segmental Arteries -> Subsegmental Arteries -> Arterioles -> Pulmonary Capillaries.
    • Systemic Circulation Arteries (Major Examples & Branches):
      • Aorta:
        • Ascending Aorta: Right & Left Coronary Arteries.
        • Aortic Arch:
          • Brachiocephalic Trunk -> Right Common Carotid Artery, Right Subclavian Artery.
          • Left Common Carotid Artery.
          • Left Subclavian Artery.
        • Descending Aorta:
          • Thoracic Aorta: Bronchial arteries, Esophageal arteries, Posterior Intercostal arteries, Superior Phrenic arteries, Pericardial arteries, Mediastinal arteries.
          • Abdominal Aorta (T12-L4):
            • Unpaired Visceral Branches: Celiac Trunk, Superior Mesenteric Artery (SMA), Inferior Mesenteric Artery (IMA).
            • Paired Visceral Branches: Middle Suprarenal arteries, Renal arteries, Gonadal (Testicular/Ovarian) arteries.
            • Paired Parietal Branches: Inferior Phrenic arteries, Lumbar arteries (4 pairs).
            • Unpaired Parietal Branch: Median Sacral artery.
            • Bifurcation: Right & Left Common Iliac Arteries (at L4).
      • Arteries of Head & Neck (from Common Carotids & Subclavians):
        • Common Carotid Artery ->
          • Internal Carotid Artery (ICA): Carotid sinus (baroreceptor), Carotid body (chemoreceptor). Enters skull via carotid canal. Branches: Ophthalmic artery (Central retinal artery), Anterior Cerebral Artery (ACA), Middle Cerebral Artery (MCA), Posterior Communicating Artery. (ACA & MCA are terminal branches). Part of Circle of Willis.
          • External Carotid Artery (ECA): Branches (Superior thyroid, Ascending pharyngeal, Lingual, Facial, Occipital, Posterior auricular, Maxillary, Superficial temporal - "Some Anatomists Like Freaking Out Poor Medical Students").
            • Maxillary artery branches: Middle meningeal, Inferior alveolar, Buccal, Infraorbital, Sphenopalatine, etc.
        • Subclavian Artery -> (becomes Axillary artery at lateral border of 1st rib)
          • Vertebral Artery (ascends through transverse foramina of C6-C1, enters skull via foramen magnum, unites to form Basilar artery).
          • Internal Thoracic Artery (Mammary artery): Anterior intercostal arteries, Pericardiophrenic artery, Musculophrenic artery, Superior epigastric artery.
          • Thyrocervical Trunk: Inferior thyroid artery, Suprascapular artery, Transverse cervical artery (Superficial & Deep branches/Dorsal Scapular a.).
          • Costocervical Trunk: Deep cervical artery, Supreme/Superior intercostal artery.
          • Dorsal Scapular Artery (can arise from subclavian or transverse cervical).
        • Circle of Willis (Cerebral Arterial Circle): Anastomosis at base of brain. Formed by: Anterior Communicating Artery, Anterior Cerebral Arteries, Internal Carotid Arteries (supraclinoid segment), Posterior Communicating Arteries, Posterior Cerebral Arteries (from basilar).
        • Basilar Artery (from union of vertebrals): Pontine arteries, Labyrinthine (Internal Auditory) artery, Anterior Inferior Cerebellar Artery (AICA), Superior Cerebellar Artery (SCA), bifurcates into Posterior Cerebral Arteries (PCA).
      • Arteries of Upper Limb (continuation of Subclavian):
        • Axillary Artery (3 parts defined by Pectoralis Minor): Branches - Superior thoracic, Thoracoacromial (Pectoral, Acromial, Clavicular, Deltoid branches), Lateral thoracic, Subscapular (Circumflex scapular, Thoracodorsal), Anterior circumflex humeral, Posterior circumflex humeral.
        • Brachial Artery (continuation of axillary, in arm): Profunda brachii (Deep brachial) artery (with radial nerve), Superior/Inferior ulnar collateral arteries. Bifurcates into Radial & Ulnar arteries in cubital fossa.
        • Radial Artery: Radial recurrent, Palmar carpal branch, Superficial palmar branch, Dorsal carpal branch, Principalis pollicis, Radialis indicis. Forms deep palmar arch.
        • Ulnar Artery: Ulnar recurrent (anterior/posterior), Common interosseous artery (-> Anterior & Posterior interosseous arteries), Palmar/Dorsal carpal branches. Forms superficial palmar arch.
        • Palmar Arches: Superficial Palmar Arch (mainly ulnar a.), Deep Palmar Arch (mainly radial a.). Digital arteries from these.
      • Arteries of Lower Limb (from Common Iliac):
        • Common Iliac Artery ->
          • Internal Iliac Artery (supplies pelvis, perineum, gluteal region, medial thigh):
            • Anterior Division: Obturator, Umbilical (-> Superior vesical), Inferior vesical (male) / Vaginal (female), Middle rectal, Internal pudendal (-> Inferior rectal, Perineal, Dorsal a. of penis/clitoris), Inferior gluteal, Uterine (female).
            • Posterior Division: Iliolumbar, Lateral sacral, Superior gluteal.
          • External Iliac Artery (becomes Femoral artery past inguinal ligament): Inferior epigastric artery, Deep circumflex iliac artery.
        • Femoral Artery (in femoral triangle, adductor canal): Superficial circumflex iliac, Superficial epigastric, Superficial/Deep external pudendal, Profunda femoris (Deep femoral) artery (-> Medial/Lateral circumflex femoral, Perforating arteries). Descending genicular artery.
        • Popliteal Artery (continuation of femoral, posterior to knee): Genicular arteries (Superior medial/lateral, Middle, Inferior medial/lateral). Bifurcates into Anterior & Posterior Tibial arteries.
        • Anterior Tibial Artery (passes to anterior compartment of leg): Anterior/Posterior tibial recurrent. Becomes Dorsalis Pedis artery on dorsum of foot.
        • Posterior Tibial Artery (in posterior compartment): Fibular (Peroneal) artery (major branch), Medial/Lateral plantar arteries (terminal branches in foot). Circumflex fibular artery.
        • Dorsalis Pedis Artery: Lateral tarsal, Arcuate artery (-> Dorsal metatarsal arteries -> Dorsal digital arteries), Deep plantar artery (to complete plantar arch).
        • Plantar Arteries (Medial & Lateral): Deep Plantar Arch (mainly from lateral plantar a. and deep plantar branch of dorsalis pedis). Plantar metatarsal arteries -> Plantar digital arteries.
  • C. Veins (carry blood towards heart)
    • General Structure: Similar tunics to arteries but thinner walls, less muscle/elastin, larger lumens. Valves present in many veins (especially limb veins) to prevent backflow.
    • Venules: Smallest veins, collect blood from capillaries. Postcapillary venules are important for leukocyte extravasation.
    • Pulmonary Circulation Veins:
      • Pulmonary Capillaries -> Pulmonary Venules -> Segmental Veins -> Lobar Veins -> Superior & Inferior Pulmonary Veins (2 from each lung, carry oxygenated blood to Left Atrium).
    • Systemic Circulation Veins (Major Examples & Tributaries):
      • Veins Draining into Superior Vena Cava (SVC):
        • SVC formed by union of Right & Left Brachiocephalic Veins. Azygos vein drains into SVC.
        • Brachiocephalic Veins formed by union of Internal Jugular Vein & Subclavian Vein.
        • Internal Jugular Vein (IJV): Drains brain (via dural sinuses), face, neck. Tributaries: Sigmoid sinus, Inferior petrosal sinus, Facial v., Lingual v., Pharyngeal v., Superior/Middle thyroid v.
        • Subclavian Vein: Continuation of Axillary vein. External Jugular Vein (EJV - drains scalp, superficial face, drains into subclavian or IJV).
        • Dural Venous Sinuses (between layers of dura mater, drain brain): Superior Sagittal, Inferior Sagittal, Straight, Transverse, Sigmoid, Occipital, Cavernous, Superior/Inferior Petrosal, Sphenoparietal. (Eventually drain into IJVs). Emissary veins connect dural sinuses with extracranial veins. Diploic veins (in skull bones).
        • Vertebral Veins. Internal Thoracic Veins.
        • Azygos System (drains posterior thoracic & abdominal walls):
          • Azygos Vein (right side, arches over root of right lung to SVC). Tributaries: Right posterior intercostal veins, Hemiazygos vein, Accessory Hemiazygos vein, Bronchial veins, Esophageal veins.
          • Hemiazygos Vein (left lower thorax).
          • Accessory Hemiazygos Vein (left upper thorax).
      • Veins of Upper Limb:
        • Superficial: Cephalic Vein (lateral, drains into axillary v.), Basilic Vein (medial, joins brachial v. to form axillary v.), Median Cubital Vein (connects cephalic & basilic in cubital fossa - common for venipuncture). Dorsal venous network of hand.
        • Deep (accompany arteries): Radial veins, Ulnar veins, Brachial veins, Axillary vein. Palmar venous arches.
      • Veins Draining into Inferior Vena Cava (IVC):
        • IVC formed by union of Right & Left Common Iliac Veins (at L5).
        • Common Iliac Veins formed by union of Internal & External Iliac Veins.
        • Internal Iliac Vein: Tributaries correspond to branches of internal iliac artery (Superior/Inferior gluteal, Internal pudendal, Obturator, etc.). Pelvic venous plexuses (Rectal, Vesical, Prostatic, Uterine, Vaginal).
        • External Iliac Vein: Continuation of Femoral vein.
        • Lumbar Veins (ascending lumbar veins connect to azygos/hemiazygos).
        • Right Gonadal (Testicular/Ovarian) Vein (Left gonadal v. drains into Left Renal v.).
        • Renal Veins (Left is longer, receives left suprarenal & gonadal veins).
        • Right Suprarenal Vein (Left suprarenal v. drains into Left Renal v.).
        • Inferior Phrenic Veins.
        • Hepatic Veins (drain liver into IVC).
      • Hepatic Portal System (drains GI tract, spleen, pancreas, gallbladder to liver):
        • Hepatic Portal Vein formed by union of Superior Mesenteric Vein & Splenic Vein.
        • Superior Mesenteric Vein (SMV): Drains small intestine, cecum, ascending/transverse colon. Tributaries: Jejunal, Ileal, Ileocolic, Right colic, Middle colic veins.
        • Splenic Vein: Drains spleen, pancreas, stomach. Tributaries: Short gastric, Left gastro-omental (gastroepiploic), Pancreatic veins. Inferior Mesenteric Vein (IMV) usually drains into Splenic Vein.
        • Inferior Mesenteric Vein (IMV): Drains descending/sigmoid colon, rectum. Tributaries: Left colic, Sigmoid, Superior rectal veins.
        • Gastric Veins (Right & Left - drain directly into portal vein). Cystic Vein (from gallbladder, often to portal vein).
        • (Porto-caval anastomoses: sites where portal and systemic venous systems connect, e.g., esophageal, umbilical, rectal, retroperitoneal).
      • Veins of Lower Limb:
        • Superficial:
          • Great Saphenous Vein (longest vein, medial side, drains into femoral vein in femoral triangle - saphenous opening).
          • Small Saphenous Vein (posterior leg, drains into popliteal vein).
          • Dorsal venous arch of foot.
        • Deep (accompany arteries): Anterior/Posterior Tibial veins, Fibular (Peroneal) veins, Popliteal vein, Femoral vein (becomes external iliac). Deep plantar venous arch.
  • D. Microcirculation:
    • Arterioles: Terminal arterioles, Metarterioles (with precapillary sphincters regulating flow into true capillaries). Thoroughfare channel.
    • Capillaries: Smallest blood vessels, site of exchange. Single layer of endothelium and basal lamina.
      • Types:
        • Continuous Capillaries (most common, e.g., muscle, skin, CNS - tight junctions, intercellular clefts, pericytes). Blood-Brain Barrier has very tight junctions.
        • Fenestrated Capillaries (endothelial cells have pores/fenestrations, often with diaphragms; e.g., kidneys, small intestine, endocrine glands - for absorption/filtration).
        • Sinusoidal Capillaries (Discontinuous Capillaries): Larger lumens, irregular shape, large fenestrations, incomplete basal lamina, large intercellular clefts. E.g., liver, spleen, bone marrow, adrenal medulla. Allow passage of large molecules and cells.
    • Venules: Postcapillary venules (receive blood from capillaries, primary site of leukocyte emigration), Muscular venules.
  • E. Vasa Vasorum: "Vessels of the vessels" - tiny blood vessels that supply the walls of larger arteries and veins (tunica adventitia and outer media).
  • F. Nervi Vasorum (Vasomotor Nerves): Autonomic nerve fibers (mostly sympathetic) that innervate smooth muscle in vessel walls, controlling vasoconstriction/vasodilation.

V. Organs & Parts of Organs (by System)

A. Nervous System

1. Central Nervous System (CNS)
  • Brain (Encephalon):
    • Cerebrum (Telencephalon):
      • Cerebral Hemispheres (Right & Left), separated by Longitudinal Fissure, connected by Corpus Callosum.
      • Cerebral Cortex (Gray Matter - neuron cell bodies, dendrites, unmyelinated axons, synapses, glia):
        • Lobes:
          • Frontal Lobe: Precentral gyrus (Primary Motor Cortex - Brodmann Area 4), Premotor Cortex (Area 6), Supplementary Motor Area, Frontal Eye Fields (Area 8), Prefrontal Cortex (executive functions, personality, working memory - Areas 9,10,11,12,46,47), Broca's Area (motor speech - Areas 44,45, usually left hemisphere).
          • Parietal Lobe: Postcentral gyrus (Primary Somatosensory Cortex - Areas 3,1,2), Somatosensory Association Cortex (Areas 5,7), Parts of Wernicke's Area (language comprehension), Angular gyrus, Supramarginal gyrus.
          • Temporal Lobe: Superior temporal gyrus (Primary Auditory Cortex - Areas 41,42), Auditory Association Cortex (Area 22), Wernicke's Area (Area 22, usually left hemisphere), Olfactory Cortex (uncus), Hippocampal formation (memory), Amygdala (emotion). Middle and Inferior temporal gyri.
          • Occipital Lobe: Primary Visual Cortex (Area 17, along calcarine sulcus), Visual Association Areas (Areas 18,19).
          • Insula (Insular Lobe): Deep to lateral sulcus; gustatory cortex, visceral sensation, emotion, consciousness.
        • Gyri (folds) and Sulci (grooves): Central Sulcus (of Rolando - separates frontal/parietal), Lateral Sulcus (Sylvian Fissure - separates temporal from frontal/parietal), Parieto-occipital Sulcus. Many others (e.g., superior/middle/inferior frontal gyri, cingulate gyrus).
      • Cerebral White Matter (myelinated axons):
        • Association Fibers (connect areas within same hemisphere): Short association fibers (arcuate fibers), Long association fibers (e.g., Superior/Inferior longitudinal fasciculi, Uncinate fasciculus, Cingulum).
        • Commissural Fibers (connect corresponding areas of two hemispheres): Corpus Callosum (Rostrum, Genu, Body, Splenium), Anterior Commissure, Posterior Commissure, Hippocampal Commissure (Fornix).
        • Projection Fibers (connect cortex with lower brain/spinal cord): Internal Capsule (Anterior limb, Genu, Posterior limb - contains corticospinal, corticobulbar, thalamocortical fibers), Corona Radiata, External Capsule, Extreme Capsule.
      • Basal Ganglia (Nuclei - subcortical gray matter, motor control, cognition, emotion):
        • Corpus Striatum:
          • Caudate Nucleus (Head, Body, Tail).
          • Lentiform Nucleus: Putamen (lateral), Globus Pallidus (medial - External segment GPe, Internal segment GPi).
        • Functionally related: Subthalamic Nucleus (in diencephalon), Substantia Nigra (in midbrain - Pars compacta SNc dopaminergic, Pars reticulata SNr GABAergic). Claustrum. Nucleus Accumbens (part of ventral striatum, reward).
      • Limbic System (emotion, memory, motivation): Cingulate gyrus, Parahippocampal gyrus, Hippocampus (Dentate gyrus, Ammon's horn CA1-CA4, Subiculum), Amygdala, Fornix (connects hippocampus to mammillary bodies), Mammillary bodies (hypothalamus), Septal nuclei, Anterior thalamic nuclei, Olfactory bulbs/tracts. Papez circuit.
    • Diencephalon (surrounds third ventricle):
      • Thalamus (paired oval masses, relay station for sensory info, motor activity, consciousness): Divided by internal medullary lamina. Nuclei groups: Anterior, Medial (Mediodorsal MD), Lateral (Ventral Anterior VA, Ventral Lateral VL, Ventral Posterior VPL/VPM, Lateral Dorsal LD, Lateral Posterior LP, Pulvinar), Intralaminar (Centromedian CM, Parafascicular Pf), Midline, Reticular. Medial & Lateral Geniculate Bodies/Nuclei (MGN - auditory, LGN - visual).
      • Hypothalamus (inferior to thalamus, regulates ANS, endocrine system, body temp, hunger, thirst, sleep-wake, emotion): Mammillary bodies, Infundibulum (stalk connecting to pituitary), Tuber cinereum. Numerous nuclei: Supraoptic, Paraventricular (produce ADH, oxytocin), Suprachiasmatic (circadian rhythm), Anterior, Posterior, Lateral, Ventromedial, Arcuate, Dorsomedial.
      • Epithalamus (posterior-superior to thalamus): Pineal Gland (Body) (melatonin), Habenular Nuclei (Habenula), Stria Medullaris Thalami.
      • Subthalamus (inferior to thalamus, lateral to hypothalamus): Subthalamic Nucleus. Zona incerta. Fields of Forel.
    • Cerebellum (posterior to pons/medulla, inferior to occipital lobe, in posterior cranial fossa): Coordinates voluntary movements, posture, balance, equilibrium, motor learning.
      • Two Cerebellar Hemispheres, connected by Vermis (midline).
      • Cortex (Gray matter - highly folded into Folia): Molecular layer, Purkinje cell layer, Granule cell layer.
      • Arbor Vitae (White matter - branched pattern).
      • Deep Cerebellar Nuclei (embedded in white matter): Dentate (largest, lateral), Emboliform, Globose (Emboliform + Globose = Interposed nuclei), Fastigial (most medial).
      • Lobes: Anterior, Posterior, Flocculonodular Lobe (vestibulocerebellum).
      • Cerebellar Peduncles (connect cerebellum to brainstem): Superior (to midbrain), Middle (to pons), Inferior (to medulla).
    • Brainstem (connects cerebrum/diencephalon to spinal cord):
      • Midbrain (Mesencephalon):
        • Cerebral Peduncles (ventral - Crus cerebri/Basis pedunculi containing motor tracts, Tegmentum).
        • Tectum (dorsal - "roof"): Corpora Quadrigemina (Superior Colliculi - visual reflexes, Inferior Colliculi - auditory reflexes).
        • Substantia Nigra (Pars compacta, Pars reticulata).
        • Red Nucleus.
        • Periaqueductal Gray (PAG - pain modulation).
        • Cranial Nerve Nuclei (III, IV). Reticular formation.
        • Cerebral Aqueduct (of Sylvius - connects 3rd and 4th ventricles).
      • Pons ("bridge" - anterior to cerebellum):
        • Basilar Pons (ventral - pontine nuclei, transverse pontine fibers/middle cerebellar peduncle).
        • Pontine Tegmentum (dorsal - cranial nerve nuclei, tracts, reticular formation).
        • Cranial Nerve Nuclei (V, VI, VII, VIII - vestibular/cochlear).
        • Respiratory centers (Pneumotaxic, Apneustic). Locus Coeruleus.
      • Medulla Oblongata (Myelencephalon - continuous with spinal cord at foramen magnum):
        • Pyramids (ventral - corticospinal tracts, Decussation of pyramids).
        • Olives (lateral to pyramids - Inferior Olivary Nucleus).
        • Dorsal Column Nuclei: Nucleus Gracilis, Nucleus Cuneatus.
        • Cranial Nerve Nuclei (VIII, IX, X, XI, XII).
        • Vital Centers: Cardiovascular center (cardiac, vasomotor), Respiratory rhythmicity center. Other centers: vomiting, swallowing, coughing, sneezing.
        • Reticular Formation (extends through brainstem - consciousness, arousal, sleep, motor control, pain modulation). Raphe nuclei (serotonin), Gigantocellular reticular nucleus.
    • Ventricles (CSF-filled cavities):
      • Lateral Ventricles (2, one in each cerebral hemisphere): Anterior horn, Body, Posterior horn, Inferior horn. Interventricular Foramen (of Monro) connects to Third Ventricle.
      • Third Ventricle (midline, in diencephalon).
      • Cerebral Aqueduct (of Sylvius - in midbrain) connects to Fourth Ventricle.
      • Fourth Ventricle (between pons/medulla and cerebellum). Lateral apertures (Foramina of Luschka - 2), Median aperture (Foramen of Magendie - 1) allow CSF to enter subarachnoid space. Continuous with central canal of spinal cord.
    • Choroid Plexus: Vascular structures in ventricles that produce Cerebrospinal Fluid (CSF). Ependymal cells.
    • Meninges (protective CT coverings):
      • Dura Mater (tough outer layer):
        • Periosteal Layer (adheres to skull).
        • Meningeal Layer (inner layer, forms dural folds/septa).
        • Dural Folds: Falx Cerebri (in longitudinal fissure), Falx Cerebelli (between cerebellar hemispheres), Tentorium Cerebelli (separates cerebrum from cerebellum, tentorial notch), Diaphragma Sellae (covers pituitary fossa).
        • Dural Venous Sinuses (located between periosteal and meningeal layers, or within folds of meningeal layer - see Veins).
        • Epidural Space (potential space between skull and dura). Subdural Space (potential space between dura and arachnoid).
      • Arachnoid Mater (middle web-like layer):
        • Subarachnoid Space (between arachnoid and pia, contains CSF, cerebral arteries/veins). Arachnoid trabeculae. Cisterns (enlarged subarachnoid spaces, e.g., Cisterna Magna, Pontine Cistern).
        • Arachnoid Granulations (Villi) (project into dural sinuses for CSF reabsorption).
      • Pia Mater (delicate inner layer, adheres to brain surface, follows gyri/sulci).
  • Spinal Cord:
    • Location: Extends from foramen magnum to L1/L2 vertebra (Conus Medullaris).
    • Enlargements: Cervical Enlargement (nerves to upper limbs), Lumbar (Lumbosacral) Enlargement (nerves to lower limbs).
    • Cauda Equina ("horse's tail" - bundle of lumbar/sacral/coccygeal nerve roots below conus medullaris).
    • Filum Terminale (extension of pia mater from conus medullaris to coccyx - Internum within dural sac, Externum outside).
    • Meninges (continuous with cranial meninges): Spinal Dura Mater (single layer, separated from vertebrae by Epidural Space containing fat and venous plexus), Arachnoid Mater, Pia Mater (Denticulate ligaments - extensions of pia anchoring cord to dura). Subarachnoid space contains CSF. Lumbar Cistern (subarachnoid space L2-S2, site for lumbar puncture).
    • Cross-Sectional Anatomy:
      • Gray Matter (H-shaped or butterfly-shaped core):
        • Dorsal (Posterior) Horns: Sensory neurons (interneurons, terminals of afferent fibers). Substantia Gelatinosa. Nucleus Proprius. Clarke's Column (Nucleus Dorsalis of Clarke - T1-L2, proprioception).
        • Ventral (Anterior) Horns: Motor neurons (somatic efferent). Medial/Lateral motor neuron groups.
        • Lateral Horns (T1-L2, S2-S4): Autonomic neurons (preganglionic sympathetic T1-L2; preganglionic parasympathetic S2-S4).
        • Gray Commissure (connects gray matter on R/L sides, surrounds Central Canal).
        • Laminae of Rexed (I-X): Cytoarchitectural organization of gray matter.
      • White Matter (surrounds gray matter, myelinated axons in tracts/fasciculi):
        • Dorsal (Posterior) Funiculi/Columns: Ascending sensory tracts. Fasciculus Gracilis (medial, lower body), Fasciculus Cuneatus (lateral, upper body - above T6). (Convey proprioception, fine touch, vibration).
        • Lateral Funiculi/Columns: Ascending and Descending tracts. E.g., Lateral Spinothalamic tract (pain, temp), Lateral Corticospinal tract (voluntary motor), Spinocerebellar tracts (proprioception to cerebellum).
        • Ventral (Anterior) Funiculi/Columns: Ascending and Descending tracts. E.g., Anterior Spinothalamic tract (crude touch, pressure), Anterior Corticospinal tract, Vestibulospinal, Reticulospinal tracts.
      • Anterior Median Fissure, Posterior Median Sulcus.
    • Spinal Segments (31): 8 Cervical, 12 Thoracic, 5 Lumbar, 5 Sacral, 1 Coccygeal. (Each gives rise to a pair of spinal nerves).
2. Peripheral Nervous System (PNS)
  • Nerves (Bundles of axons in CT sheaths):
    • Connective Tissue Layers: Endoneurium (around individual axon/myelin), Perineurium (around fascicle of axons - forms blood-nerve barrier), Epineurium (around entire nerve).
  • Cranial Nerves (12 pairs, arise from brain/brainstem):
    • CN I: Olfactory Nerve (Sensory - smell). Olfactory receptor cells in nasal mucosa -> Olfactory bulb -> Olfactory tract.
    • CN II: Optic Nerve (Sensory - vision). Retinal ganglion cells -> Optic chiasm -> Optic tract -> LGN (thalamus).
    • CN III: Oculomotor Nerve (Motor - most extrinsic eye muscles: SR, IR, MR, IO, Levator Palpebrae Superioris; Parasympathetic - sphincter pupillae, ciliary muscle via ciliary ganglion). Nuclei in midbrain.
    • CN IV: Trochlear Nerve (Motor - Superior Oblique muscle of eye). Nucleus in midbrain. Only CN to exit dorsally.
    • CN V: Trigeminal Nerve (Mixed - Sensory to face, scalp, oral/nasal cavity, cornea; Motor to muscles of mastication). Nuclei in pons/medulla.
      • Trigeminal Ganglion (Semilunar/Gasserian - sensory cell bodies).
      • V1 (Ophthalmic Division - Sensory): Frontal, Lacrimal, Nasociliary nerves. Enters orbit via superior orbital fissure.
      • V2 (Maxillary Division - Sensory): Zygomatic, Infraorbital, Superior alveolar, Palatine nerves. Exits skull via foramen rotundum. Pterygopalatine ganglion (parasympathetic for lacrimal/nasal glands, fibers from CN VII).
      • V3 (Mandibular Division - Mixed): Auriculotemporal, Buccal, Lingual (sensory to ant 2/3 tongue), Inferior alveolar (-> Mental n.) nerves; Motor branches to masseter, temporalis, pterygoids, tensor tympani, tensor veli palatini, mylohyoid, ant. belly digastric. Exits skull via foramen ovale. Otic ganglion (parasympathetic for parotid, fibers from CN IX). Submandibular ganglion (parasympathetic for submandibular/sublingual glands, fibers from CN VII via chorda tympani).
    • CN VI: Abducens Nerve (Motor - Lateral Rectus muscle of eye). Nucleus in pons.
    • CN VII: Facial Nerve (Mixed - Motor to muscles of facial expression, stapedius, stylohyoid, post. belly digastric; Sensory - taste ant 2/3 tongue via chorda tympani; Parasympathetic - lacrimal, submandibular, sublingual glands). Nuclei in pons. Geniculate ganglion (sensory/taste).
      • Branches: Greater Petrosal n. (to pterygopalatine ganglion), Chorda Tympani (joins lingual n.), Nerve to Stapedius, Posterior Auricular n., Terminal motor branches (Temporal, Zygomatic, Buccal, Marginal Mandibular, Cervical - "Two Zebras Bit My Carrot").
    • CN VIII: Vestibulocochlear Nerve (Sensory - hearing from cochlea, balance/equilibrium from vestibule/semicircular canals). Nuclei in pons/medulla. Vestibular ganglion (Scarpa's), Spiral (Cochlear) ganglion.
    • CN IX: Glossopharyngeal Nerve (Mixed - Motor to stylopharyngeus; Sensory - taste post 1/3 tongue, general sensation from pharynx, tonsils, middle ear, carotid body/sinus; Parasympathetic - parotid gland via otic ganglion). Nuclei in medulla. Superior/Inferior (Petrosal) ganglia (sensory).
    • CN X: Vagus Nerve ("Wanderer" - Mixed - Motor to pharynx, larynx, soft palate; Sensory - taste from epiglottis, sensation from pharynx, larynx, thoracic/abdominal viscera, external ear; Parasympathetic - thoracic/abdominal viscera up to splenic flexure of colon). Nuclei in medulla. Superior (Jugular)/Inferior (Nodose) ganglia (sensory).
      • Branches: Pharyngeal, Superior Laryngeal (Internal/External branches), Recurrent Laryngeal, Cardiac, Pulmonary, Esophageal, Gastric, Intestinal branches.
    • CN XI: Accessory Nerve (Motor - Sternocleidomastoid, Trapezius muscles). Cranial root (from medulla, joins vagus), Spinal root (from C1-C5 spinal cord, ascends through foramen magnum, exits via jugular foramen).
    • CN XII: Hypoglossal Nerve (Motor - Intrinsic & Extrinsic muscles of tongue, except palatoglossus). Nucleus in medulla.
  • Spinal Nerves (31 pairs, arise from spinal cord segments):
    • Roots: Dorsal (Posterior) Root (Sensory afferent fibers, with Dorsal Root Ganglion - DRG, containing cell bodies of sensory neurons), Ventral (Anterior) Root (Motor efferent fibers).
    • Spinal Nerve Proper: Formed by union of dorsal and ventral roots (mixed sensory/motor). Exits via intervertebral foramen.
    • Rami (Branches):
      • Dorsal (Posterior) Ramus: Innervates deep back muscles and skin of back.
      • Ventral (Anterior) Ramus: Innervates anterolateral trunk, limbs. (Larger).
      • Meningeal Branch (Sinuvertebral nerve): Innervates vertebrae, ligaments, dura, blood vessels.
      • Rami Communicantes (connect to sympathetic chain ganglia): White Ramus (preganglionic sympathetic, T1-L2), Gray Ramus (postganglionic sympathetic, all levels).
    • Nerve Plexuses (networks formed by ventral rami of spinal nerves, primarily for limbs):
      • Cervical Plexus (C1-C4, part of C5):
        • Cutaneous Branches: Lesser Occipital, Greater Auricular, Transverse Cervical, Supraclavicular nerves.
        • Motor Branches: Ansa Cervicalis (innervates infrahyoid muscles), Phrenic Nerve (C3,C4,C5 - "keeps the diaphragm alive"). Segmental branches to prevertebral muscles.
      • Brachial Plexus (C5-T1, supplies upper limb): Roots -> Trunks (Superior, Middle, Inferior) -> Divisions (Anterior, Posterior) -> Cords (Lateral, Medial, Posterior) -> Branches (Terminal & Collateral nerves).
        • Supraclavicular Branches (from Roots/Trunks): Dorsal Scapular n., Long Thoracic n., Nerve to Subclavius, Suprascapular n.
        • Infraclavicular Branches (from Cords/Terminal):
          • Lateral Cord: Lateral Pectoral n., Musculocutaneous n., Lateral root of Median n.
          • Medial Cord: Medial Pectoral n., Medial Cutaneous n. of Arm, Medial Cutaneous n. of Forearm, Ulnar n., Medial root of Median n.
          • Posterior Cord: Upper Subscapular n., Thoracodorsal (Nerve to Latissimus Dorsi) n., Lower Subscapular n., Axillary n., Radial n.
        • Terminal Nerves: Musculocutaneous, Axillary, Radial, Median, Ulnar.
      • Lumbar Plexus (L1-L4, within Psoas Major muscle):
        • Branches: Iliohypogastric n., Ilioinguinal n., Genitofemoral n., Lateral Femoral Cutaneous n., Obturator n., Femoral n. Lumbosacral Trunk (L4,L5 contribution to sacral plexus).
      • Sacral Plexus (L4-S4, anterior to Piriformis muscle):
        • Branches: Superior Gluteal n., Inferior Gluteal n., Nerve to Piriformis, Nerve to Obturator Internus & Gemellus Superior, Nerve to Quadratus Femoris & Gemellus Inferior, Posterior Femoral Cutaneous n., Sciatic Nerve (largest nerve, Tibial division & Common Peroneal/Fibular division), Pudendal Nerve (S2,S3,S4 - "keeps the P. off the floor" - perineum).
      • Coccygeal Plexus (S4, S5, Co1): Anococcygeal nerves.
    • Intercostal Nerves (Ventral rami of T1-T11, run in costal grooves). Subcostal Nerve (T12). Supply intercostal muscles, skin of thoracic/abdominal wall.
    • Dermatomes: Area of skin supplied by sensory fibers of a single spinal nerve.
    • Myotomes: Group of muscles supplied by motor fibers of a single spinal nerve.
  • Autonomic Nervous System (ANS) - components in both CNS & PNS: Regulates involuntary functions. Sympathetic (fight/flight), Parasympathetic (rest/digest).
    • Sympathetic Division (Thoracolumbar outflow T1-L2):
      • Preganglionic Neurons: Cell bodies in lateral horns of T1-L2 spinal cord. Axons (myelinated) travel via ventral roots -> spinal nerves -> White rami communicantes -> Sympathetic Chain Ganglia.
      • Sympathetic Chain Ganglia (Paravertebral ganglia - paired, alongside vertebral column): Cervical (Superior, Middle, Inferior/Stellate), Thoracic, Lumbar, Sacral, Ganglion Impar.
      • Preganglionic axon can: 1) Synapse in chain ganglion at same level. 2) Ascend/descend in chain to synapse at different level. 3) Pass through chain without synapsing to reach Prevertebral (Collateral) Ganglia.
      • Prevertebral (Collateral) Ganglia (anterior to vertebral column, near major abdominal arteries): Celiac ganglion, Superior Mesenteric ganglion, Inferior Mesenteric ganglion, Aorticorenal ganglia. (Splanchnic Nerves - Greater, Lesser, Least, Lumbar, Sacral - carry preganglionic fibers to these).
      • Postganglionic Neurons: Cell bodies in chain ganglia or prevertebral ganglia. Axons (unmyelinated) travel via: Gray rami communicantes (to spinal nerves for skin effectors), direct visceral branches (to thoracic organs), or periarterial plexuses (along arteries to abdominal/pelvic organs).
      • Adrenal Medulla: Acts as modified sympathetic ganglion, secretes epinephrine/norepinephrine into blood.
    • Parasympathetic Division (Craniosacral outflow - CN III, VII, IX, X; S2-S4):
      • Preganglionic Neurons: Cell bodies in brainstem nuclei (Edinger-Westphal for CN III, Superior Salivatory for CN VII, Inferior Salivatory for CN IX, Dorsal Motor Nucleus of Vagus for CN X) and lateral horns of S2-S4 spinal cord. Long preganglionic axons.
      • Terminal (Intramural) Ganglia: Located near or within walls of target organs.
        • Named Cranial Ganglia: Ciliary (CN III), Pterygopalatine (CN VII), Submandibular (CN VII), Otic (CN IX).
      • Postganglionic Neurons: Cell bodies in terminal ganglia. Short postganglionic axons.
    • Enteric Nervous System (ENS - "brain of the gut"): Intrinsic nervous system of GI tract. Myenteric (Auerbach's) plexus (between muscle layers - motility), Submucosal (Meissner's) plexus (in submucosa - secretion, blood flow). Can function independently but modulated by ANS.
3. Sensory Receptors
  • Classification by Stimulus Type:
    • Mechanoreceptors: Touch, pressure, vibration, stretch (e.g., Meissner's corpuscles, Pacinian corpuscles, Merkel cells/discs, Ruffini endings, baroreceptors, proprioceptors).
    • Thermoreceptors: Temperature (Warm receptors, Cold receptors - free nerve endings).
    • Photoreceptors: Light (Rods, Cones in retina).
    • Chemoreceptors: Chemicals (e.g., taste buds, olfactory receptors, osmoreceptors, carotid/aortic bodies for O2/CO2/pH).
    • Nociceptors: Pain (damaging stimuli - free nerve endings, respond to mechanical, thermal, chemical).
  • Classification by Location:
    • Exteroceptors: Stimuli from outside body (touch, pain, temp, special senses).
    • Interoceptors (Visceroceptors): Stimuli from internal viscera/blood vessels (chemical changes, stretch, temp).
    • Proprioceptors: Internal stimuli regarding body position/movement (in skeletal muscles, tendons, joints, ligaments, CT coverings - Muscle spindles, Golgi tendon organs, Joint kinesthetic receptors).
  • Classification by Structure:
    • Simple Receptors (modified dendritic endings of sensory neurons, most general senses):
      • Unencapsulated: Free nerve endings, Merkel discs, Hair follicle receptors.
      • Encapsulated: Meissner's (Tactile) corpuscles, Pacinian (Lamellar) corpuscles, Ruffini endings (Bulbous corpuscles), Muscle spindles, Golgi tendon organs, Krause end bulbs.
    • Complex Receptors (Special Sense Organs): Vision, hearing, equilibrium, smell, taste.

B. Special Sensory Organs

  • 1. Eye (Organ of Vision) & Associated Structures:
    • Eyeball (Bulbus Oculi):
      • Fibrous Tunic (Outer layer):
        • Sclera ("white of eye" - dense CT, posterior 5/6). Lamina cribrosa (where optic nerve exits).
        • Cornea (transparent anterior 1/6, avascular, refracts light). Epithelium (stratified squamous), Bowman's layer, Stroma (collagen), Descemet's membrane, Endothelium. Limbus (corneoscleral junction).
      • Vascular Tunic (Uvea - Middle layer):
        • Choroid (posterior, highly vascular, pigmented - melanin).
        • Ciliary Body (anterior continuation of choroid, ring around lens):
          • Ciliary Muscle (smooth muscle - meridional, radial, circular fibers; accommodation of lens via parasympathetic CN III).
          • Ciliary Processes (folds on inner surface, secrete aqueous humor).
          • Suspensory Ligaments (Zonules of Zinn - attach ciliary body to lens).
        • Iris (colored part, anterior to lens, diaphragm controlling pupil size):
          • Pupil (central opening).
          • Sphincter Pupillae muscle (circular, constricts pupil - parasympathetic CN III).
          • Dilator Pupillae muscle (radial, dilates pupil - sympathetic).
          • Pigmented epithelium (posterior), Stroma (connective tissue, melanocytes).
      • Inner Tunic (Retina - Neural layer):
        • Pigmented Layer (Retinal Pigment Epithelium - RPE): Outer single layer, absorbs light, phagocytizes photoreceptor discs, vitamin A metabolism.
        • Neural Layer (sensory part, converts light to nerve impulses):
          • Photoreceptor Layer: Rods (dim light, peripheral vision, B&W), Cones (bright light, color vision - S, M, L types for blue, green, red). Outer/Inner segments, synaptic terminals.
          • Outer Nuclear Layer (ONL): Cell bodies of rods/cones.
          • Outer Plexiform Layer (OPL): Synapses between photoreceptors, bipolar cells, horizontal cells.
          • Inner Nuclear Layer (INL): Cell bodies of Bipolar cells, Horizontal cells, Amacrine cells. Müller glial cells.
          • Inner Plexiform Layer (IPL): Synapses between bipolar cells, amacrine cells, ganglion cells.
          • Ganglion Cell Layer (GCL): Cell bodies of Ganglion cells (axons form optic nerve).
          • Nerve Fiber Layer (NFL): Axons of ganglion cells, run towards optic disc.
          • Internal Limiting Membrane.
        • Optic Disc (Blind Spot): Where optic nerve exits, no photoreceptors.
        • Macula Lutea ("yellow spot"): Lateral to optic disc, area of high cone density.
        • Fovea Centralis (central depression in macula, ~1.5mm): Highest visual acuity, only cones. Foveola (center of fovea, thinnest retina).
      • Internal Chambers & Fluids:
        • Anterior Segment (anterior to lens): Filled with Aqueous Humor (clear fluid, produced by ciliary processes, nourishes lens/cornea, maintains intraocular pressure).
          • Anterior Chamber (between cornea and iris).
          • Posterior Chamber (between iris and lens).
          • Aqueous humor drains via Trabecular Meshwork -> Scleral Venous Sinus (Canal of Schlemm) -> Episcleral veins.
        • Posterior Segment (Vitreous Chamber - posterior to lens): Filled with Vitreous Humor/Body (gel-like substance, transmits light, supports lens, holds retina in place). Hyaloid canal (remnant of hyaloid artery).
      • Lens: Biconvex, transparent, avascular, flexible structure for focusing light on retina. Lens capsule, Lens epithelium (anterior), Lens fibers (crystallins).
    • Accessory Structures of the Eye:
      • Eyebrows (Supercilia).
      • Eyelids (Palpebrae - Superior, Inferior): Skin, Subcutaneous tissue, Orbicularis oculi muscle, Tarsal plates (CT, contain Tarsal/Meibomian glands - sebaceous, prevent tear evaporation), Palpebral conjunctiva (lines inner eyelids). Palpebral fissure, Medial/Lateral canthus (commissure). Lacrimal caruncle (at medial canthus).
      • Eyelashes (Cilia): Hairs with Ciliary glands (sebaceous glands of Zeis, sweat glands of Moll).
      • Conjunctiva: Transparent mucous membrane. Palpebral conjunctiva, Bulbar conjunctiva (covers anterior sclera, not cornea). Conjunctival sac.
      • Lacrimal Apparatus (produces & drains tears/lacrimal fluid):
        • Lacrimal Gland (in superolateral orbit, secretes tears).
        • Lacrimal Ducts (Excretory ducts - convey tears to conjunctival sac).
        • Lacrimal Puncta (openings on medial margin of each eyelid).
        • Lacrimal Canaliculi (Superior, Inferior - drain tears from puncta).
        • Lacrimal Sac (in lacrimal fossa).
        • Nasolacrimal Duct (drains tears into inferior nasal meatus of nasal cavity).
      • Extrinsic Eye Muscles (move eyeball, originate from bony orbit, insert on sclera):
        • Rectus Muscles: Superior Rectus, Inferior Rectus, Medial Rectus, Lateral Rectus (innervated by CN VI Abducens). (Common tendinous ring/Annulus of Zinn - origin).
        • Oblique Muscles: Superior Oblique (passes through Trochlea, innervated by CN IV Trochlear), Inferior Oblique.
        • (SR, IR, MR, IO, LPS innervated by CN III Oculomotor).
      • Orbital Fat (cushions eye). Fascia Bulbi (Tenon's capsule). Check ligaments.
  • 2. Ear (Organ of Hearing & Equilibrium):
    • Outer (External) Ear (collects sound waves):
      • Auricle (Pinna): Elastic cartilage covered by skin. Helix, Antihelix, Tragus, Antitragus, Lobule (earlobe), Concha.
      • External Acoustic Meatus (Auditory Canal): S-shaped tube from auricle to tympanic membrane. Outer 1/3 cartilaginous (with hairs, Sebaceous glands, Ceruminous glands - produce cerumen/earwax), Inner 2/3 bony (temporal bone).
      • Tympanic Membrane (Eardrum): Thin, translucent CT membrane separating external ear from middle ear. Vibrates in response to sound. Umbo (attachment of malleus handle). Pars flaccida, Pars tensa.
    • Middle Ear (Tympanic Cavity - air-filled cavity in petrous temporal bone): Transmits vibrations to inner ear.
      • Auditory Ossicles (smallest bones, transmit/amplify vibrations):
        • Malleus (Hammer): Handle attached to tympanic membrane, Head articulates with incus.
        • Incus (Anvil): Body articulates with malleus, Long process articulates with stapes.
        • Stapes (Stirrup): Head articulates with incus, Footplate fits into Oval Window (Fenestra Vestibuli).
      • Synovial joints between ossicles. Ligaments suspend ossicles.
      • Muscles of Middle Ear (dampen loud sounds - tympanic reflex):
        • Tensor Tympani (inserts on malleus handle, CN V3).
        • Stapedius (inserts on stapes neck, CN VII - smallest skeletal muscle).
      • Openings:
        • Oval Window (Fenestra Vestibuli): Stapes footplate here, leads to Scala Vestibuli of cochlea.
        • Round Window (Fenestra Cochleae): Membrane-covered, leads from Scala Tympani of cochlea (pressure relief).
        • Opening of Pharyngotympanic (Auditory/Eustachian) Tube: Connects middle ear to nasopharynx, equalizes pressure.
      • Epitympanic Recess (superior part of cavity). Mastoid Antrum & Air Cells (posterior).
    • Inner Ear (Labyrinth - fluid-filled cavities in petrous temporal bone): Houses sensory organs for hearing & equilibrium.
      • Bony Labyrinth (series of bony channels filled with Perilymph):
        • Vestibule: Central egg-shaped cavity. Contains Saccule & Utricle of membranous labyrinth. Oval window on lateral wall.
        • Semicircular Canals (3 - Anterior/Superior, Posterior, Lateral/Horizontal): Oriented in 3 planes. Ampulla (swelling at one end of each canal). Detect rotational acceleration.
        • Cochlea ("snail shell" - spiral bony chamber, ~2.75 turns around Modiolus - bony core): Contains Cochlear Duct of membranous labyrinth.
          • Scala Vestibuli (superior chamber, abuts oval window, continuous with Scala Tympani at Helicotrema - apex).
          • Scala Tympani (inferior chamber, terminates at round window).
      • Membranous Labyrinth (series of membranous sacs/ducts within bony labyrinth, filled with Endolymph - high K+):
        • Saccule & Utricle (in vestibule): House Maculae (sensory receptors for static equilibrium - head position relative to gravity, linear acceleration).
          • Macula: Patch of hair cells and supporting cells. Otolithic Membrane (gelatinous, studded with Otoliths/Otoconia - calcium carbonate crystals) rests on hair cells (stereocilia, kinocilium). Movement bends hairs, stimulates nerve fibers.
        • Semicircular Ducts (within semicircular canals): Communicate with utricle. Ampulla of each duct houses a Crista Ampullaris.
          • Crista Ampullaris: Sensory receptor for dynamic equilibrium (rotational acceleration). Hair cells, supporting cells. Cupula (gelatinous cap) covers crista, moved by endolymph flow.
        • Cochlear Duct (Scala Media - triangular duct within cochlea, between scala vestibuli/tympani): Houses Spiral Organ (Organ of Corti) - receptor for hearing.
          • Roof: Vestibular Membrane (Reissner's membrane) - separates cochlear duct from scala vestibuli.
          • Floor: Basilar Membrane - supports Organ of Corti, vibrates at different frequencies along its length (tonotopic organization). Separates cochlear duct from scala tympani. Osseous spiral lamina.
          • Spiral Organ (Organ of Corti):
            • Inner Hair Cells (IHCs - single row, primary sensory receptors for hearing).
            • Outer Hair Cells (OHCs - three rows, cochlear amplifier - modulate IHC sensitivity).
            • Supporting Cells (e.g., Pillar cells forming inner tunnel, Deiters' cells).
            • Tectorial Membrane (gelatinous, overlies hair cells, stereocilia of OHCs embedded in it).
            • Hair cells have Stereocilia (microvilli) that bend against tectorial membrane.
      • Vestibulocochlear Nerve (CN VIII) branches: Vestibular nerve (from maculae, cristae via Vestibular ganglion), Cochlear nerve (from Organ of Corti via Spiral ganglion in modiolus).
  • 3. Olfactory Epithelium (Organ of Smell):
    • Location: Roof of nasal cavity, along superior nasal conchae and superior nasal septum.
    • Cell Types:
      • Olfactory Receptor Neurons (Bipolar neurons): Dendrite extends to epithelial surface, ends in Olfactory Cilia (non-motile, contain odorant receptors). Axons form Olfactory Nerve (CN I) filaments, pass through Cribriform Plate of ethmoid bone to synapse in Olfactory Bulbs.
      • Supporting (Sustentacular) Cells: Columnar epithelial cells, surround/support receptor cells. Produce mucus.
      • Basal Cells: Stem cells, divide to replace receptor cells.
    • Olfactory (Bowman's) Glands: In underlying lamina propria, secrete mucus that dissolves odorants.
  • 4. Taste Buds (Organ of Gustation):
    • Location: Mostly on tongue papillae (Fungiform, Vallate/Circumvallate, Foliate), also on soft palate, epiglottis, pharynx.
    • Structure: Flask-shaped group of cells.
    • Cell Types:
      • Gustatory (Taste) Receptor Cells: Modified epithelial cells (not neurons) with Gustatory Hairs (microvilli) projecting through Taste Pore to surface. Synapse with sensory nerve fibers (CN VII, IX, X). Detect sweet, sour, salty, bitter, umami tastes.
      • Supporting (Sustentacular) Cells: Insulate gustatory cells.
      • Basal Cells: Stem cells, replace gustatory and supporting cells.
    • Papillae of Tongue:
      • Filiform Papillae: Most numerous, conical, keratinized, provide friction (no taste buds).
      • Fungiform Papillae: Mushroom-shaped, scattered over tongue surface, contain taste buds.
      • Vallate (Circumvallate) Papillae: Large, 8-12 in V-shape at back of tongue, surrounded by trench, taste buds on lateral walls. Von Ebner's glands (serous) wash trench.
      • Foliate Papillae: Folds on lateral posterior tongue, taste buds degenerate in childhood.

C. Digestive System

  • 1. Oral Cavity (Mouth, Buccal Cavity):
    • Boundaries: Anteriorly by Lips (Labia), Laterally by Cheeks (Buccae), Superiorly by Palate, Inferiorly by floor of mouth (Mylohyoid muscle, tongue), Posteriorly by Oropharyngeal Isthmus (Fauces).
    • Lips (Labia - Superior & Inferior): Core of skeletal muscle (Orbicularis Oris).
      • External Aspect: Keratinized stratified squamous epithelium (skin). Philtrum (vertical groove on upper lip), Vermilion Border (red margin, transition zone), Vermilion Zone (thinly keratinized, translucent, rich in capillaries). Labial Frenulum (Superior & Inferior - midline mucosal folds).
    • Cheeks (Buccae): Form lateral walls. Buccinator muscle, Buccal fat pad. Lined by non-keratinized stratified squamous epithelium. Openings of Parotid ducts (opposite upper 2nd molar).
    • Oral Vestibule: Slit-like space between lips/cheeks and teeth/gingivae.
    • Oral Cavity Proper: Space internal to teeth/gingivae.
    • Palate (Roof of mouth):
      • Hard Palate (anterior 2/3): Formed by Palatine processes of Maxillae and Horizontal plates of Palatine bones. Covered by keratinized/parakeratinized stratified squamous epithelium. Transverse palatine folds (Rugae). Incisive papilla (over incisive fossa).
      • Soft Palate (posterior 1/3): Mobile muscular flap, closes off nasopharynx during swallowing. Covered by non-keratinized stratified squamous epithelium orally, pseudostratified ciliated columnar nasally.
        • Uvula (Musculus uvulae): Conical projection from posterior edge.
        • Palatoglossal Arch (Anterior pillar of fauces): Mucosal fold overlying Palatoglossus muscle, from soft palate to tongue.
        • Palatopharyngeal Arch (Posterior pillar of fauces): Mucosal fold overlying Palatopharyngeus muscle, from soft palate to pharyngeal wall.
        • (Palatine tonsils located between these arches).
    • Tongue (Lingua): Muscular organ (skeletal muscle).
      • Parts: Root (posterior 1/3, in oropharynx), Body (anterior 2/3, in oral cavity proper), Apex (tip).
      • Surfaces: Dorsal (superior - rough due to papillae), Ventral (inferior - smooth, Lingual frenulum, Deep lingual veins visible).
      • Sulcus Terminalis: V-shaped groove separating anterior 2/3 from posterior 1/3 (root). Foramen Cecum (pit at apex of sulcus, remnant of thyroglossal duct).
      • Lingual Papillae (on dorsal surface of anterior 2/3):
        • Filiform Papillae: Most numerous, conical, keratinized, whitish, provide friction, no taste buds.
        • Fungiform Papillae: Mushroom-shaped, reddish (vascular core), scattered, contain taste buds.
        • Vallate (Circumvallate) Papillae: 8-12 large, circular, in V-row anterior to sulcus terminalis. Surrounded by a trench (moat). Taste buds on lateral walls. Von Ebner's glands (serous) open into trench.
        • Foliate Papillae: Vertical folds on posterolateral tongue, contain taste buds (more prominent in children).
      • Lingual Tonsil: Lymphoid tissue on posterior 1/3 (root) of tongue.
      • Muscles:
        • Intrinsic Muscles (originate/insert within tongue, change shape): Superior Longitudinal, Inferior Longitudinal, Transverse, Vertical.
        • Extrinsic Muscles (originate outside tongue, insert into it, change position): Genioglossus, Hyoglossus, Styloglossus, Palatoglossus (innervated by CN X, others by CN XII).
      • Taste Buds (see Special Senses).
    • Teeth (Dentes):
      • Sets:
        • Deciduous (Primary/Milk) Teeth: 20 total. Erupt ~6 months - 2.5 years. (Dental Formula: I 2/2, C 1/1, M 2/2 per quadrant x 2 = 20). Incisors, Canines, Molars.
        • Permanent (Secondary) Teeth: 32 total. Erupt ~6 years - adulthood. (Dental Formula: I 2/2, C 1/1, PM 2/2, M 3/3 per quadrant x 2 = 32). Incisors, Canines, Premolars (Bicuspids), Molars. (Third molars = Wisdom teeth).
      • Parts of a Tooth:
        • Crown: Exposed part above gingiva. Anatomical crown (enamel-covered), Clinical crown (visible). Cusps (on occlusal surface).
        • Neck: Constricted region at gum line.
        • Root: Part embedded in alveolar process of maxilla/mandible (in alveolus/socket). Single or multiple roots. Apex (tip of root) with Apical Foramen (opening for nerves/vessels).
      • Tissues of a Tooth:
        • Enamel: Hardest substance in body, acellular, covers crown. Composed of hydroxyapatite crystals organized into enamel rods/prisms. Produced by ameloblasts (before eruption).
        • Dentin: Bulk of tooth, underlies enamel and cementum, yellowish. Harder than bone. Contains dentinal tubules with odontoblast processes. Produced by odontoblasts (line pulp cavity). Primary, Secondary, Tertiary dentin.
        • Pulp Cavity: Central space within crown and root. Contains Dental Pulp (CT, blood vessels, nerves, lymphatics, odontoblasts). Pulp chamber (in crown), Root canal (in root).
        • Cementum: Bone-like CT covering root, attaches tooth to periodontal ligament. Cellular (apical) and Acellular (cervical) types. Produced by cementoblasts.
      • Periodontium (supporting tissues):
        • Periodontal Ligament (PDL): Fibrous CT attaching cementum to alveolar bone. Collagen fibers (Sharpey's fibers) embedded in both. Acts as shock absorber, sensory.
        • Alveolar Bone (Alveolar Process): Part of maxilla/mandible forming sockets. Lamina dura (compact bone lining socket).
        • Gingiva (Gums): Oral mucosa (keratinized stratified squamous) covering alveolar processes. Free gingiva, Attached gingiva, Interdental papilla. Gingival sulcus (crevice around neck of tooth).
    • Salivary Glands:
      • Major Salivary Glands (Paired):
        • Parotid Glands: Largest, anterior to ear, superficial to masseter. Parotid (Stensen's) Duct pierces buccinator, opens opposite upper 2nd molar. Predominantly serous acini (watery saliva, amylase). Facial nerve (CN VII) passes through (but doesn't innervate gland itself).
        • Submandibular Glands: Medial to body of mandible. Submandibular (Wharton's) Duct opens at sublingual caruncle (floor of mouth). Mixed, predominantly serous acini, some mucous.
        • Sublingual Glands: Smallest, beneath tongue in floor of mouth. Multiple small Ducts of Rivinus open onto sublingual folds; some join to form major Sublingual (Bartholin's) Duct opening with submandibular duct. Mixed, predominantly mucous acini.
      • Minor Salivary Glands: Numerous small glands in mucosa of lips, cheeks, palate, tongue (e.g., lingual glands of Von Ebner - serous, associated with vallate papillae; anterior lingual glands - Blandin & Nuhn). Mostly mucous.
      • Histology of Salivary Glands: Compound tubuloacinar glands.
        • Acini (secretory units): Serous cells (pyramidal, granular cytoplasm, secrete protein-rich fluid), Mucous cells (columnar, foamy cytoplasm with mucinogen granules), Serous demilunes (crescents of serous cells capping mucous acini in mixed glands). Myoepithelial cells (surround acini, contractile, aid secretion).
        • Duct System: Intercalated ducts (simple cuboidal), Striated ducts (simple columnar, basal striations due to mitochondria - modify saliva by ion transport), Excretory ducts (stratified cuboidal/columnar).
  • 2. Pharynx (Throat): Muscular funnel-shaped tube, common pathway for air and food.
    • Nasopharynx: Posterior to nasal cavity, superior to soft palate. Respiratory epithelium. Pharyngeal tonsil (adenoids) on posterior wall. Opening of Pharyngotympanic (Eustachian) tube (guarded by Torus Tubarius). Salpingopharyngeal fold.
    • Oropharynx: Posterior to oral cavity, from soft palate to epiglottis. Non-keratinized stratified squamous epithelium. Palatine tonsils (in tonsillar fossae between palatoglossal/palatopharyngeal arches). Lingual tonsil at base of tongue.
    • Laryngopharynx (Hypopharynx): Inferior to oropharynx, from epiglottis to esophagus (C6 level). Non-keratinized stratified squamous epithelium. Piriform recesses (fossae on either side of laryngeal inlet).
    • Muscles of Pharynx:
      • Outer Circular Layer (Constrictors - constrict pharynx during swallowing): Superior, Middle, Inferior Pharyngeal Constrictors. (Pharyngeal raphe - posterior midline insertion). Cricopharyngeus (part of inferior constrictor, acts as upper esophageal sphincter).
      • Inner Longitudinal Layer (Elevate pharynx/larynx during swallowing): Stylopharyngeus, Salpingopharyngeus, Palatopharyngeus.
  • 3. Esophagus: Muscular tube (~25 cm) from laryngopharynx to stomach. Passes through diaphragm at Esophageal Hiatus (T10).
    • Sphincters:
      • Upper Esophageal Sphincter (UES): Physiological sphincter (Cricopharyngeus muscle). Prevents air entry.
      • Lower Esophageal Sphincter (LES) / Cardiac Sphincter: Physiological sphincter at gastroesophageal junction. Prevents reflux. (Intrinsic smooth muscle tone, angle of entry, diaphragmatic crura contribute).
    • Constrictions: Cervical (at UES), Thoracic (where crossed by aortic arch & left main bronchus), Diaphragmatic (at hiatus).
    • Layers (Tunics) of Esophageal Wall:
      • Mucosa: Non-keratinized stratified squamous epithelium. Lamina propria (CT, diffuse lymphoid tissue). Muscularis Mucosae (longitudinal smooth muscle).
      • Submucosa: Dense irregular CT. Blood vessels, lymphatics, Meissner's (submucosal) plexus. Esophageal glands proper (mucous, in submucosa throughout). Esophageal cardiac glands (mucous, in lamina propria at ends of esophagus).
      • Muscularis Externa: Inner circular, Outer longitudinal layers. Auerbach's (myenteric) plexus between layers.
        • Upper 1/3: Skeletal muscle.
        • Middle 1/3: Mixed skeletal and smooth muscle.
        • Lower 1/3: Smooth muscle.
      • Adventitia: Loose CT (thoracic part). Serosa (Visceral peritoneum) below diaphragm (abdominal part).
  • 4. Stomach (Gaster): J-shaped, distensible organ in upper left abdomen. Food storage, mixing, chemical digestion (protein, fats).
    • Regions:
      • Cardia: Surrounds cardial orifice (esophageal opening).
      • Fundus: Dome-shaped region superior and lateral to cardia.
      • Body (Corpus): Main central region.
      • Pyloric Part (Pylorus): Funnel-shaped outflow region.
        • Pyloric Antrum (wider part, next to body).
        • Pyloric Canal (narrower part leading to pylorus).
        • Pyloric Sphincter (thickened circular smooth muscle, controls emptying into duodenum).
    • Curvatures: Lesser Curvature (medial, concave), Greater Curvature (lateral, convex). Angular incisure (notch on lesser curvature).
    • Omenta (peritoneal folds attached to stomach):
      • Lesser Omentum: From lesser curvature & proximal duodenum to liver. Consists of Hepatogastric ligament and Hepatoduodenal ligament (contains portal triad: portal vein, hepatic artery proper, common bile duct).
      • Greater Omentum: Large, apron-like fold from greater curvature, drapes over small intestine. Fuses with transverse mesocolon. Contains fat, lymph nodes. Gastrocolic ligament, Gastrosplenic ligament, Gastrophrenic ligament are parts.
    • Rugae (Gastric Folds): Longitudinal folds of mucosa/submucosa when stomach is empty, allow distension.
    • Layers of Stomach Wall:
      • Mucosa: Simple columnar epithelium (surface mucous cells - secrete protective alkaline mucus). Invaginates to form Gastric Pits, which lead to Gastric Glands. Lamina propria (loose CT, lymphoid cells). Muscularis Mucosae.
        • Gastric Glands (in fundus/body):
          • Mucous Neck Cells: In neck of gland, secrete acidic mucus. Progenitor cells.
          • Parietal (Oxyntic) Cells: Mainly in upper/middle gland. Large, eosinophilic, intracellular canaliculi. Secrete Hydrochloric Acid (HCl) and Intrinsic Factor (IF - for B12 absorption).
          • Chief (Zymogenic/Peptic) Cells: Mainly in lower gland. Basophilic cytoplasm. Secrete Pepsinogen (precursor to pepsin) and Gastric Lipase.
          • Enteroendocrine Cells (DNES cells): Scattered, mostly in base of gland.
            • G cells: Secrete Gastrin (stimulates acid secretion, mucosal growth). Mainly in pyloric glands.
            • D cells: Secrete Somatostatin (inhibits gastrin, acid, histamine, etc.).
            • ECL (Enterochromaffin-like) cells: Secrete Histamine (stimulates acid secretion).
        • Cardiac Glands (in cardia): Mucous secreting.
        • Pyloric Glands (in pyloric antrum): Mucous secreting, many G cells. Deeper pits.
      • Submucosa: CT, blood vessels, lymphatics, Meissner's plexus.
      • Muscularis Externa: Three layers of smooth muscle (unique to stomach). Outer Longitudinal, Middle Circular, Inner Oblique. Auerbach's plexus.
      • Serosa (Visceral peritoneum).
  • 5. Small Intestine: Longest part of alimentary canal (~6-7 meters in cadaver). Primary site of digestion and absorption.
    • Features increasing surface area: Plicae Circulares, Villi, Microvilli.
      • Plicae Circulares (Valves of Kerckring): Permanent circular folds of mucosa and submucosa. Most numerous in jejunum.
      • Villi: Finger-like projections of mucosa (epithelium + lamina propria). Core contains capillary network and a Lacteal (lymphatic capillary for fat absorption).
      • Microvilli (Striated/Brush Border): Microscopic projections of apical plasma membrane of absorptive epithelial cells (Enterocytes). Contain digestive enzymes (brush border enzymes).
    • Regions:
      • Duodenum (~25 cm, C-shaped, mostly retroperitoneal):
        • Parts: Superior (1st, duodenal cap/bulb, intraperitoneal), Descending (2nd, retroperitoneal), Horizontal/Inferior (3rd, retroperitoneal), Ascending (4th, retroperitoneal).
        • Duodenojejunal Flexure (ligament of Treitz/suspensory muscle of duodenum attaches here).
        • Major Duodenal Papilla (Ampulla of Vater): Opening for common bile duct and main pancreatic duct in descending part. Sphincter of Oddi (Hepatopancreatic sphincter) controls release.
        • Minor Duodenal Papilla: Opening for accessory pancreatic duct (if present), proximal to major.
        • Brunner's Glands: In submucosa of duodenum only. Secrete alkaline mucus to neutralize chyme.
      • Jejunum (~2.5 m, intraperitoneal, mainly LUQ):
        • Thicker wall, wider lumen, more prominent plicae circulares, longer villi, more vascular (redder) than ileum. Fewer Peyer's patches. Less fat in mesentery.
      • Ileum (~3.5 m, intraperitoneal, mainly RLQ):
        • Thinner wall, narrower lumen, less prominent plicae, shorter villi than jejunum. More Peyer's patches. More fat in mesentery.
        • Ileocecal Valve (Sphincter): Guards opening into large intestine (cecum). Prevents reflux.
    • Mesentery Proper: Fan-shaped peritoneal fold attaching jejunum and ileum to posterior abdominal wall.
    • Histology of Small Intestine Wall:
      • Mucosa:
        • Epithelium: Simple columnar.
          • Enterocytes (Absorptive cells): Tall columnar, microvilli, absorb nutrients. Produce brush border enzymes.
          • Goblet Cells: Secrete mucus, increase in number distally.
          • Paneth Cells: At base of crypts, eosinophilic granules. Secrete lysozyme, defensins (antibacterial).
          • Enteroendocrine Cells: Secrete hormones (e.g., CCK, Secretin, GIP, Motilin).
          • M (Microfold) Cells: Overlie Peyer's patches, transport antigens.
        • Intestinal Glands (Crypts of Lieberkühn): Tubular glands between villi, extend to muscularis mucosae. Contain stem cells.
        • Lamina Propria: Loose CT, blood/lymphatic capillaries (lacteals), lymphoid cells, GALT (Gut-Associated Lymphoid Tissue). Peyer's Patches (aggregated lymphoid nodules, prominent in ileum submucosa/lamina propria).
        • Muscularis Mucosae: Thin smooth muscle layer.
      • Submucosa: CT, larger blood/lymph vessels, Meissner's plexus. (Brunner's glands in duodenum).
      • Muscularis Externa: Inner Circular, Outer Longitudinal smooth muscle layers. Auerbach's plexus.
      • Serosa (Visceral peritoneum) (except for retroperitoneal parts of duodenum).
  • 6. Large Intestine: ~1.5 m long. Absorption of water/electrolytes, formation/storage of feces, vitamin synthesis (by bacteria).
    • Distinctive Features:
      • Taeniae Coli: Three longitudinal bands of smooth muscle (outer layer of muscularis externa concentrated).
      • Haustra: Sacculations of colon wall between taeniae.
      • Epiploic (Omental) Appendages: Small, fat-filled pouches of visceral peritoneum on outer surface.
    • Regions:
      • Cecum: Blind pouch in RLQ, below ileocecal valve. Intraperitoneal.
        • Vermiform Appendix: Narrow, blind-ended tube (~2-20 cm) attached to posteromedial cecum. Contains abundant lymphoid tissue. Mesoappendix (its mesentery).
      • Colon:
        • Ascending Colon: Retroperitoneal, extends up right side to Hepatic (Right Colic) Flexure.
        • Transverse Colon: Intraperitoneal (suspended by Transverse Mesocolon), extends across abdomen from hepatic to Splenic (Left Colic) Flexure (higher than hepatic).
        • Descending Colon: Retroperitoneal, extends down left side.
        • Sigmoid Colon: S-shaped, intraperitoneal (suspended by Sigmoid Mesocolon), in LLQ, leads to rectum.
      • Rectum: ~15 cm, anterior to sacrum/coccyx. Retroperitoneal (upper part has some peritoneum).
        • Rectal Valves (Transverse Folds of Houston): 3-4 internal semilunar folds.
        • Rectal Ampulla (dilated lower part, stores feces).
      • Anal Canal: ~3-4 cm, terminal part, from pelvic diaphragm to anus.
        • Anal Columns (of Morgagni): Longitudinal mucosal folds in upper half.
        • Anal Sinuses (Valves): Depressions between columns. Secrete mucus.
        • Pectinate (Dentate) Line: Irregular line at level of anal valves. Marks transition in epithelium, blood/nerve supply, lymphatic drainage (embryological boundary).
        • Sphincters:
          • Internal Anal Sphincter: Smooth muscle (thickening of inner circular layer), involuntary.
          • External Anal Sphincter: Skeletal muscle (Puborectalis part of levator ani, superficial/deep parts), voluntary.
        • Anus: External opening.
    • Histology of Large Intestine Wall:
      • Mucosa: Simple columnar epithelium (colonocytes for absorption, abundant Goblet cells for mucus). No villi, no plicae circulares. Straight, tubular Intestinal Glands (Crypts of Lieberkühn) deeper than in small intestine. Lamina propria rich in lymphoid cells. Muscularis mucosae.
      • Submucosa: CT, vessels, Meissner's plexus. Lymphoid nodules.
      • Muscularis Externa: Inner circular layer. Outer longitudinal layer concentrated into Taeniae Coli (except in rectum/anal canal where it's continuous). Auerbach's plexus.
      • Serosa (Visceral peritoneum) or Adventitia (for retroperitoneal parts - ascending, descending colon, rectum).
      • Anal Canal Histology: Upper part (above pectinate line) - simple columnar/cuboidal. Transition zone. Lower part (below pectinate line) - non-keratinized stratified squamous, becoming keratinized at anus.
  • 7. Liver (Hepar): Largest gland, RUQ, mainly under diaphragm. Numerous metabolic functions.
    • Surfaces: Diaphragmatic (superior, anterior, posterior - smooth, convex), Visceral (posteroinferior - flatter, impressions from organs).
    • Bare Area: On posterior diaphragmatic surface, not covered by peritoneum, direct contact with diaphragm.
    • Lobes:
      • Anatomical Lobes (based on external features): Right Lobe (larger), Left Lobe, Caudate Lobe (posterior, between IVC and fissure for ligamentum venosum), Quadrate Lobe (inferior, between gallbladder and fissure for ligamentum teres).
      • Functional Lobes/Segments (Couinaud Segments I-VIII): Based on portal venous and hepatic arterial supply, and biliary drainage. Used in surgery. Right and Left functional lobes separated by Cantlie's line (from IVC fossa to gallbladder fossa).
    • Fissures/Grooves: For IVC, gallbladder, ligamentum teres, ligamentum venosum.
    • Ligaments (peritoneal folds or remnants):
      • Falciform Ligament: Attaches liver to anterior abdominal wall and diaphragm, separates R/L anatomical lobes.
      • Ligamentum Teres Hepatis (Round Ligament of Liver): Fibrous remnant of fetal umbilical vein, in free edge of falciform ligament.
      • Coronary Ligament (Anterior & Posterior layers): Attaches superior liver to diaphragm, encloses bare area.
      • Triangular Ligaments (Right & Left): Formed at ends of coronary ligament.
      • Lesser Omentum: Attaches to porta hepatis and fissure for ligamentum venosum.
      • Ligamentum Venosum: Fibrous remnant of fetal ductus venosus, shunted blood from umbilical vein to IVC.
    • Porta Hepatis (Hilum of liver) on visceral surface: Transverse fissure where Portal Triad structures enter/exit:
      • Hepatic Portal Vein (posteriorly, brings nutrient-rich blood from GI tract).
      • Hepatic Artery Proper (anteriorly/left, brings oxygenated blood, branch of celiac trunk).
      • Common Hepatic Duct (anteriorly/right, formed by R/L hepatic ducts, carries bile out).
      • Lymphatics, Nerves also pass through.
    • Microscopic Anatomy:
      • Capsule of Glisson: Fibrous CT capsule covering liver, thicker at porta hepatis.
      • Hepatocytes: Main functional cells, polyhedral, eosinophilic cytoplasm, often binucleate. Arranged in radiating Plates/Cords (one cell thick) separated by sinusoids. Functions: bile synthesis, protein synthesis (albumin, clotting factors), detoxification, metabolism, storage.
      • Sinusoids: Modified capillaries between hepatocyte plates. Lined by discontinuous fenestrated endothelium. Mix portal venous and hepatic arterial blood. Blood flows towards central vein.
      • Kupffer Cells (Stellate Macrophages): Phagocytic cells within sinusoid lining.
      • Space of Disse (Perisinusoidal Space): Between hepatocytes and sinusoidal endothelium. Site of exchange. Contains microvilli of hepatocytes, plasma.
      • Hepatic Stellate Cells (Ito Cells): In Space of Disse. Store Vitamin A. Can produce collagen (fibrosis in chronic injury). Pit cells (liver-specific NK cells).
      • Bile Canaliculi: Tiny channels between adjacent hepatocytes (formed by their plasma membranes). Collect bile produced by hepatocytes. Bile flows countercurrent to blood, towards portal triads.
      • Bile Ductules (Canals of Hering): Lined by cuboidal cholangiocytes. Connect canaliculi to interlobular bile ducts.
      • Interlobular Bile Ducts (in portal triads): Lined by cuboidal/columnar cholangiocytes. Join to form R/L Hepatic Ducts.
      • Liver Lobules (organization models):
        • Classic Lobule: Hexagonal, centered on a Central Vein (Terminal Hepatic Venule). Portal Triads at corners. Blood flows from triads to central vein. Emphasizes endocrine function (blood processing).
        • Portal Lobule: Triangular, centered on a Portal Triad. Bile drains from hepatocytes in the triangle towards the central bile duct. Emphasizes exocrine function (bile secretion).
        • Liver Acinus (of Rappaport): Diamond/oval-shaped, centered on terminal branches of portal triad vessels running along short axis. Long axis connects two central veins. Divides hepatocytes into 3 zones based on proximity to blood supply (Zone 1 periportal - best oxygenated, first to see toxins; Zone 2 midzonal; Zone 3 centrilobular/perivenular - poorest oxygenated, last to see toxins, first to show ischemic necrosis, main site of drug detox). Best correlates structure with function.
      • Central Veins -> Sublobular Veins -> Collecting Veins -> Hepatic Veins (2-3 large) -> Inferior Vena Cava.
  • 8. Gallbladder: Pear-shaped sac on visceral surface of liver (in gallbladder fossa). Stores and concentrates bile.
    • Parts: Fundus (projects beyond inferior liver border), Body, Neck (tapers, continuous with cystic duct).
    • Cystic Duct (~4 cm): Connects neck of gallbladder to Common Hepatic Duct. Contains Spiral Valve of Heister (mucosal folds, keeps duct open).
    • Bile Duct System: Right & Left Hepatic Ducts (from liver) -> Common Hepatic Duct + Cystic Duct (from gallbladder) -> Common Bile Duct (Bile Duct) -> (usually joins Main Pancreatic Duct to form) Hepatopancreatic Ampulla (of Vater) -> opens into Duodenum at Major Duodenal Papilla.
    • Histology of Gallbladder Wall:
      • Mucosa: Simple columnar epithelium (absorptive cells for concentrating bile). Highly folded into Rugae (when empty). No goblet cells. Lamina propria. No muscularis mucosae, no submucosa.
      • Muscularis (Externa): Smooth muscle fibers (randomly oriented), contract to expel bile (stimulated by CCK).
      • Serosa (Visceral peritoneum) on free surface. Adventitia where attached to liver.
      • Rokitansky-Aschoff Sinuses: Deep invaginations of mucosa into muscularis (can be pathological).
  • 9. Pancreas: Elongated gland, retroperitoneal (except tail), posterior to stomach. Both exocrine and endocrine functions.
    • Parts:
      • Head: Expanded part in C-curve of duodenum. Uncinate Process (posterior projection, hooks behind SMA/SMV).
      • Neck: Constricted part anterior to SMA/SMV and portal vein formation.
      • Body: Extends to left, posterior to stomach.
      • Tail: Tapers, reaches hilum of spleen. Intraperitoneal (within splenorenal ligament).
    • Exocrine Pancreas (produces pancreatic juice with digestive enzymes):
      • Pancreatic Acini: Clusters of serous acinar cells (pyramidal, basophilic base, apical zymogen granules - contain inactive enzymes like trypsinogen, chymotrypsinogen, procarboxypeptidase, proelastase; and active enzymes like amylase, lipase, nuclease). Centroacinar cells (duct cells extending into acinus lumen, secrete bicarbonate).
      • Duct System:
        • Intercalated Ducts (from acini, lined by simple cuboidal, secrete bicarbonate).
        • Intralobular Ducts.
        • Interlobular Ducts (in CT septa).
        • Main Pancreatic Duct (of Wirsung): Runs length of pancreas, joins common bile duct to form hepatopancreatic ampulla (or opens separately).
        • Accessory Pancreatic Duct (of Santorini): Drains part of head, opens into duodenum at minor papilla (variable patency).
    • Endocrine Pancreas (Islets of Langerhans - see Endocrine System). Scattered clusters of endocrine cells.
  • 10. Peritoneum & Peritoneal Cavity:
    • Peritoneum: Extensive serous membrane (simple squamous epithelium/mesothelium + underlying CT).
      • Parietal Peritoneum: Lines inner surface of abdominopelvic wall.
      • Visceral Peritoneum: Covers (invests) surfaces of abdominal organs.
      • Peritoneal Fluid: Thin film of serous fluid in peritoneal cavity, lubricates.
    • Peritoneal Cavity: Potential space between parietal and visceral layers.
      • Greater Sac: Main, larger part of peritoneal cavity.
      • Lesser Sac (Omental Bursa): Smaller space posterior to stomach, lesser omentum, and liver. Communicates with greater sac via Epiploic Foramen (of Winslow / Omental Foramen) - bounded by portal triad, IVC, caudate lobe, 1st part of duodenum.
    • Peritoneal Formations (folds of peritoneum):
      • Mesentery: Double layer of peritoneum attaching parts of intestine (jejunum, ileum, transverse colon, sigmoid colon, appendix) to posterior abdominal wall. Contains blood vessels, nerves, lymphatics. (e.g., The Mesentery/Mesentery Proper, Transverse Mesocolon, Sigmoid Mesocolon, Mesoappendix).
      • Omentum: Double (or quadruple) layer attaching stomach to other organs.
        • Greater Omentum (see Stomach).
        • Lesser Omentum (see Stomach).
      • Peritoneal Ligaments: Connect organs to each other or to abdominal wall (e.g., Falciform, Coronary, Triangular ligaments of liver; Gastrosplenic, Splenorenal, Phrenicocolic ligaments).
    • Intraperitoneal Organs: Almost completely covered by visceral peritoneum (e.g., stomach, spleen, liver, jejunum, ileum, transverse colon, sigmoid colon, tail of pancreas). Suspended by mesentery/omentum.
    • Retroperitoneal Organs: Lie posterior to parietal peritoneum, only partially covered (usually anteriorly).
      • Primarily Retroperitoneal: Developed and remain behind peritoneum (e.g., kidneys, adrenal glands, ureters, aorta, IVC, esophagus (thoracic part), rectum (lower part)).
      • Secondarily Retroperitoneal: Were initially intraperitoneal with mesentery, but mesentery fused with posterior abdominal wall during development (e.g., duodenum (parts 2-4), pancreas (head, neck, body), ascending colon, descending colon).
    • Subdivisions of Peritoneal Cavity: Supracolic compartment (above transverse mesocolon), Infracolic compartment (below transverse mesocolon - further divided by mesentery proper). Paracolic gutters (lateral to ascending/descending colon). Pelvic cavity.

D. Respiratory System

  • 1. Nose (Nasus) & Nasal Cavity:
    • External Nose:
      • Framework: Nasal bones, Frontal processes of maxillae, Nasal part of frontal bone, and various cartilages (Septal cartilage, Lateral nasal cartilages, Major alar cartilages, Minor alar cartilages).
      • Parts: Root (between eyebrows), Bridge, Dorsum Nasi, Apex (tip), Alae (wings, bound nares laterally), Nares (Nostrils - external openings). Philtrum (of lip, inferior to nose).
      • Skin: Contains sebaceous glands. Vibrissae (hairs in vestibule).
    • Nasal Cavity: Extends from nares to choanae (posterior nasal apertures leading to nasopharynx). Divided by Nasal Septum.
      • Nasal Septum (midline):
        • Anterior: Septal cartilage.
        • Posterior: Perpendicular plate of Ethmoid bone (superiorly), Vomer (inferiorly).
        • Minor contributions from nasal crests of maxillae and palatine bones.
        • Mucosa: Olfactory (superior part), Respiratory (most of it). Kiesselbach's Plexus (Little's Area) on anterior septum - common site of epistaxis (nosebleed).
      • Floor: Hard palate (Palatine processes of maxillae, Horizontal plates of palatine bones).
      • Roof: Narrow, formed by Nasal bones, Frontal bone (nasal spine), Cribriform plate of Ethmoid bone, Body of Sphenoid bone.
      • Lateral Walls: Complex, formed by parts of Ethmoid (Superior & Middle conchae, uncinate process), Maxilla (nasal surface), Lacrimal bone, Inferior Nasal Concha (separate bone), Palatine bone (perpendicular plate), Medial pterygoid plate of Sphenoid.
        • Nasal Conchae (Turbinates - scroll-like bony projections covered by mucosa, increase surface area, create turbulence):
          • Superior Nasal Concha (part of ethmoid).
          • Middle Nasal Concha (part of ethmoid).
          • Inferior Nasal Concha (separate bone).
        • Nasal Meatuses (passageways inferior to each concha):
          • Superior Meatus: Sphenoethmoidal recess (above superior concha) receives opening of Sphenoidal sinus. Superior meatus receives openings of Posterior ethmoidal air cells.
          • Middle Meatus: Receives openings of Frontal sinus (via frontonasal duct or ethmoidal infundibulum), Maxillary sinus (ostium), Anterior ethmoidal air cells, Middle ethmoidal air cells (via ethmoidal bulla). Semilunar hiatus, Ethmoidal bulla.
          • Inferior Meatus: Receives opening of Nasolacrimal duct.
      • Nasal Vestibule: Dilated area just inside nares, lined by skin with vibrissae.
      • Choanae (Posterior Nasal Apertures): Openings from nasal cavity into nasopharynx.
      • Mucosa:
        • Olfactory Mucosa: In roof, upper septum, and superior conchae. Contains olfactory receptor cells (see Special Senses). Yellowish-brown.
        • Respiratory Mucosa: Lines most of nasal cavity. Pseudostratified ciliated columnar epithelium with Goblet cells (Respiratory Epithelium). Lamina propria rich in blood vessels (venous plexuses - for warming air) and seromucous glands. Swell bodies (venous erectile tissue, alternate congestion).
    • Paranasal Sinuses: Air-filled cavities in certain skull bones, lined by respiratory mucosa, drain into nasal cavity. Lighten skull, resonate voice, produce mucus.
      • Frontal Sinuses (in frontal bone, above orbits). Drain into middle meatus via frontonasal duct.
      • Ethmoidal Air Cells/Sinuses (in ethmoidal labyrinths, between orbit and nasal cavity):
        • Anterior Group: Drains into middle meatus (infundibulum/hiatus semilunaris).
        • Middle Group: Drains into middle meatus (on ethmoidal bulla).
        • Posterior Group: Drains into superior meatus.
      • Sphenoidal Sinuses (in body of sphenoid bone, posterior to upper nasal cavity). Drain into sphenoethmoidal recess.
      • Maxillary Sinuses (largest, in maxillae, lateral to nasal cavity). Ostium high on medial wall, drains into middle meatus (hiatus semilunaris).
  • 2. Pharynx (Throat): (Described in detail under Digestive System - Nasopharynx, Oropharynx, Laryngopharynx. Shared pathway).
  • 3. Larynx (Voice Box): Connects laryngopharynx to trachea. Air passage, voice production, prevents food entry into trachea. Located C3-C6 vertebrae.
    • Laryngeal Cartilages (9 total - 3 large unpaired, 3 small paired):
      • Unpaired:
        • Thyroid Cartilage (largest, hyaline): Two laminae fuse anteriorly to form Laryngeal Prominence ("Adam's apple"). Superior thyroid notch, Inferior thyroid notch. Superior/Inferior horns (cornua). Oblique line (for muscle attachment).
        • Cricoid Cartilage (hyaline): Complete ring, signet-ring shaped (narrow arch anteriorly, broad lamina posteriorly). Inferior to thyroid cartilage. Articulates with thyroid and arytenoid cartilages.
        • Epiglottis (elastic cartilage): Leaf-shaped, superior to larynx, posterior to root of tongue. Stalk (petiolus) attached to inner thyroid cartilage. Covers laryngeal inlet during swallowing. Valleculae (depressions between epiglottis and tongue base).
      • Paired:
        • Arytenoid Cartilages (2, hyaline mostly, some elastic): Pyramidal, sit on superior-posterior border of cricoid lamina.
          • Apex (superiorly, supports corniculate cartilage).
          • Muscular Process (posterolateral, for muscle attachment).
          • Vocal Process (anterior, for attachment of vocal ligament).
        • Corniculate Cartilages (2, of Santorini, elastic): Small, conical, on apices of arytenoid cartilages. In aryepiglottic folds.
        • Cuneiform Cartilages (2, of Wrisberg, elastic): Small, club-shaped, anterior to corniculate cartilages, within aryepiglottic folds. Support/stiffen folds.
    • Laryngeal Joints: Cricothyroid joints, Cricoarytenoid joints (allow movement of cartilages for vocal cord tension/abduction/adduction).
    • Ligaments & Membranes:
      • Extrinsic:
        • Thyrohyoid Membrane: Connects thyroid cartilage to hyoid bone. Median/Lateral thyrohyoid ligaments. Triticeal cartilage (small, in lateral thyrohyoid ligament).
        • Hyoepiglottic Ligament: Connects epiglottis to hyoid bone.
        • Cricotracheal Ligament: Connects cricoid cartilage to first tracheal ring.
      • Intrinsic:
        • Conus Elasticus (Cricovocal Membrane): From superior border of cricoid to vocal ligaments. Upper free edge forms Vocal Ligaments.
        • Quadrangular Membrane: From lateral epiglottis/thyroid to arytenoid/corniculate cartilages. Inferior free edge forms Vestibular Ligaments. Superior free edge in aryepiglottic fold.
    • Laryngeal Cavity (Lumen):
      • Laryngeal Inlet (Aditus): Opening from laryngopharynx into larynx. Bounded by epiglottis, aryepiglottic folds, interarytenoid notch.
      • Vestibule: Superior part, from inlet to vestibular folds.
      • Vestibular Folds (False Vocal Cords): Mucosal folds overlying vestibular ligaments. Pinkish. Superior to vocal folds.
      • Ventricle (Sinus) of Larynx: Recess between vestibular and vocal folds. Saccule of Larynx (mucous gland containing pouch extending from ventricle).
      • Vocal Folds (True Vocal Cords): Mucosal folds overlying vocal ligaments (elastic fibers) and vocalis muscle. Pearly white. Vibrate to produce sound.
      • Glottis: Comprises the vocal folds and the Rima Glottidis (space between them). Shape changes with phonation/respiration.
      • Infraglottic Cavity: Inferior part, from vocal folds to trachea.
    • Muscles of the Larynx (Intrinsic): Modify length/tension of vocal folds, open/close rima glottidis. (All innervated by Recurrent Laryngeal Nerve branch of Vagus, except Cricothyroid by External Laryngeal Nerve branch of Vagus).
      • Abductors of Vocal Folds (open glottis): Posterior Cricoarytenoid (only one).
      • Adductors of Vocal Folds (close glottis): Lateral Cricoarytenoid, Transverse Arytenoid, Oblique Arytenoid (Aryepiglottic part closes inlet).
      • Tensors of Vocal Folds: Cricothyroid (lengthens/tenses), Vocalis (part of Thyroarytenoid - fine adjustments of tension).
      • Relaxers of Vocal Folds: Thyroarytenoid (main body - shortens/relaxes).
    • Mucosa: Respiratory epithelium (pseudostratified ciliated columnar) below vocal folds and in parts above. Non-keratinized stratified squamous epithelium on vocal folds, epiglottis (lingual surface), aryepiglottic folds (areas of abrasion).
  • 4. Trachea (Windpipe): Flexible tube (~10-12 cm long, 2.5 cm diameter) from larynx (C6) to primary bronchi (T4/T5 - Carina). Anterior to esophagus.
    • Framework: 16-20 C-shaped rings of Hyaline Cartilage, open posteriorly. Annular Ligaments (fibroelastic CT) connect rings.
    • Trachealis Muscle: Smooth muscle and elastic tissue spanning posterior gap in cartilage rings. Allows esophageal expansion, controls tracheal diameter.
    • Carina: Ridge of cartilage at bifurcation of trachea into R/L main bronchi. Mucosa highly sensitive (cough reflex).
    • Layers of Tracheal Wall:
      • Mucosa: Respiratory epithelium (pseudostratified ciliated columnar with goblet cells, brush cells, basal cells, small granule cells/neuroendocrine cells). Lamina propria (loose CT, elastic fibers, lymphoid tissue).
      • Submucosa: CT with Seromucous Tracheal Glands.
      • Cartilaginous Layer (Hyaline cartilage rings).
      • Adventitia: Outer CT layer, blends with surrounding tissues.
  • 5. Bronchial Tree (Conducting Airways within Lungs): Series of branching tubes.
    • Main (Primary) Bronchi (Right & Left): From tracheal bifurcation to hilum of lung.
      • Right Main Bronchus: Wider, shorter, more vertical than left (aspirated objects more likely to enter here). Gives off superior lobar bronchus before entering hilum.
      • Left Main Bronchus: Narrower, longer, more horizontal. Passes inferior to aortic arch, anterior to esophagus.
      • Structure similar to trachea (cartilage rings, less complete as they branch).
    • Lobar (Secondary) Bronchi: Supply lobes of lungs.
      • Right: 3 (Superior, Middle, Inferior).
      • Left: 2 (Superior, Inferior).
      • Cartilage plates instead of rings.
    • Segmental (Tertiary) Bronchi: Supply bronchopulmonary segments.
      • Right: 10 (e.g., Apical, Posterior, Anterior in superior lobe).
      • Left: 8-10 (e.g., Apicoposterior, Anterior in superior lobe; Lingular superior/inferior).
      • Each segmental bronchus supplies a functionally independent unit of lung (Bronchopulmonary Segment) with its own artery/vein.
    • Bronchioles (diameter < 1 mm):
      • Conducting Bronchioles (larger, many generations of branching). Cartilage absent, replaced by smooth muscle. Epithelium changes from pseudostratified ciliated columnar to simple ciliated columnar/cuboidal. Goblet cells decrease. Clara cells (Club cells) appear.
      • Terminal Bronchioles: Smallest conducting bronchioles (<0.5 mm). Simple cuboidal epithelium, many Clara cells (secrete surfactant-like substance, detoxify, stem cells), few cilia, no goblet cells. Prominent smooth muscle.
    • Respiratory Zone (Site of Gas Exchange):
      • Respiratory Bronchioles: Transition zone. Branch from terminal bronchioles. Thin walls, simple cuboidal epithelium (some ciliated, some Clara). Scattered Alveoli bud off their walls.
      • Alveolar Ducts: Long, thin-walled passages, completely lined by alveoli. Simple squamous epithelium. Scant smooth muscle in knobs between alveoli.
      • Alveolar Sacs: Clusters of alveoli at end of alveolar ducts (like bunch of grapes).
      • Alveoli (singular: Alveolus): Microscopic air sacs (~300 million per lung), primary site of gas exchange.
        • Wall Structure:
          • Type I Alveolar Cells (Type I Pneumocytes): Simple squamous epithelial cells, form ~95% of alveolar surface. Extremely thin, for gas diffusion.
          • Type II Alveolar Cells (Type II Pneumocytes/Septal Cells): Cuboidal epithelial cells, scattered among Type I. Secrete Pulmonary Surfactant (lipoprotein complex, reduces surface tension, prevents alveolar collapse). Can proliferate and differentiate into Type I cells (progenitors). Lamellar bodies (surfactant storage).
          • Alveolar Macrophages (Dust Cells): Phagocytes in alveolar lumen and interstitium, clear debris/pathogens.
        • Interalveolar Septum: Wall between adjacent alveoli. Contains elastic fibers, reticular fibers, extensive capillary network (from pulmonary arteries). Pores of Kohn (small openings in septa, allow air passage/collateral ventilation between alveoli).
        • Respiratory Membrane (Alveolocapillary Barrier - for gas exchange, ~0.5 µm thick):
          • Type I Alveolar cell cytoplasm.
          • Fused basal laminae of alveolar cell and capillary endothelial cell.
          • Capillary endothelial cell cytoplasm.
          • (Surfactant layer on alveolar surface).
  • 6. Lungs (Pulmones): Paired, cone-shaped organs in thoracic cavity, separated by mediastinum.
    • Parts:
      • Apex: Superior tip, extends into root of neck (above clavicle).
      • Base: Inferior concave surface, rests on diaphragm.
      • Costal Surface: Large, convex, adjacent to ribs/costal cartilages.
      • Mediastinal (Medial) Surface: Concave, faces mediastinum. Contains Hilum.
        • Hilum (Root of Lung): Region where bronchi, pulmonary vessels, lymphatic vessels, nerves enter/exit lung. Structures collectively form the Root.
      • Borders: Anterior, Posterior, Inferior.
    • Right Lung: Shorter (due to liver), broader. 3 Lobes, 2 Fissures.
      • Lobes: Superior, Middle, Inferior.
      • Fissures:
        • Oblique Fissure: Separates inferior lobe from superior and middle lobes.
        • Horizontal Fissure: Separates superior lobe from middle lobe.
      • Cardiac Impression (less prominent than left). Grooves for SVC, azygos vein, esophagus.
    • Left Lung: Longer, narrower. 2 Lobes, 1 Fissure.
      • Lobes: Superior, Inferior.
      • Fissure: Oblique Fissure: Separates superior and inferior lobes.
      • Cardiac Notch: Indentation for heart on anterior border of superior lobe.
      • Lingula: Tongue-like projection of superior lobe, inferior to cardiac notch (homologous to middle lobe of right lung).
      • Deep Cardiac Impression. Groove for aortic arch and descending aorta.
    • Bronchopulmonary Segments: Anatomically and functionally distinct units within each lobe, supplied by a segmental bronchus and a tertiary branch of pulmonary artery. Drained by intersegmental pulmonary veins. Surrounded by CT septa. Surgically resectable. (10 in right lung, 8-10 in left lung).
    • Pleurae & Pleural Cavity: Serous membranes surrounding lungs.
      • Visceral Pleura: Covers lung surface, dips into fissures. Inseparable from lung tissue.
      • Parietal Pleura: Lines thoracic wall, diaphragm, mediastinum.
        • Costal Pleura (lines ribs, sternum, intercostal spaces).
        • Diaphragmatic Pleura (covers diaphragm).
        • Mediastinal Pleura (covers mediastinum).
        • Cervical Pleura (Cupula - extends into root of neck, covers apex of lung).
      • Pleural Cavity: Potential space between visceral and parietal pleura. Contains thin film of Pleural Fluid (lubricates, allows layers to slide, creates surface tension for lung expansion).
      • Pleural Recesses: Potential spaces where parts of parietal pleura are in contact, lungs don't fully occupy them during quiet breathing.
        • Costodiaphragmatic Recesses (largest, inferiorly between costal and diaphragmatic pleura).
        • Costomediastinal Recesses (anteriorly between costal and mediastinal pleura, especially left side near cardiac notch).
    • Blood Supply:
      • Pulmonary Circulation (for gas exchange): Pulmonary Arteries (deoxygenated blood from RV) branch alongside bronchi -> Arterioles -> Capillary plexuses around alveoli. Pulmonary Veins (oxygenated blood) -> LA.
      • Bronchial Circulation (supplies lung tissues/bronchi themselves): Bronchial Arteries (oxygenated blood, usually from thoracic aorta or intercostal arteries). Bronchial Veins (drain into pulmonary veins or azygos system).
    • Innervation: Pulmonary Plexus (anterior/posterior to root of lung). Autonomic fibers (parasympathetic from Vagus - bronchoconstriction, mucus secretion; sympathetic from T1-T4 ganglia - bronchodilation, vasoconstriction). Visceral afferent fibers (stretch, irritation).
    • Lymphatic Drainage: Superficial (subpleural) and Deep lymphatic plexuses -> Bronchopulmonary (Hilar) nodes -> Tracheobronchial nodes (Superior, Inferior/Carinal) -> Paratracheal nodes -> Bronchomediastinal trunks.

E. Urinary System

  • 1. Kidneys (Renes): Paired, bean-shaped organs, retroperitoneal, on posterior abdominal wall (T12-L3 vertebrae). Right kidney usually slightly lower than left (due to liver).
    • External Anatomy:
      • Surfaces: Anterior, Posterior.
      • Borders: Superior, Inferior, Medial (concave), Lateral (convex).
      • Hilum (Renal Hilum): Vertical cleft on medial border. Entry/exit point for Renal Artery, Renal Vein, Ureter, nerves, lymphatics. Renal Pelvis is posterior-most structure in hilum (Vein-Artery-Pelvis: VAP from anterior to posterior).
      • Coverings (Capsules):
        • Renal Capsule (Fibrous Capsule): Innermost, transparent fibrous CT layer, adheres directly to kidney surface. Protects from trauma/infection.
        • Adipose Capsule (Perirenal Fat / Perinephric Fat): Middle layer of fatty tissue, cushions kidney.
        • Renal Fascia (Gerota's Fascia): Outer dense irregular CT layer, anchors kidney and adrenal gland to surrounding structures. Encloses kidney and adipose capsule. Pararenal fat (Paranephric fat) is external to renal fascia.
    • Internal Anatomy (Coronal Section):
      • Renal Cortex: Outer, light-colored, granular region. Extends inwards as Renal Columns between pyramids.
        • Cortical Labyrinth: Contains renal corpuscles, proximal/distal convoluted tubules.
        • Medullary Rays (Ferrein's pyramids): Bundles of collecting ducts and loops of Henle extending from medulla into cortex.
      • Renal Medulla: Inner, darker, striated region. Consists of:
        • Renal Pyramids (Medullary Pyramids): 8-18 cone-shaped structures. Base faces cortex, Apex (Renal Papilla) points internally towards minor calyces. Contain loops of Henle, collecting ducts.
        • Renal Columns (of Bertin): Inward extensions of cortical tissue separating pyramids.
      • Renal Lobe: A renal pyramid + overlying cortical tissue + half of adjacent renal columns. Functional unit.
      • Renal Papilla: Apex of each renal pyramid, projects into a minor calyx. Ducts of Bellini (papillary ducts) open here. Area Cribrosa (perforated surface of papilla).
      • Minor Calyces (singular: Calyx): Cup-shaped structures enclosing renal papillae, collect urine. 8-18 per kidney.
      • Major Calyces: Formed by union of 2-3 minor calyces. 2-3 per kidney.
      • Renal Pelvis: Funnel-shaped expansion at upper end of ureter, within renal sinus. Formed by union of major calyces.
      • Renal Sinus: Fat-filled cavity within kidney, medial to hilum, contains renal pelvis, calyces, vessels, nerves.
    • Microscopic Anatomy (Nephron & Collecting System):
      • Nephron: Functional unit of kidney, responsible for filtration, reabsorption, secretion. ~1 million per kidney.
        • Renal Corpuscle (Malpighian Corpuscle): Filtration unit.
          • Glomerulus: Tuft of fenestrated capillaries. Supplied by Afferent Arteriole, drained by Efferent Arteriole.
          • Glomerular (Bowman's) Capsule: Double-walled epithelial cup surrounding glomerulus.
            • Parietal Layer: Outer simple squamous epithelium.
            • Visceral Layer: Inner layer, modified epithelial cells called Podocytes. Podocytes have foot processes (Pedicels) that interdigitate, forming Filtration Slits (covered by slit diaphragms - nephrin).
            • Bowman's Space (Capsular/Urinary Space): Space between visceral and parietal layers, collects filtrate.
          • Filtration Membrane (Barrier): From capillary lumen to Bowman's space:
            1. Fenestrated endothelium of glomerulus (prevents passage of cells).
            2. Glomerular Basement Membrane (GBM - fused basal laminae of endothelium and podocytes; thick, negatively charged, restricts large proteins). Lamina rara interna, lamina densa, lamina rara externa.
            3. Filtration slits between pedicels of podocytes (covered by slit diaphragms, restrict medium-sized proteins).
          • Mesangium: Mesangial Cells (intraglomerular and extraglomerular) and matrix. Provide structural support, phagocytosis, contractility (regulate GFR), secrete prostaglandins.
        • Renal Tubule: Coiled and straight tube, modifies filtrate into urine.
          • Proximal Convoluted Tubule (PCT): Longest, most coiled part. Arises from Bowman's capsule (at urinary pole). Lined by simple cuboidal epithelium with prominent Brush Border (microvilli - for reabsorption), abundant mitochondria (basal striations). Majority of reabsorption occurs here.
          • Loop of Henle (Nephron Loop): U-shaped. Dips into medulla.
            • Descending Limb:
              • Thick Descending Limb (Pars Recta of PCT): Similar to PCT.
              • Thin Descending Limb: Simple squamous epithelium. Permeable to water, less to solutes.
            • Ascending Limb:
              • Thin Ascending Limb: Simple squamous epithelium. Impermeable to water, permeable to solutes (passive NaCl reabsorption).
              • Thick Ascending Limb (TAL / Pars Recta of DCT / Diluting Segment): Simple cuboidal epithelium, no brush border, many mitochondria. Actively reabsorbs NaCl, impermeable to water. Contributes to medullary osmotic gradient. Macula Densa located here.
          • Distal Convoluted Tubule (DCT): Shorter, less coiled than PCT. Lined by simple cuboidal epithelium, fewer microvilli, no distinct brush border. Reabsorbs Na+, Cl-, Ca2+ (under PTH influence), secretes K+, H+.
        • Types of Nephrons:
          • Cortical Nephrons (~85%): Short loops of Henle, mainly in cortex, may dip slightly into outer medulla. Peritubular capillaries.
          • Juxtamedullary Nephrons (~15%): Long loops of Henle extending deep into inner medulla. Essential for concentrating urine. Efferent arterioles form Vasa Recta.
      • Juxtaglomerular Apparatus (JGA) / Complex: Region where initial DCT contacts afferent (and sometimes efferent) arteriole of its own glomerulus (at vascular pole). Regulates blood pressure and GFR.
        • Macula Densa: Specialized patch of tall, narrow epithelial cells in TAL/initial DCT. Chemoreceptors sensitive to NaCl concentration in tubular fluid.
        • Juxtaglomerular (JG) Cells (Granular Cells): Modified smooth muscle cells in wall of afferent arteriole (and sometimes efferent). Mechanoreceptors sensitive to blood pressure. Synthesize, store, secrete Renin.
        • Extraglomerular Mesangial Cells (Lacis Cells / Goormaghtigh Cells): In angle between afferent/efferent arterioles and macula densa. Function uncertain (possibly signal transmission).
      • Collecting System: Transports and modifies urine from nephrons to minor calyces.
        • Connecting Tubules (Arcade Tubules): Link DCTs to collecting ducts.
        • Collecting Ducts: Receive filtrate from several nephrons. Run through cortex (Cortical Collecting Ducts) and medulla (Medullary Collecting Ducts).
          • Lined by simple cuboidal (cortex) to simple columnar (medulla) epithelium.
          • Principal Cells: Most numerous. Reabsorb Na+ and water (ADH-sensitive aquaporins), secrete K+.
          • Intercalated Cells (Alpha & Beta types): Regulate acid-base balance by secreting H+ (alpha) or HCO3- (beta).
        • Papillary Ducts (of Bellini): Large ducts formed by merging of medullary collecting ducts. Open at renal papilla into minor calyx. Lined by tall columnar epithelium.
    • Blood Supply: Kidneys receive ~20-25% of cardiac output.
      • Renal Artery (from abdominal aorta) -> Segmental Arteries (supply segments) -> Interlobar Arteries (in renal columns) -> Arcuate Arteries (arch over bases of pyramids, at corticomedullary junction) -> Cortical Radiate Arteries (Interlobular Arteries - ascend into cortex) -> Afferent Arterioles -> Glomeruli -> Efferent Arterioles ->
        • Peritubular Capillaries (around PCT/DCT in cortex, from efferent arterioles of cortical nephrons).
        • Vasa Recta (long, straight capillary loops in medulla, alongside loops of Henle, from efferent arterioles of juxtamedullary nephrons - countercurrent exchange).
      • Venous Drainage (parallels arterial supply): Peritubular Capillaries/Vasa Recta -> Cortical Radiate Veins (Interlobular Veins) -> Arcuate Veins -> Interlobar Veins -> Renal Vein (to IVC). (No segmental veins).
    • Innervation: Renal Plexus (sympathetic fibers from celiac plexus, T10-L1 - regulate blood flow, renin release; parasympathetic from vagus - function uncertain). Sensory fibers.
  • 2. Ureters: Paired muscular tubes (~25-30 cm long) conveying urine from renal pelvis to urinary bladder. Retroperitoneal.
    • Course: Descend from renal pelvis, pass over pelvic brim (crossing common iliac artery bifurcation), run along lateral pelvic wall, enter bladder posteroinferiorly at an oblique angle (prevents reflux).
    • Constrictions (sites of potential stone impaction):
      • Ureteropelvic Junction (UPJ).
      • Where ureter crosses pelvic brim / iliac vessels.
      • Ureterovesical Junction (UVJ - where ureter enters bladder wall).
    • Layers of Ureter Wall:
      • Mucosa: Transitional Epithelium (Urothelium - stratified, specialized for distension and protection from urine). Lamina propria (dense CT).
      • Muscularis: Smooth muscle layers, propel urine via peristalsis.
        • Upper 2/3: Inner Longitudinal, Outer Circular (opposite of GI tract).
        • Lower 1/3: Adds a 3rd Outer Longitudinal layer.
      • Adventitia: Outer loose CT layer.
  • 3. Urinary Bladder: Hollow, distensible muscular organ in pelvis, posterior to pubic symphysis. Stores urine.
    • Shape/Location: Pyramidal when empty (tetrahedral - apex, base, superior surface, 2 inferolateral surfaces). Ovoid when full, expands superiorly into abdominal cavity.
    • Parts:
      • Apex: Points anteriorly towards pubic symphysis. Median Umbilical Ligament (urachus remnant) extends from apex.
      • Body: Main central part.
      • Fundus (Base): Posterior wall, triangular. Receives ureters.
      • Neck: Inferior constricted part, leads to urethra. Surrounded by internal urethral sphincter.
    • Trigone: Smooth triangular area on inner posterior wall, between ureteric orifices (superolaterally) and internal urethral orifice (inferiorly). Mucosa firmly adherent. Sensitive to stretch.
    • Ureteric Orifices: Slit-like openings of ureters.
    • Internal Urethral Orifice: Opening into urethra.
    • Rugae: Folds of mucosa (except in trigone) when bladder is empty.
    • Ligaments: Median umbilical ligament, Medial umbilical ligaments (obliterated umbilical arteries), Pubovesical ligaments (female) / Puboprostatic ligaments (male), Lateral ligaments of bladder.
    • Layers of Bladder Wall:
      • Mucosa: Transitional Epithelium (Urothelium). Lamina propria.
      • Submucosa (not always distinct from lamina propria).
      • Muscularis (Detrusor Muscle): Three poorly differentiated layers of smooth muscle (inner/outer longitudinal, middle circular). Contracts during micturition. Fibers around neck form Internal Urethral Sphincter (involuntary).
      • Serosa (Visceral peritoneum) on superior surface and upper parts of lateral surfaces. Adventitia on other parts.
  • 4. Urethra: Muscular tube conveying urine from bladder out of body. Also pathway for semen in males.
    • Female Urethra: ~3-4 cm long. Extends from internal urethral orifice (bladder neck) to External Urethral Orifice (in vestibule, anterior to vaginal opening, posterior to clitoris).
      • Passes through urogenital diaphragm (deep perineal pouch), where it's surrounded by External Urethral Sphincter (skeletal muscle, voluntary).
      • Mucosa: Transitional epithelium near bladder, then pseudostratified/stratified columnar, then stratified squamous near external orifice. Urethral glands (mucous).
      • Muscularis: Smooth muscle layers.
    • Male Urethra: ~18-22 cm long. S-shaped.
      • Three Parts:
        • Prostatic Urethra (~3-4 cm): Passes through prostate gland. Widest, most dilatable part. Receives openings of Ejaculatory Ducts (carrying sperm and seminal fluid) and Ducts of Prostate Gland. Urethral Crest (median ridge on posterior wall), Prostatic Sinuses (grooves on either side of crest where prostatic ducts open), Prostatic Utricle (small pouch on crest, homolog of uterus/vagina), Openings of ejaculatory ducts (on either side of utricle).
        • Membranous (Intermediate) Urethra (~1-2 cm): Passes through urogenital diaphragm (deep perineal pouch). Narrowest, least distensible part. Surrounded by External Urethral Sphincter (skeletal muscle, voluntary). Bulbourethral (Cowper's) glands are posterolateral, ducts open into spongy urethra.
        • Spongy (Penile) Urethra (~15 cm): Passes through corpus spongiosum of penis. Extends from bulb of penis to External Urethral Orifice on glans penis. Receives ducts of Bulbourethral glands (in bulb). Urethral Lacunae (small mucosal recesses), Glands of Littré (mucous glands in submucosa). Navicular Fossa (dilated part within glans penis before external orifice).
      • Mucosa: Transitional epithelium in prostatic part (near bladder), then pseudostratified/stratified columnar, then stratified squamous in navicular fossa/external orifice.
      • Submucosa: Erectile vascular tissue (part of corpus spongiosum).
      • Muscularis: Smooth muscle layers (continuous with bladder).
    • Internal Urethral Sphincter: Smooth muscle at bladder neck (more defined in males, prevents semen reflux into bladder).
    • External Urethral Sphincter: Skeletal muscle, part of urogenital diaphragm, voluntary control.

F. Endocrine System

  • General Characteristics: Glands that secrete hormones (chemical messengers) directly into interstitial fluid, then into bloodstream to target cells/organs. Highly vascular.
  • 1. Pituitary Gland (Hypophysis): Small gland (~1 cm diameter) located in Sella Turcica of sphenoid bone. Connected to hypothalamus by Infundibulum (Pituitary Stalk).
    • Two Major Lobes with different embryological origins:
      • Anterior Pituitary (Adenohypophysis): Glandular tissue, derived from oral ectoderm (Rathke's Pouch). Comprises ~75% of gland.
        • Pars Distalis (Anterior Lobe proper): Largest part. Cells arranged in cords/clusters around fenestrated capillaries.
          • Chromophils (stain intensely):
            • Acidophils (stain with acidic dyes, e.g., eosin):
              • Somatotrophs: Secrete Growth Hormone (GH / Somatotropin). Most abundant type.
              • Lactotrophs (Mammotrophs): Secrete Prolactin (PRL). Increase during pregnancy/lactation.
            • Basophils (stain with basic dyes, e.g., hematoxylin):
              • Corticotrophs: Secrete Adrenocorticotropic Hormone (ACTH / Corticotropin) and Melanocyte-Stimulating Hormone (MSH) (derived from Pro-opiomelanocortin - POMC).
              • Thyrotrophs: Secrete Thyroid-Stimulating Hormone (TSH / Thyrotropin).
              • Gonadotrophs: Secrete Follicle-Stimulating Hormone (FSH) and Luteinizing Hormone (LH).
          • Chromophobes (stain poorly): Degranulated chromophils, undifferentiated stem cells, or follicular cells.
        • Pars Tuberalis: Wraps around infundibulum. Contains some gonadotrophs, thyrotrophs. Highly vascular.
        • Pars Intermedia: Thin wedge between pars distalis and pars nervosa (posterior pituitary). Poorly developed in adult humans, often contains colloid-filled cysts (Rathke's cysts). Produces MSH (from POMC, mainly in fetal life and certain conditions).
        • Hypothalamo-Hypophyseal Portal System: Vascular connection between hypothalamus and anterior pituitary.
          • Superior Hypophyseal Arteries (from internal carotid) form Primary Capillary Plexus in median eminence/infundibulum.
          • Hypothalamic releasing/inhibiting hormones (e.g., GHRH, GnRH, CRH, TRH, Somatostatin, Dopamine) secreted here.
          • Portal Veins (Hypophyseal Portal Veins) carry these hormones down infundibulum to Secondary Capillary Plexus in pars distalis, where they regulate anterior pituitary hormone secretion.
      • Posterior Pituitary (Neurohypophysis): Neural tissue, derived from neuroectoderm (downgrowth of hypothalamus). Comprises ~25% of gland. Does not synthesize hormones; stores and releases hormones produced by hypothalamus.
        • Pars Nervosa (Posterior Lobe proper): Largest part. Contains:
          • Axons (unmyelinated) of neurosecretory cells whose cell bodies are in Supraoptic and Paraventricular Nuclei of hypothalamus. These form the Hypothalamo-Hypophyseal Tract.
          • Herring Bodies: Dilations along axons, especially at terminals, containing stored neurosecretory vesicles (with Oxytocin or Antidiuretic Hormone/ADH/Vasopressin bound to neurophysin).
          • Pituicytes: Specialized glial cells (similar to astrocytes), support axons.
        • Infundibulum (Pituitary Stalk): Connects pars nervosa to hypothalamus. Contains axons of hypothalamo-hypophyseal tract and vessels of portal system. Median Eminence (part of hypothalamus at base of infundibulum).
        • Hormones released:
          • Antidiuretic Hormone (ADH / Vasopressin): Synthesized mainly in Supraoptic Nucleus. Increases water reabsorption in kidney collecting ducts.
          • Oxytocin: Synthesized mainly in Paraventricular Nucleus. Stimulates uterine contractions, milk ejection.
        • Blood supply: Inferior Hypophyseal Arteries.
  • 2. Thyroid Gland: Butterfly-shaped gland in anterior neck, inferior to larynx, anterior and lateral to trachea (C5-T1 level). Largest pure endocrine gland.
    • Lobes: Right and Left Lateral Lobes, connected by a median Isthmus (anterior to 2nd-4th tracheal rings).
    • Pyramidal Lobe (variable): Superior extension from isthmus, remnant of thyroglossal duct.
    • Capsule: Thin fibrous capsule, and an outer pretracheal fascia.
    • Microscopic Anatomy:
      • Thyroid Follicles: Spherical structures, functional units of thyroid. Lined by simple cuboidal/squamous (inactive) to columnar (active) Follicular Cells. Lumen filled with Colloid (gel-like substance, mainly Thyroglobulin - iodinated glycoprotein, storage form of thyroid hormones).
        • Follicular Cells: Synthesize thyroglobulin and thyroid hormones (Thyroxine/T4, Triiodothyronine/T3) under TSH stimulation. Apical microvilli. Rich in RER, Golgi, lysosomes.
      • Parafollicular Cells (C Cells / Clear Cells): Larger, pale-staining cells located in clusters between follicles or within follicular epithelium (but not reaching lumen). Neuroendocrine origin (neural crest). Secrete Calcitonin (Peptide hormone, lowers blood Ca2+ levels, inhibits osteoclast activity).
    • Blood Supply: Highly vascular. Superior Thyroid Arteries (from external carotids), Inferior Thyroid Arteries (from thyrocervical trunks). Extensive capillary network around follicles.
    • Venous Drainage: Superior, Middle, Inferior Thyroid Veins.
  • 3. Parathyroid Glands: Small, ovoid glands, typically 4 in number (Superior & Inferior pairs), embedded in posterior surface of thyroid gland (usually outside thyroid capsule but within its fascial sheath). Variable in number and location.
    • Microscopic Anatomy:
      • Chief (Principal) Cells: Most numerous, small, polygonal, slightly eosinophilic/clear cytoplasm. Synthesize and secrete Parathyroid Hormone (PTH / Parathormone) - major regulator of blood Ca2+ levels (increases Ca2+ by stimulating osteoclasts, enhancing kidney reabsorption, promoting Vitamin D activation). Contain PTH granules.
      • Oxyphil Cells: Larger, strongly eosinophilic (abundant mitochondria), less numerous than chief cells. Appear around puberty, increase with age. Function uncertain (possibly inactive/modified chief cells, or source of some PTH). Often in clusters.
      • Adipocytes: Increase with age, can constitute >50% of gland in older adults.
    • Blood Supply: Mainly from Inferior Thyroid Arteries.
  • 4. Adrenal (Suprarenal) Glands: Paired, pyramid-shaped (right) or crescent-shaped (left) glands, superior to kidneys (like caps). Retroperitoneal. Enclosed within renal fascia and adipose capsule.
    • Structure: Outer Cortex and Inner Medulla (distinct embryologically, structurally, functionally).
      • Adrenal Cortex (outer ~80-90%): Glandular tissue, derived from mesoderm. Secretes steroid hormones (Corticosteroids). Zonation:
        • Zona Glomerulosa (outermost, ~15%): Cells arranged in rounded clusters/arches. Secrete Mineralocorticoids (mainly Aldosterone - regulates Na+/K+ balance, blood pressure/volume, stimulated by Angiotensin II and K+).
        • Zona Fasciculata (middle, thickest, ~75-80%): Cells arranged in long, straight cords (fascicles) separated by sinusoidal capillaries. Cells large, foamy cytoplasm (Spongiocytes - due to lipid droplets, cholesterol precursors). Secrete Glucocorticoids (mainly Cortisol/Hydrocortisone - stress response, metabolism, anti-inflammatory; also Corticosterone, Cortisone). Regulated by ACTH.
        • Zona Reticularis (innermost, ~5-10%): Cells arranged in irregular, anastomosing cords. Cells smaller, more eosinophilic than fasciculata cells. Secrete small amounts of Glucocorticoids and Adrenal Androgens (e.g., Dehydroepiandrosterone/DHEA, Androstenedione - precursors to sex hormones, contribute to libido, secondary sex characteristics, especially in females). Regulated by ACTH.
      • Adrenal Medulla (inner ~10-20%): Neuroendocrine tissue, derived from neural crest cells (modified sympathetic ganglion).
        • Chromaffin Cells (Medullary Cells): Large, epithelioid cells arranged in clusters/cords around medullary veins and sinuses. Modified postganglionic sympathetic neurons (lack axons). Cytoplasm contains granules storing Catecholamines.
          • Epinephrine (Adrenaline) - secreting cells (most numerous, ~80%): Smaller granules.
          • Norepinephrine (Noradrenaline) - secreting cells (~20%): Larger, denser granules.
          • Secretion stimulated by preganglionic sympathetic fibers (acetylcholine). Involved in acute stress response ("fight or flight").
        • Ganglion Cells (sympathetic) may be present.
    • Blood Supply: Rich. Superior, Middle, Inferior Suprarenal Arteries (from inferior phrenic, aorta, renal arteries respectively). Form capsular plexus -> cortical sinusoids -> medullary sinusoids.
    • Venous Drainage: Single large Central Adrenomedullary Vein (or Suprarenal Vein) drains each gland (Right Suprarenal Vein to IVC; Left Suprarenal Vein to Left Renal Vein). Cortical blood (rich in corticosteroids) bathes medullary cells (influences catecholamine synthesis, e.g., cortisol induces PNMT for epinephrine synthesis).
  • 5. Pineal Gland (Epiphysis Cerebri): Small, pine cone-shaped gland (~5-8 mm) located in posterior epithalamus, near roof of third ventricle. Part of diencephalon.
    • Cells:
      • Pinealocytes: Main cells, modified neurons, arranged in clusters/cords. Large, irregular nuclei, basophilic cytoplasm. Produce Melatonin (derived from serotonin - regulates circadian rhythms, sleep-wake cycles, possibly reproductive functions). Secretion stimulated by darkness, inhibited by light (via retinohypothalamic tract).
      • Interstitial (Glial) Cells: Astrocyte-like supporting cells.
    • Corpora Arenacea ("Brain Sand"): Calcified concretions (calcium phosphates/carbonates) in extracellular space. Increase with age. Characteristic histological feature.
    • Innervation: Primarily from postganglionic sympathetic fibers (Superior Cervical Ganglion).
  • 6. Pancreatic Islets (Islets of Langerhans): Clusters of endocrine cells scattered throughout exocrine pancreas (more numerous in tail). ~1-2 million islets.
    • Cell Types (identified by immunocytochemistry/granule morphology):
      • Alpha (α) Cells (~15-20%): Usually peripheral. Secrete Glucagon (increases blood glucose by stimulating glycogenolysis/gluconeogenesis in liver).
      • Beta (β) Cells (~70-80%): Usually central. Secrete Insulin (lowers blood glucose by promoting glucose uptake/utilization/storage by cells) and Amylin (co-secreted, satiety, slows gastric emptying).
      • Delta (δ) Cells (~5-10%): Scattered. Secrete Somatostatin (paracrine - inhibits insulin/glucagon secretion; endocrine - inhibits GH, TSH, GI functions).
      • PP (Gamma/F) Cells (~1-2%): Mainly in head of pancreas. Secrete Pancreatic Polypeptide (inhibits pancreatic exocrine secretion, gallbladder contraction, influences gut motility, satiety).
      • Epsilon (ε) Cells (<1%): Secrete Ghrelin (stimulates appetite, GH release).
    • Highly vascularized by fenestrated capillaries. Innervated by autonomic fibers.
  • 7. Ovaries & Testes (Gonads): (Also primary reproductive organs - detailed structure in Reproductive System). Produce steroid sex hormones and gametes.
    • Ovaries (Female):
      • Hormones: Estrogens (e.g., Estradiol - from developing follicles, granulosa/theca cells), Progesterone (from Corpus Luteum - granulosa lutein/theca lutein cells), Inhibin, Activin, Relaxin.
    • Testes (Male):
      • Hormones: Androgens (mainly Testosterone - from Interstitial (Leydig) Cells in CT between seminiferous tubules), Inhibin, Activin (from Sertoli cells).
  • 8. Endocrine Cells in Other Organs (Diffuse Neuroendocrine System - DNES / APUD cells): Isolated or small clusters of hormone-secreting cells in various organs.
    • Digestive Tract (Enteroendocrine Cells / Gastroenteropancreatic - GEP cells):
      • Stomach: G cells (Gastrin), D cells (Somatostatin), ECL cells (Histamine), Ghrelin cells.
      • Small Intestine: I cells (CCK), S cells (Secretin), K cells (GIP), L cells (GLP-1, PYY), Mo cells (Motilin), D cells (Somatostatin), Serotonin-producing cells (Enterochromaffin cells).
      • Large Intestine: Similar to small intestine but fewer.
    • Kidneys:
      • Juxtaglomerular (JG) Cells: Secrete Renin (part of RAAS system for blood pressure regulation).
      • Erythropoietin (EPO) - producing cells (Peritubular interstitial fibroblasts in cortex): Secrete EPO (stimulates RBC production).
      • Cells involved in Vitamin D activation (Calcitriol production - proximal tubule cells).
    • Heart:
      • Atrial Myocytes (in atria): Secrete Atrial Natriuretic Peptide (ANP / ANF - lowers blood pressure/volume by promoting Na+ excretion, vasodilation).
      • Ventricular Myocytes: Secrete Brain Natriuretic Peptide (BNP - similar effects to ANP, marker for heart failure).
    • Liver: Secretes Angiotensinogen (precursor to angiotensin), Thrombopoietin (stimulates platelet production), Insulin-like Growth Factor 1 (IGF-1 / Somatomedin C - mediates GH effects).
    • Placenta (Temporary endocrine organ during pregnancy): Secretes Human Chorionic Gonadotropin (hCG), Human Placental Lactogen (hPL / Chorionic Somatomammotropin), Estrogens, Progesterone, Relaxin.
    • Adipose Tissue: Secretes Leptin (satiety, energy balance), Adiponectin (insulin sensitivity, anti-inflammatory), Resistin.
    • Thymus: Secretes Thymosins, Thymopoietin, Thymulin (involved in T-cell maturation).
    • Skin: Produces Cholecalciferol (Vitamin D3) upon UV exposure.
    • Lungs: Neuroendocrine cells (Pulmonary neuroendocrine cells - PNECs / Kulchitsky cells) in bronchial epithelium, often in clusters (Neuroepithelial bodies - NEBs). Secrete serotonin, bombesin, calcitonin gene-related peptide (CGRP). Role in lung development, hypoxia sensing.

G. Male Reproductive System

  • 1. Scrotum: Dual-chambered sac of skin and superficial fascia, hanging outside abdominopelvic cavity at root of penis. Supports and houses testes.
    • Skin: Thin, pigmented, rugose (wrinkled). Median Raphe (ridge, marks fusion of urethral folds).
    • Dartos Muscle: Layer of smooth muscle in superficial fascia, responsible for wrinkling of scrotal skin (helps regulate temperature by changing surface area).
    • Cremaster Muscle: Bands of skeletal muscle (extension of internal oblique abdominal muscle) that elevate testes (closer to body for warmth, or during sexual arousal/danger - cremasteric reflex).
    • Scrotal Septum: Internally divides scrotum into two compartments, one for each testis.
    • Temperature Regulation: Location outside body cavity and muscular actions maintain scrotal temperature ~2-3°C lower than core body temperature, essential for viable sperm production. Pampiniform plexus (venous network) also contributes via countercurrent heat exchange.
  • 2. Testes (Testicles / Male Gonads - Paired): Primary reproductive organs, produce sperm and androgens (testosterone). Oval-shaped, ~4-5 cm long. Suspended in scrotum by spermatic cord.
    • Coverings:
      • Tunica Vaginalis (serous membrane, derived from peritoneum, carried down during testicular descent):
        • Parietal Layer (outer).
        • Visceral Layer (inner, adheres to testis).
        • Cavity of Tunica Vaginalis (potential space between layers, contains serous fluid).
      • Tunica Albuginea: Tough, white, fibrous CT capsule deep to visceral tunica vaginalis. Thickens posteriorly to form Mediastinum Testis.
    • Internal Structure:
      • Mediastinum Testis: Posterior thickening of tunica albuginea, projects into testis. Blood vessels, lymphatics, ducts enter/exit here.
      • Septa (Fibrous septa extend from tunica albuginea/mediastinum testis inward, dividing testis into ~250-300 Lobules).
      • Lobules: Each contains 1-4 tightly coiled Seminiferous Tubules.
      • Seminiferous Tubules: Site of spermatogenesis. ~50-80 cm long if uncoiled.
        • Epithelium (Stratified, complex):
          • Sertoli (Sustentacular) Cells: Large, columnar, non-dividing supporting cells. Extend from basal lamina to lumen. Form Blood-Testis Barrier (via tight junctions). Nourish/support developing sperm cells, phagocytize excess cytoplasm, secrete inhibin, activin, androgen-binding protein (ABP), Mullerian-inhibiting substance (MIS) during development.
          • Spermatogenic Cells (Germ cells - in various stages of development, arranged in layers between Sertoli cells, from basal lamina to lumen): Spermatogonia (stem cells, diploid), Primary Spermatocytes (diploid, undergo meiosis I), Secondary Spermatocytes (haploid, undergo meiosis II), Spermatids (haploid, undergo spermiogenesis to become spermatozoa), Spermatozoa (Sperm - released into lumen).
        • Basal Lamina: Surrounds seminiferous tubule.
        • Peritubular (Myoid) Cells: Smooth muscle-like cells outside basal lamina, contractile, help move sperm/fluid through tubules.
      • Interstitial Tissue (Connective Tissue Stroma): In spaces between seminiferous tubules. Contains:
        • Interstitial (Leydig) Cells: Large, polygonal, eosinophilic cells. Secrete Testosterone (stimulated by LH). Rich in SER, lipid droplets, mitochondria.
        • Blood vessels, lymphatics, nerves, macrophages.
      • Tubuli Recti (Straight Tubules): Short, straight tubules connecting ends of seminiferous tubules to rete testis. Lined by Sertoli cells initially, then simple cuboidal.
      • Rete Testis: Network of anastomosing channels within mediastinum testis. Lined by simple cuboidal/low columnar epithelium (some ciliated). Collects sperm from tubuli recti.
    • Descent of Testes: Develop retroperitoneally in abdomen, descend into scrotum during fetal development, guided by Gubernaculum. Pass through inguinal canal.
  • 3. Epididymis (Paired): Comma-shaped, highly coiled tube (~6 meters if uncoiled) on posterior surface of testis. Site of sperm maturation and storage.
    • Parts:
      • Head (Caput Epididymis): Superior, largest part. Receives sperm from rete testis via Efferent Ductules.
      • Body (Corpus Epididymis): Middle part.
      • Tail (Cauda Epididymis): Inferior, tapering part. Stores sperm. Continuous with Ductus Deferens.
    • Duct of the Epididymis: Single, highly coiled tube forming body and tail. Lined by pseudostratified columnar epithelium with long Stereocilia (non-motile microvilli - absorb excess fluid, secrete substances for sperm maturation). Surrounded by smooth muscle (peristalsis).
    • Efferent Ductules: 12-20 short, coiled ducts connecting rete testis to head of epididymis. Lined by alternating ciliated columnar cells (propel sperm) and non-ciliated cuboidal cells (absorb fluid). Scalloped lumen appearance.
  • 4. Ductus Deferens (Vas Deferens - Paired): Muscular tube (~45 cm long) conveying sperm from epididymis tail to ejaculatory duct. Part of Spermatic Cord.
    • Course: Ascends from scrotum through inguinal canal, enters pelvis, passes superior and then posterior to bladder, medial to ureter and seminal vesicle. Terminal part dilates to form Ampulla of Ductus Deferens (storage).
    • Layers of Wall:
      • Mucosa: Pseudostratified columnar epithelium with sparse stereocilia (may be simple columnar distally). Narrow, stellate lumen.
      • Muscularis: Very thick, three layers of smooth muscle (Inner Longitudinal, Middle Circular, Outer Longitudinal). Powerful peristalsis during ejaculation.
      • Adventitia: Outer CT layer.
  • 5. Spermatic Cord (Paired): Collection of structures passing to/from testis, through inguinal canal. Suspends testis in scrotum.
    • Contents: Ductus Deferens, Testicular Artery (from aorta), Pampiniform Plexus of veins ( testicular veins, thermoregulation), Cremasteric artery & vein, Artery to ductus deferens, Genital branch of Genitofemoral nerve (motor to cremaster, sensory to scrotum), Sympathetic nerve fibers, Lymphatic vessels.
    • Coverings (Fascial layers derived from abdominal wall):
      • External Spermatic Fascia (from external oblique aponeurosis).
      • Cremasteric Fascia & Muscle (from internal oblique muscle/fascia).
      • Internal Spermatic Fascia (from transversalis fascia).
  • 6. Ejaculatory Ducts (Paired): Short tubes (~2 cm) formed by union of ampulla of ductus deferens and duct of seminal vesicle. Pass through posterior prostate gland to open into prostatic urethra (on either side of prostatic utricle). Lined by pseudostratified/simple columnar epithelium.
  • 7. Urethra (Male): (Described under Urinary System - Prostatic, Membranous, Spongy. Common pathway for urine and semen).
  • 8. Accessory Sex Glands (produce seminal fluid, which mixes with sperm to form semen):
    • Seminal Vesicles (Seminal Glands - Paired): Convoluted, pouch-like glands posterior to bladder, lateral to ampullae of ductus deferens. Ducts join ductus deferens to form ejaculatory ducts.
      • Secretion: Viscous, alkaline fluid (~60-70% of semen volume). Contains Fructose (sperm energy), Prostaglandins (stimulate uterine motility, suppress female immune response), Fibrinogen-like proteins (coagulation of semen), yellow pigment.
      • Histology: Highly folded mucosa (honeycomb appearance) lined by pseudostratified columnar/cuboidal epithelium (secretory). Smooth muscle layer. Fibrous capsule.
    • Prostate Gland (Single): Chestnut-sized gland inferior to bladder, surrounding prostatic urethra.
      • Lobes/Zones: Traditionally described as anterior, posterior, lateral, median lobes. Clinically more relevant zones:
        • Peripheral Zone (PZ): Largest (~70%), posterior and lateral. Site of most carcinomas.
        • Central Zone (CZ): Surrounds ejaculatory ducts (~25%). Resistant to carcinoma.
        • Transition Zone (TZ): Surrounds urethra proximal to ejaculatory ducts (~5%). Site of benign prostatic hyperplasia (BPH).
        • Anterior Fibromuscular Stroma (non-glandular).
      • Secretion: Thin, milky, slightly acidic fluid (~20-30% of semen volume). Contains Citric acid (nutrient), Seminalplasmin (antibiotic), Prostate-Specific Antigen (PSA - enzyme, liquefies semen coagulum), other enzymes (acid phosphatase, fibrinolysin).
      • Histology: Tubuloacinar glands embedded in fibromuscular stroma. Glands lined by simple columnar/cuboidal or pseudostratified epithelium. Ducts converge to open into prostatic urethra. Corpora Amylacea (calcified prostatic concretions, increase with age). Fibrous capsule.
    • Bulbourethral (Cowper's) Glands (Paired): Pea-sized glands posterolateral to membranous urethra, within urogenital diaphragm. Ducts (~2.5 cm) open into proximal spongy urethra (in bulb of penis).
      • Secretion: Thick, clear, alkaline mucus (pre-ejaculate). Lubricates urethra, neutralizes acidic urine traces prior to ejaculation.
      • Histology: Tubuloacinar glands, mucous secreting cells. Skeletal muscle fibers from urogenital diaphragm in capsule.
  • 9. Penis: External male genital organ, common outlet for urine and semen.
    • Parts:
      • Root: Attached part in superficial perineal pouch. Consists of Bulb of Penis (expanded proximal part of corpus spongiosum, attached to perineal membrane) and Crura of Penis (2, proximal diverging parts of corpora cavernosa, attached to ischiopubic rami). Bulbospongiosus muscle covers bulb, Ischiocavernosus muscles cover crura.
      • Body (Shaft): Free, pendulous part. Cylindrical. Contains three columns of erectile tissue.
      • Glans Penis: Expanded conical tip, formed by distal expansion of corpus spongiosum. Corona Glandis (projecting posterior margin). External Urethral Orifice (meatus) at tip. Neck of glans (constriction proximal to corona).
    • Erectile Tissues (spongy network of CT, smooth muscle, and vascular sinuses/lacunae, become engorged with blood during erection):
      • Corpora Cavernosa (2, dorsal): Paired columns, form bulk of penis. Surrounded by thick Tunica Albuginea (fibrous sheath). Fused medially, separated by Septum Penis (incomplete).
      • Corpus Spongiosum (1, ventral): Surrounds spongy urethra. Thinner tunica albuginea. Expands proximally to form Bulb, distally to form Glans.
    • Coverings:
      • Skin: Thin, loose, pigmented.
      • Superficial (Dartos) Fascia: Contains smooth muscle.
      • Deep (Buck's) Fascia: Dense CT sheath enclosing all three erectile columns.
    • Prepuce (Foreskin): Retractable fold of skin and fascia covering glans penis (in uncircumcised males). Frenulum of Prepuce (median fold on ventral surface, connects prepuce to glans).
    • Suspensory Ligament of Penis (from pubic symphysis to deep fascia). Fundiform Ligament of Penis (from linea alba).
  • 10. Semen (Seminal Fluid): Composite fluid of sperm and secretions from accessory glands. Milky white, alkaline. Contains sperm, fructose, prostaglandins, enzymes, citric acid, mucus, etc. Provides transport medium, nutrients, protection for sperm. Typical ejaculate volume 2-5 ml, >20 million sperm/ml.

H. Female Reproductive System

  • 1. Ovaries (Female Gonads - Paired): Primary reproductive organs, produce ova (eggs) and female sex hormones (estrogens, progesterone). Almond-shaped, ~3-5 cm long. Located in ovarian fossae on lateral pelvic walls. Intraperitoneal (but not covered by peritoneum where germinal epithelium is).
    • Ligaments (suspend ovaries):
      • Broad Ligament of Uterus (peritoneal fold): Mesovarium (part of broad ligament attaching to anterior ovary, suspends it).
      • Ovarian Ligament (proper): Connects ovary (medial pole) to uterus (lateral aspect, inferior to uterine tube).
      • Suspensory Ligament of the Ovary (Infundibulopelvic ligament): Attaches ovary (lateral pole) to lateral pelvic wall. Contains ovarian artery, vein, nerves, lymphatics.
    • Surface: Covered by Germinal Epithelium (misnomer - simple cuboidal/squamous epithelium, continuous with mesothelium of peritoneum, not source of germ cells).
    • Tunica Albuginea: Dense CT layer deep to germinal epithelium.
    • Internal Structure (distinct Cortex and Medulla):
      • Ovarian Cortex (Outer region): Dense CT stroma. Contains Ovarian Follicles in various stages of development, and Corpora Lutea/Albicantia.
        • Ovarian Follicles (each contains an Oocyte - immature egg):
          • Primordial Follicle: Primary oocyte (arrested in prophase I of meiosis) surrounded by a single layer of flat Follicular (Granulosa) Cells. Most numerous type, present from birth.
          • Primary Follicle (Unilaminar -> Multilaminar): Oocyte enlarges. Follicular cells become cuboidal (Unilaminar), then proliferate to form stratified Granulosa Cell layer (Multilaminar). Zona Pellucida (glycoprotein layer) forms around oocyte. Theca Folliculi (CT stroma organizes around follicle) begins to form.
          • Secondary (Antral) Follicle: Fluid-filled spaces (Antral Follicles) appear between granulosa cells, coalesce to form a single Antrum (filled with Liquor Folliculi). Granulosa cells differentiate into Membrana Granulosa (lines antrum) and Cumulus Oophorus (mound supporting oocyte). Theca Folliculi differentiates into Theca Interna (vascular, endocrine - produces androgens converted to estrogen by granulosa cells) and Theca Externa (fibrous CT).
          • Graafian (Mature/Vesicular/Tertiary) Follicle: Large (~2-2.5 cm), dominant follicle that bulges from ovarian surface, ready for ovulation. Antrum is large. Oocyte (now completed meiosis I, arrested in metaphase II - secondary oocyte) surrounded by Zona Pellucida and Corona Radiata (innermost layer of cumulus cells).
        • Corpus Luteum ("Yellow Body"): Temporary endocrine structure formed from remnants of ovulated Graafian follicle (granulosa and theca interna cells transform into Granulosa Lutein Cells and Theca Lutein Cells). Secretes Progesterone and Estrogens to prepare/maintain uterus for pregnancy.
        • Corpus Albicans ("White Body"): Fibrous scar tissue, remnant of degenerated corpus luteum (if no pregnancy).
        • Atretic Follicles: Degenerated follicles (most follicles undergo atresia at various stages).
      • Ovarian Medulla (Inner region): Loose CT, highly vascular (helical arteries), nerves, lymphatics. No follicles. Hilum (where vessels/nerves enter, continuous with mesovarium). Rete Ovarii (homolog of rete testis, often vestigial).
  • 2. Uterine (Fallopian) Tubes / Oviducts (Paired): Muscular tubes (~10-12 cm long) extending laterally from superolateral uterus to ovaries. Transport ova from ovary to uterus, site of fertilization. Within mesosalpinx (part of broad ligament).
    • Parts:
      • Infundibulum: Funnel-shaped distal end, opens into peritoneal cavity near ovary.
        • Fimbriae: Finger-like projections fringing infundibulum. One large Fimbria Ovarica is attached to ovary. Sweep over ovary to capture ovulated oocyte.
      • Ampulla: Widest, longest part (~2/3 of length). Usual site of fertilization.
      • Isthmus: Narrower, thicker-walled part, joins uterus.
      • Intramural (Interstitial/Uterine) Part: Passes through uterine wall, opens into uterine cavity at Uterine Ostium.
    • Layers of Wall:
      • Mucosa: Simple columnar epithelium. Highly folded (plicae), especially in ampulla.
        • Ciliated Cells: Beat towards uterus, propel oocyte/zygote.
        • Peg (Secretory) Cells: Non-ciliated, provide nutrition for oocyte/sperm/zygote.
      • Muscularis: Inner Circular, Outer Longitudinal smooth muscle layers. Peristalsis aids transport.
      • Serosa (Visceral peritoneum, part of broad ligament).
  • 3. Uterus (Womb): Hollow, thick-walled, pear-shaped muscular organ in pelvis, anterior to rectum, posterosuperior to bladder. Site of menstruation, implantation of fertilized ovum, development of fetus, labor.
    • Position: Usually Anteverted (tipped anterosuperiorly relative to axis of vagina) and Anteflexed (body flexed anteriorly relative to cervix). Can be retroverted/retroflexed.
    • Parts:
      • Fundus: Rounded superior part, above entry of uterine tubes.
      • Body (Corpus): Main central part. Uterine Cavity (triangular, flattened AP).
      • Isthmus: Constricted region (~1 cm) between body and cervix (Lower Uterine Segment in pregnancy).
      • Cervix (Neck): Inferior, narrow, cylindrical part projecting into vagina.
        • Supravaginal Part (above vagina).
        • Vaginal Part (Portio Vaginalis - protrudes into vagina).
        • Cervical Canal: Connects uterine cavity to vagina. Internal Os (opening to uterine cavity), External Os (opening to vagina - round/oval in nulliparous, slit-like in parous women).
        • Endocervical Mucosa: Simple columnar epithelium with mucous-secreting Cervical Glands (produce cervical mucus - consistency changes during cycle). Nabothian Cysts (blocked glands).
        • Ectocervical Mucosa (on vaginal part): Non-keratinized stratified squamous epithelium. Transformation Zone (Squamocolumnar Junction - SCJ) at external os, where columnar meets squamous epithelium (site of most cervical cancers).
    • Ligaments (support uterus):
      • Broad Ligament (Paired, peritoneal fold): Extends from lateral uterus to lateral pelvic walls.
        • Mesometrium (largest part, surrounds body of uterus).
        • Mesosalpinx (suspends uterine tube).
        • Mesovarium (suspends ovary).
      • Round Ligaments of Uterus (Paired): Extend from superolateral uterus (near tube attachment), through inguinal canal, to labia majora. Maintain anteversion. Remnant of gubernaculum.
      • Cardinal (Transverse Cervical / Mackenrodt's) Ligaments (Paired): Extend from cervix/lateral vaginal fornices to lateral pelvic walls. Major support. Contain uterine artery/vein.
      • Uterosacral Ligaments (Paired): Extend from posterolateral cervix/upper vagina to sacrum. Maintain anteversion.
      • Pubocervical Ligaments (Paired): Extend from cervix to posterior pubis.
    • Layers of Uterine Wall:
      • Perimetrium (Outer serous layer): Visceral peritoneum, part of broad ligament. Becomes adventitia laterally/inferiorly where peritoneum is absent.
      • Myometrium (Middle muscular layer): Thickest layer, three poorly defined layers of smooth muscle (outer/inner longitudinal/oblique, middle circular - rich in vessels, Stratum Vasculare). Hypertrophies greatly during pregnancy. Contracts during labor/orgasm.
      • Endometrium (Inner mucosal layer): Simple columnar epithelium with underlying lamina propria (Endometrial Stroma - specialized CT). Tubular Uterine Glands extend into stroma. Highly vascular. Undergoes cyclical changes during menstrual cycle (stimulated by ovarian hormones).
        • Stratum Functionalis (Functional Layer): Superficial ~2/3, shed during menstruation. Supplied by Spiral Arteries.
        • Stratum Basalis (Basal Layer): Deeper ~1/3, retained during menstruation, regenerates functionalis. Supplied by Straight Arteries.
    • Blood Supply: Uterine Arteries (from internal iliacs) -> Arcuate arteries (in myometrium) -> Radial arteries (to endometrium) -> Straight arteries (to stratum basalis) & Spiral arteries (to stratum functionalis - responsive to hormones, constrict/rupture during menstruation).
  • 4. Vagina: Fibromuscular tube (~7-10 cm long) extending from cervix to vestibule. Female organ of copulation, outlet for menstrual flow, birth canal. Posterior to bladder/urethra, anterior to rectum.
    • Fornix (plural: Fornices): Recess surrounding cervix in upper vagina (Anterior, Posterior - deepest, Lateral fornices).
    • Vaginal Orifice (Introitus): Opening into vestibule. May be partially covered by Hymen (mucosal fold) in virgins.
    • Rugae: Transverse folds of mucosa, allow distension.
    • Layers of Vaginal Wall:
      • Mucosa: Non-keratinized stratified squamous epithelium (contains glycogen, metabolized by bacteria to lactic acid, creating acidic environment). Lamina propria (elastic fibers, rich venous plexus, no glands - lubricated by cervical mucus and transudate).
      • Muscularis: Two poorly defined smooth muscle layers (Inner Circular, Outer Longitudinal). Skeletal muscle (Bulbospongiosus) at orifice.
      • Adventitia: Outer fibroelastic CT, blends with surrounding structures.
  • 5. External Genitalia (Vulva / Pudendum):
    • Mons Pubis: Rounded fatty eminence anterior to pubic symphysis, covered with pubic hair (after puberty).
    • Labia Majora (Paired): Prominent longitudinal folds of skin and adipose tissue, extending from mons pubis to posterior commissure. Homologous to male scrotum. Outer surface pigmented, hairy; inner surface smooth, hairless, sebaceous/sweat glands.
    • Labia Minora (Paired): Smaller, hairless folds of skin medial to labia majora. Enclose vestibule. Rich in sebaceous glands, melanocytes. Unite anteriorly to form Prepuce of Clitoris and Frenulum of Clitoris. Unite posteriorly to form Fourchette (posterior commissure, often torn during childbirth).
    • Clitoris: Erectile structure at anterior junction of labia minora. Homologous to male penis.
      • Body: Contains two small Corpora Cavernosa.
      • Glans Clitoridis: Small, rounded, highly sensitive tip, exposed.
      • Prepuce (Hood) of Clitoris: Formed by anterior fusion of labia minora, covers body/glans.
      • Crura (2): Proximal diverging parts of corpora cavernosa, attached to ischiopubic rami.
    • Vestibule: Space enclosed by labia minora. Contains:
      • External Urethral Orifice (anteriorly).
      • Vaginal Orifice (Introitus) (posteriorly).
      • Openings of Ducts of Greater Vestibular (Bartholin's) Glands: Paired, pea-sized mucous glands posterolateral to vaginal orifice (in superficial perineal pouch). Homologous to male bulbourethral glands. Secrete mucus for lubrication during arousal.
      • Openings of Ducts of Lesser Vestibular (Skene's / Paraurethral) Glands: Small mucous glands flanking urethra. Homologous to male prostate.
    • Bulb of the Vestibule (Paired): Masses of erectile tissue deep to labia on either side of vaginal orifice. Homologous to bulb of penis/corpus spongiosum. Covered by Bulbospongiosus muscles.
  • 6. Perineum (Female): Diamond-shaped region inferior to pelvic diaphragm, between pubic symphysis (anterior), ischial tuberosities (lateral), coccyx (posterior).
    • Divided by imaginary line between ischial tuberosities into:
      • Urogenital Triangle (Anterior): Contains external genitalia, urethral/vaginal orifices. Perineal Membrane (fibrous sheet in deep perineal pouch).
      • Anal Triangle (Posterior): Contains anal canal and anus. Ischioanal (Ischiorectal) Fossae (fat-filled spaces).
    • Perineal Body (Central Tendon of Perineum): Fibromuscular mass between urogenital and anal triangles. Convergence point for several muscles (bulbospongiosus, external anal sphincter, superficial/deep transverse perineal). Important support structure, often torn during childbirth (episiotomy site).
  • 7. Mammary Glands (Breasts): Modified sweat glands (apocrine type) located in superficial fascia of pectoral region. Produce milk for lactation. Present in both sexes, functional in females after puberty/pregnancy.
    • External Structure:
      • Nipple: Conical projection, surrounded by areola. Contains openings of Lactiferous Ducts. Smooth muscle fibers (erection of nipple). Richly innervated.
      • Areola: Pigmented circular area surrounding nipple. Contains Areolar (Montgomery's) Glands (modified sebaceous glands, secrete lubricant, enlarge during pregnancy). Smooth muscle fibers (wrinkling).
    • Internal Structure:
      • Lobes: 15-20 lobes radiating from nipple, separated by adipose tissue and fibrous CT (Suspensory Ligaments of Cooper - attach breast to dermis and deep fascia, provide support).
      • Lobules: Subdivisions of lobes, contain clusters of Alveoli (Grapelike secretory units, lined by milk-secreting cuboidal epithelial cells - develop fully during pregnancy/lactation). Myoepithelial cells surround alveoli (contract to eject milk).
      • Duct System: Alveoli -> Intralobular Ducts -> Interlobular Ducts -> Lactiferous Ducts (one per lobe) -> Lactiferous Sinus (dilated portion of duct deep to nipple, milk reservoir) -> Opens onto surface of nipple.
    • Adipose Tissue: Determines size/shape of non-lactating breast.
    • Blood Supply: Branches from internal thoracic, axillary (lateral thoracic, thoracoacromial), and intercostal arteries.
    • Lymphatic Drainage: Crucial for spread of breast cancer. Mainly to Axillary lymph nodes (pectoral, subscapular, humeral, central, apical groups). Also to Parasternal (Internal Thoracic) nodes, Supraclavicular nodes.

I. Integumentary System

  • 1. Skin (Cutis): Covers entire external surface.
    • Layers:
      • Epidermis (Outer layer): Keratinized stratified squamous epithelium. Avascular. Thickness varies (e.g., thin on eyelids, thick on palms/soles).
        • Cell Types:
          • Keratinocytes (~90%): Most abundant. Produce Keratin (tough, fibrous, protective protein). Arranged in strata. Connected by Desmosomes. Undergo keratinization (maturation and migration from basal layer to surface).
          • Melanocytes (~8%): In stratum basale. Neural crest derived. Synthesize Melanin pigment (packaged into Melanosomes, transferred to keratinocytes). Protects against UV radiation. Skin color depends on amount/type of melanin produced (not number of melanocytes).
          • Langerhans (Dendritic) Cells: In stratum spinosum. Immune cells (antigen-presenting). Bone marrow derived. Star-shaped.
          • Merkel (Tactile) Cells: In stratum basale. Mechanoreceptors for light touch. Associated with sensory nerve endings (Merkel disc).
        • Strata (Layers - from deep to superficial):
          • Stratum Basale (Stratum Germinativum): Deepest single layer of cuboidal/columnar keratinocytes. Actively mitotic (stem cells). Contains melanocytes, Merkel cells. Attached to basement membrane by Hemidesmosomes.
          • Stratum Spinosum ("Prickly Layer"): 8-10 layers of polyhedral keratinocytes. Cells appear spiny due to desmosomes shrinking but remaining attached during tissue preparation. Contains Langerhans cells. Cells begin to produce lamellar granules.
          • Stratum Granulosum ("Granular Layer"): 3-5 layers of flattened keratinocytes undergoing apoptosis. Cytoplasm contains:
            • Keratohyalin Granules (darkly staining, non-membrane bound): Involved in keratinization process (assemble keratin filaments).
            • Lamellar Granules (membrane-bound): Release lipid-rich secretion into extracellular space, forming water-repellent barrier.
          • Stratum Lucidum ("Clear Layer"): Only present in thick skin (palms, soles). 2-3 layers of clear, flat, dead keratinocytes. Contains Eleidin (translucent protein derived from keratohyalin).
          • Stratum Corneum ("Horny Layer"): Outermost 20-30 (or more) layers of anucleated, dead, flattened, keratin-filled keratinocytes (Corneocytes/Squames). Continuously shed (desquamation). Water-resistant (but not waterproof). Thickness varies greatly.
        • Epidermal Ridges (e.g., Fingerprints/Dermatoglyphs): Downward projections of epidermis into dermis (interdigitate with dermal papillae). Genetically determined, unique patterns on fingers, palms, toes, soles. Increase friction/grip.
      • Dermis (Corium - Inner layer): Strong, flexible connective tissue layer deep to epidermis. Vascularized, innervated. Contains blood vessels, nerves, sensory receptors, hair follicles, glands.
        • Layers:
          • Papillary Layer (Superficial ~20%): Loose areolar CT.
            • Dermal Papillae: Finger-like projections extending into overlying epidermis. Contain capillary loops (blood supply for epidermis), Meissner's (tactile) corpuscles (touch receptors), free nerve endings (pain, temperature).
            • Forms dermal ridges in thick skin.
          • Reticular Layer (Deeper ~80%): Dense irregular CT. Bundles of collagen fibers (strength, resilience) and elastic fibers (stretch, recoil) arranged in networks. Contains larger blood vessels, nerves, Pacinian (lamellar) corpuscles (deep pressure/vibration), Ruffini endings (stretch), hair follicles, sebaceous/sweat glands.
            • Cleavage (Langer's) Lines: Separations between collagen bundles, run in specific directions in different body regions. Important for surgical incisions (incisions parallel to lines heal better).
            • Flexure Lines: Dermal folds at/near joints where dermis is tightly secured to deeper structures (e.g., creases on palms, fingers, soles).
        • Dermal Plexuses (Blood Supply):
          • Cutaneous Plexus: Network of arteries/veins at junction of dermis and hypodermis. Supplies hypodermis, deep dermis (hair follicles, glands).
          • Papillary Plexus (Subpapillary Plexus): Branches from cutaneous plexus, supplies dermal papillae and epidermis (via diffusion). Important for thermoregulation (vasoconstriction/dilation).
      • Hypodermis (Subcutaneous Layer / Superficial Fascia): Not strictly part of skin, but lies deep to dermis. Loose areolar CT and Adipose Tissue. Anchors skin to underlying structures (muscles, bones), allows skin to slide. Stores fat (insulation, energy reserve), contains large blood vessels, nerves, Pacinian corpuscles. Thickness varies.
  • 2. Appendages of the Skin (Epidermal Derivatives):
    • Hair (Pili - singular: Pilus): Flexible strands of dead, keratinized cells. Produced by Hair Follicles.
      • Structure of a Hair:
        • Shaft: Part projecting above skin surface. Shape determines hair texture (round=straight, oval=wavy/curly).
        • Root: Part embedded in skin (within follicle).
        • Three Concentric Layers (in coarse hairs like terminal hairs):
          • Medulla: Central core, large cells, air spaces. Absent in fine hairs (vellus).
          • Cortex: Middle layer, several layers of flattened cells, contains pigment (melanin). Forms bulk of hair.
          • Cuticle: Outermost single layer of overlapping, shingle-like cells. Heavily keratinized.
      • Hair Follicle: Invagination of epidermis extending into dermis (or hypodermis).
        • Epithelial Root Sheath:
          • Internal Root Sheath: Surrounds hair root, derived from matrix cells. Thins out superiorly.
          • External Root Sheath: Downward continuation of epidermis (stratum basale/spinosum).
        • Connective Tissue (Dermal) Root Sheath: Derived from dermis, surrounds epithelial root sheath.
        • Hair Bulb: Expanded deep end of follicle.
          • Hair Papilla (Dermal Papilla): Nipple-like invagination of CT into bulb. Contains capillaries (nourish matrix), nerves.
          • Hair Matrix: Actively dividing epithelial cells (derived from stratum basale) surrounding papilla. Produce hair cells. Melanocytes here transfer pigment to hair cells.
      • Arrector Pili Muscle: Small bundle of smooth muscle attached to CT sheath of follicle and papillary layer of dermis. Contracts in response to cold/fear, pulls hair upright ("goosebumps"), squeezes sebaceous gland.
      • Hair Root Plexus (Nerve endings): Sensory nerves surrounding hair bulb. Detect hair movement.
      • Types of Hair:
        • Lanugo: Fine, soft, unpigmented hair on fetus.
        • Vellus Hair: Short, fine, pale "peach fuzz" body hair (children, adult females).
        • Terminal Hair: Longer, coarser, often pigmented hair (scalp, eyebrows, eyelashes; axillary/pubic regions after puberty; face, chest, limbs in males).
      • Hair Growth Cycle: Anagen (growth phase), Catagen (regressive phase), Telogen (resting phase).
    • Nails (Ungues): Scale-like modifications of epidermis on dorsal distal surfaces of fingers/toes. Hard keratin.
      • Nail Plate (Body): Visible attached part.
      • Free Edge: Distal part extending past finger/toe tip.
      • Nail Root: Proximal part embedded under skin.
      • Nail Bed: Epidermis deep to nail plate (stratum basale/spinosum).
      • Nail Matrix: Thickened proximal part of nail bed, deep to nail root. Responsible for nail growth. Actively mitotic cells.
      • Lunula (Lunule): White, crescent-shaped area at proximal end of nail plate (visible part of matrix).
      • Nail Folds (Lateral & Proximal): Folds of skin overlapping nail borders.
      • Eponychium (Cuticle): Proximal nail fold projecting onto nail plate.
      • Hyponychium: Thickened stratum corneum deep to free edge, secures nail to fingertip.
    • Glands (Cutaneous Glands):
      • Sebaceous (Oil) Glands: Simple branched alveolar glands. Usually associated with hair follicles (ducts empty into follicle). Some open directly onto skin surface (lips, glans penis, tarsal glands of eyelids). Holocrine secretion (cells rupture).
        • Secrete Sebum (oily substance - triglycerides, cholesterol, proteins, salts). Lubricates hair/skin, prevents drying, bactericidal.
        • Absent in thick skin of palms/soles.
      • Sudoriferous (Sweat) Glands: ~3-4 million. Distributed over entire skin surface except nipples, parts of external genitalia.
        • Eccrine (Merocrine) Sweat Glands: Most numerous type, abundant on palms, soles, forehead. Simple coiled tubular glands. Duct opens directly onto skin surface via a Pore.
          • Secretory Portion: In dermis/hypodermis. Coiled, lined by cuboidal/columnar cells (clear cells - secrete watery part; dark cells - secrete glycoprotein). Myoepithelial cells surround secretory portion (contract to expel sweat).
          • Duct: Coiled, extends to surface. Lined by stratified cuboidal epithelium. Reabsorbs some Na+, Cl-.
          • Sweat (Perspiration): Hypotonic filtrate of blood (99% water, salts, vitamin C, antibodies, dermcidin, metabolic wastes like urea/uric acid/ammonia). Main function: thermoregulation (evaporative cooling). Also excretion, protection.
        • Apocrine Sweat Glands: Larger than eccrine glands. Ducts empty into hair follicles (usually). Found in axillary, anogenital areas, areolae. Become functional at puberty.
          • Secretory Portion: Coiled, larger lumen than eccrine. Lined by cuboidal/columnar cells. Myoepithelial cells. Merocrine secretion mechanism (despite name "apocrine" which implies pinching off).
          • Sweat: Viscous, milky/yellowish, odorless. Contains fatty substances, proteins (in addition to eccrine components). Bacterial decomposition on skin surface produces body odor. Function uncertain (pheromones?).
        • Ceruminous Glands: Modified apocrine sweat glands in lining of external acoustic meatus (ear canal).
          • Secrete Cerumen (Earwax) - sticky, bitter substance (with sebum from nearby sebaceous glands). Deters insects, traps foreign material, lubricates eardrum.
        • Mammary Glands: (Described under Female Reproductive System). Modified apocrine sweat glands specialized for milk production.

VI. Tissues (Fundamental Types & Subtypes)

A. Epithelial Tissue

  • Covering/Lining Epithelium:
    • Simple Squamous (e.g., endothelium, mesothelium, alveoli).
    • Simple Cuboidal (e.g., kidney tubules, gland ducts).
    • Simple Columnar (e.g., GI tract lining - can be ciliated or non-ciliated, may have goblet cells).
    • Pseudostratified Columnar (e.g., trachea - usually ciliated with goblet cells).
    • Stratified Squamous (e.g., epidermis - keratinized; oral cavity, esophagus - non-keratinized).
    • Stratified Cuboidal (e.g., ducts of larger glands).
    • Stratified Columnar (rare, e.g., parts of male urethra, large ducts).
    • Transitional (Urothelium) (e.g., bladder, ureters).
  • Glandular Epithelium: (Exocrine, Endocrine - forms glands listed above).

B. Connective Tissue

  • Connective Tissue Proper:
    • Loose CT: Areolar, Adipose, Reticular.
    • Dense CT: Dense Regular (e.g., tendons, ligaments), Dense Irregular (e.g., dermis, organ capsules), Elastic (e.g., large artery walls).
  • Specialized Connective Tissue:
    • Cartilage: Hyaline (e.g., articular surfaces, costal cartilages, tracheal rings), Elastic (e.g., external ear, epiglottis), Fibrocartilage (e.g., intervertebral discs, pubic symphysis, menisci). (Chondrocytes in lacunae, matrix).
    • Bone (Osseous) Tissue: Compact bone (osteons, Haversian systems), Spongy (cancellous) bone (trabeculae). (Osteocytes, Osteoblasts, Osteoclasts, matrix).
    • Blood: Plasma, Erythrocytes (RBCs), Leukocytes (WBCs - Neutrophils, Lymphocytes, Monocytes, Eosinophils, Basophils), Thrombocytes (Platelets).
    • Lymph.

C. Muscle Tissue

  • Skeletal Muscle Tissue (striated, voluntary). (Muscle fibers/cells, sarcomeres).
  • Cardiac Muscle Tissue (striated, involuntary, intercalated discs). (Cardiomyocytes).
  • Smooth Muscle Tissue (non-striated, involuntary). (Spindle-shaped cells).

D. Nervous Tissue

  • Neurons (nerve cells): Cell body (soma), Dendrites, Axon (myelinated or unmyelinated), Axon terminals. Synapses.
  • Neuroglia (Glial cells):
    • CNS: Astrocytes, Oligodendrocytes, Microglia, Ependymal cells.
    • PNS: Schwann cells (neurolemmocytes), Satellite cells.

VII. "Artefacts" / Incidental Structures / Secretions / Products / Features

  • A. Bodily Fluids & Secretions (Beyond simple glandular products):
    • Exudates: Serous, Fibrinous, Purulent (Pus), Hemorrhagic.
    • Transudates.
    • Amniotic Fluid.
    • Vaginal Discharge (Physiological).
    • Lochia.
    • Meconium.
    • Vernix Caseosa.
    • Smegma.
    • Rheum ("Sleep," "Eye Gunk").
    • Interstitial Fluid (Tissue Fluid).
    • Synovial Fluid (Synovia).
    • Cerebrospinal Fluid (CSF).
    • Perilymph & Endolymph.
    • Aqueous Humor & Vitreous Humor.
    • Pleural Fluid, Pericardial Fluid, Peritoneal Fluid.
    • Chyle.
    • Menstrual Fluid.
  • B. Cellular Inclusions & Stored Materials (Distinct from organelles):
    • Glycogen Granules.
    • Lipid Droplets (Adiposomes).
    • Pigment Granules: Melanin Granules (Melanosomes), Hemosiderin Granules, Lipofuscin (Lipochrome / "Age Pigment"), Bilirubin, Dust Particles/Carbon Particles.
    • Crystals & Concretions: Crystals of Reinke, Charcot-Leyden Crystals, Corpora Amylacea, Corpora Arenacea ("Brain Sand"), Psammoma Bodies, Otoliths (Otoconia), Calculi (Stones - pathological).
    • Secretory Granules/Vesicles.
  • C. Extracellular Matrix Components (Beyond basic fibers/ground substance):
    • Basement Membrane (Basal Lamina + Reticular Lamina): Lamina Lucida/Rara, Lamina Densa, Reticular Lamina.
    • Elastic Laminae (Internal & External).
    • Dental Plaque (Biofilm).
    • Calculus (Tartar).
    • Fibrin Clot Components.
    • Scar Tissue (Cicatrix).
    • Amyloid.
  • D. Surface Features, Patterns & Transient Structures:**
    • Fingerprints (Dermatoglyphs).
    • Skin Lines/Creases: Cleavage (Langer's) Lines, Flexure Lines, Wrinkles (Rhytides).
    • Lanugo.
    • Deciduous Teeth (Baby Teeth).
    • Epiphyseal Plates (Growth Plates).
    • Fontanelles.
    • Umbilical Cord & Remnants: Umbilical Cord, Ligamentum Teres Hepatis, Medial Umbilical Ligaments, Median Umbilical Ligament, Umbilicus.
    • Fetal Circulation Structures & Remnants: Ductus Venosus -> Ligamentum Venosum, Foramen Ovale -> Fossa Ovalis, Ductus Arteriosus -> Ligamentum Arteriosum.
    • Corpus Luteum & Corpus Albicans.
    • Decidua.
    • Placenta.
    • Atherosclerotic Plaque.
    • Thrombus.
    • Embolus.
    • Callus (Bony).
    • Callus (Skin).
    • Corn (Clavus).
    • Blister (Vesicle/Bulla).
    • Scab (Eschar).
    • Cuticle (Nail - Eponychium).
    • Cuticle (Hair).
    • Vibrissae (Nasal Hairs).
    • Milk Line (Mammary Ridge).
    • Linea Alba.
    • Linea Nigra.
    • Striae (Stretch Marks).
    • Freckles (Ephelides).
    • Lentigines (Age/Liver Spots).
    • Moles (Nevi - Melanocytic Nevi).

PART 2: BODILY FUNCTIONS (Macro to Molecular)

I. Organismal Level (Macro) Functions & Integrated Responses

  • A. Homeostasis & Regulation (Maintenance of a Stable Internal Environment):
    • Thermoregulation: Heat Production (Basal metabolic rate, shivering, non-shivering thermogenesis), Heat Loss (Radiation, conduction, convection, evaporation), Control Mechanisms (Hypothalamic thermoregulatory center, thermoreceptors, ANS responses, behavioral adaptations).
    • Osmoregulation & Fluid Balance: Maintenance of total body water volume and osmolarity, Regulation of water intake (thirst mechanism), Regulation of water output (urine concentration via ADH, insensible losses, fecal water), Electrolyte balance.
    • Acid-Base Balance (pH Homeostasis): Buffer Systems (Bicarbonate, Phosphate, Protein/Hemoglobin), Respiratory Regulation (CO2 control), Renal Regulation (H+/HCO3- excretion/reabsorption, new HCO3- synthesis).
    • Blood Glucose Regulation: Hormonal Control (Insulin, Glucagon, Epinephrine, Cortisol, Growth Hormone), Liver's Role (Glycogenesis, glycogenolysis, gluconeogenesis), Peripheral tissue glucose uptake.
    • Blood Pressure Regulation: Short-Term (Neural - Baroreceptor reflex, Chemoreceptor reflex), Intermediate-Term (Hormonal - RAAS, ADH, Epinephrine/Norepinephrine, ANP), Long-Term (Renal - blood volume regulation).
    • Regulation of Blood Oxygen & Carbon Dioxide Levels.
    • Stress Response: Acute Stress (Alarm Reaction - Sympathetic, Adrenal Medulla), Chronic Stress (Resistance Reaction - HPA axis, Cortisol), Exhaustion Phase.
  • B. Metabolism (Sum of All Chemical Processes):
    • Anabolism: Protein synthesis, glycogenesis, lipogenesis, nucleic acid synthesis.
    • Catabolism: Proteolysis, glycogenolysis, lipolysis, glycolysis, citric acid cycle, oxidative phosphorylation.
    • Energy Metabolism: ATP production, storage, and utilization.
    • Nutrient Processing & Interconversion.
  • C. Movement, Locomotion & Responsiveness:
    • Skeletal Muscle Contraction for Locomotion & Manipulation.
    • Posture Maintenance & Balance.
    • Internal Movement: Peristalsis, ciliary action, cardiac contraction, vasomotion.
    • Sensing & Responding to Stimuli (Irritability): Detection, transduction, transmission, integration, motor output.
  • D. Growth, Development & Differentiation:
    • Cellular Growth & Division (Mitosis).
    • Tissue Growth & Repair.
    • Organ Development (Organogenesis).
    • Embryonic & Fetal Development: Gastrulation, neurulation, morphogenesis.
    • Postnatal Growth & Maturation: Infancy, childhood, adolescence, adulthood.
    • Cellular Differentiation.
    • Aging (Senescence).
  • E. Reproduction & Inheritance:
    • Gametogenesis (Spermatogenesis, Oogenesis).
    • Sexual Intercourse & Fertilization.
    • Pregnancy (Gestation) & Embryonic/Fetal Support.
    • Childbirth (Parturition).
    • Lactation.
    • Genetic Inheritance.
  • F. Defense & Immunity: Protection against pathogens, toxins, abnormal cells. Inflammation and repair.
  • G. Excretion & Elimination: Metabolic wastes (urine), CO2 (respiration), indigestible residues (feces), toxins.
  • H. Communication & Integration (Organism-wide): Nervous System, Endocrine System, Neuroendocrine Integration, Paracrine & Autocrine Signaling.

II. System Level Functions

  • A. Integumentary System Functions: Protection (Physical, Chemical, Biological Barrier), Thermoregulation, Cutaneous Sensation, Vitamin D Synthesis, Excretion (Minor), Absorption (Limited), Blood Reservoir, Water Resistance, Non-verbal Communication.
  • B. Skeletal System & Articular System Functions: Support & Framework, Protection of Internal Organs, Movement Facilitation (Leverage), Mineral Homeostasis (Calcium, Phosphorus storage), Hematopoiesis, Triglyceride (Fat) Storage, Hormone Production (Osteocalcin), Shock Absorption, Joint Stability & Mobility.
  • C. Muscular System Functions: Producing Body Movements, Stabilizing Body Positions (Posture), Generating Heat (Thermogenesis), Storing & Moving Substances Within the Body (Sphincter control, Propulsion), Protecting Viscera, Controlling Body Entrances & Exits, Supporting Soft Tissues, Energy Source (Indirectly).
  • D. Nervous System Functions: Sensory Input (Afferent), Integration (Processing, Perception, Memory, Thought, Learning, Emotion), Motor Output (Efferent), Control & Coordination of Body Activities, Higher Mental Functions, Autonomic Nervous System Specific Functions (Sympathetic, Parasympathetic, Enteric), Establishment & Maintenance of Sleep-Wake Cycles.
  • E. Endocrine System Functions: Regulation of Long-Term Processes (Growth, Development, Reproduction, Metabolism), Hormone Synthesis, Storage, & Secretion, Target Cell Activation, Maintenance of Homeostasis (Water/electrolyte balance, Blood glucose, Calcium/phosphate, Metabolic rate), Control of Reproductive Functions, Response to Stress, Regulation of Growth & Development, Modulation of Immune System Function, Control of Digestive Processes.
  • F. Cardiovascular System Functions: Transport Functions (Oxygen, Carbon Dioxide, Nutrients, Waste Products, Hormones, Heat, Immune Cells), Pumping Action of the Heart (Cardiac Output), Blood Pressure Regulation, Hemostasis (Prevention of Blood Loss), Protection Against Disease, Fluid Balance, Acid-Base Balance.
  • G. Lymphatic & Immune System Functions: Fluid Balance (Return of Interstitial Fluid), Dietary Fat Absorption, Immune Defense & Surveillance (Innate Immunity - Physical/Chemical Barriers, Phagocytosis, NK cells, Inflammation, Fever, Antimicrobial proteins; Adaptive Immunity - Cell-Mediated: T Lymphocytes - Helper, Cytotoxic, Regulatory, Memory; Humoral: B Lymphocytes - Plasma cells, Antibodies, Memory B cells), Antigen Presentation, Filtering of Lymph (Lymph nodes), Filtering of Blood (Spleen), Lymphocyte Maturation & Activation, Development of Immunological Memory & Tolerance.
  • H. Respiratory System Functions: Gas Exchange (Pulmonary Ventilation, External & Internal Respiration), Regulation of Blood pH, Vocalization (Phonation), Olfaction (Sense of Smell), Protection Against Pathogens & Irritants (Mucociliary clearance, Cough/sneeze reflexes, Alveolar macrophages), Warming & Humidifying Inspired Air, Angiotensin Conversion (ACE), Non-respiratory Air Movements.
  • I. Digestive System Functions: Ingestion, Mechanical Processing (Mastication, Deglutition, Motility - Peristalsis, Segmentation, Churning), Chemical Digestion (Enzymatic breakdown by salivary, gastric, pancreatic, brush border enzymes), Secretion (Water, acids, buffers, enzymes, mucus, hormones), Absorption (Nutrients, water, electrolytes, vitamins), Elimination (Defecation), Hormonal & Neural Regulation of Digestion, Protection (Mucus, acid, GALT), Bile Synthesis & Secretion, Metabolic Functions of Liver.
  • J. Urinary System Functions: Excretion of Metabolic Wastes, Regulation of Blood Volume & Pressure, Regulation of Blood Osmolarity, Regulation of Blood pH, Regulation of Ion Concentrations, Hormone Production (Renin, EPO, Calcitriol), Urine Formation (Glomerular Filtration, Tubular Reabsorption, Tubular Secretion), Urine Concentration & Dilution, Storage of Urine, Micturition.
  • K. Male Reproductive System Functions: Spermatogenesis, Hormone Production (Androgens - Testosterone), Sperm Maturation & Storage, Transport & Delivery of Sperm, Production of Seminal Fluid, Erection of Penis, Ejaculation, Regulation of Reproductive Functions (Hypothalamic-Pituitary-Testicular axis).
  • L. Female Reproductive System Functions: Oogenesis, Hormone Production (Estrogens, Progesterone, Inhibin, Activin, Relaxin), Ovulation, Transport of Ovum/Zygote/Embryo, Site of Fertilization, Menstrual (Uterine) Cycle, Implantation of Blastocyst, Gestation (Pregnancy), Parturition (Childbirth), Lactation, Female Sexual Response, Regulation of Reproductive Functions (Hypothalamic-Pituitary-Ovarian axis).

III. Cellular Level Functions

A. Fundamental Cellular Processes

  • Metabolism & Energy Conversion:
    • Cellular Respiration (ATP Production): Glycolysis, Pyruvate Oxidation, Citric Acid Cycle, Oxidative Phosphorylation (Electron Transport Chain, Chemiosmosis).
    • Anaerobic Respiration (Fermentation).
    • Pentose Phosphate Pathway.
    • Regulation of Metabolic Pathways.
  • Biosynthesis (Anabolism):
    • Protein Synthesis: Transcription (Gene activation, RNA polymerase, mRNA processing), Translation (Ribosomes, tRNA, genetic code, initiation, elongation, termination), Post-Translational Modification.
    • Lipid Synthesis (Lipogenesis).
    • Carbohydrate Synthesis: Gluconeogenesis, Glycogenesis.
    • Nucleic Acid Synthesis.
  • Transport Across Cell Membranes:
    • Passive Transport: Simple Diffusion, Facilitated Diffusion (Channels, Carriers), Osmosis (Aquaporins).
    • Active Transport: Primary Active Transport (Pumps - Na+/K+ ATPase), Secondary Active Transport (Symport, Antiport).
    • Bulk Transport (Vesicular Transport): Endocytosis (Phagocytosis, Pinocytosis, Receptor-Mediated Endocytosis - Clathrin, Caveolae), Exocytosis, Transcytosis.
  • Cell Adhesion & Junctions: Cell-Cell Adhesion Molecules (CAMs - Cadherins, Ig-CAMs, Selectins, Integrins), Tight Junctions, Adherens Junctions, Desmosomes, Gap Junctions, Hemidesmosomes.
  • Cell Signaling & Communication (Reception, Transduction, Response):
    • Receptor Types: Intracellular Receptors, Cell Surface Receptors (Ion Channel-Linked, G Protein-Coupled Receptors - GPCRs, Enzyme-Linked Receptors - RTKs).
    • Signal Transduction Pathways.
    • Cellular Responses.
  • Cell Growth, Division & Cycle Control: Cell Cycle (Interphase - G1, S, G2; Mitotic Phase - M), DNA Replication, Mitosis, Cytokinesis, Meiosis.
  • Apoptosis (Programmed Cell Death): Intrinsic & Extrinsic pathways, Caspases.
  • Necrosis.
  • Cellular Motility & Movement: Amoeboid Movement, Ciliary/Flagellar Movement, Muscle Contraction.
  • Maintenance of Cell Shape & Integrity (Cytoskeleton): Microfilaments (Actin), Intermediate Filaments (Keratins, etc.), Microtubules.
  • Intracellular Digestion (Lysosomes): Autophagy.
  • Detoxification (Peroxisomes, Smooth ER).
  • Secretion.
  • Absorption.
  • Excitability (Nerve & Muscle Cells).
  • Differentiation & Specialization.
  • Cellular Senescence.

B. Specialized Cellular Functions

  • Nerve Cells (Neurons): Generation & Propagation of Action Potentials, Neurotransmission, Axonal Transport, Integration of Synaptic Inputs.
  • Muscle Cells (Myocytes): Contraction (Skeletal, Cardiac, Smooth), Excitability, Extensibility & Elasticity.
  • Epithelial Cells: Barrier Function, Absorption, Secretion, Transport, Protection, Sensation.
  • Connective Tissue Cells: Fibroblasts (ECM synthesis), Chondrocytes (Cartilage matrix), Osteocytes/blasts/clasts (Bone homeostasis), Adipocytes (Lipid storage, hormone secretion), Blood Cells (Erythrocytes - O2/CO2 transport; Leukocytes - Immune defense; Platelets - Hemostasis).
  • Endocrine Cells: Hormone synthesis, storage, secretion.
  • Sensory Receptor Cells: Transduction of stimuli.
  • Germ Cells (Spermatozoa, Oocytes): Meiosis, fertilization.
  • Hepatocytes (Liver Cells): Diverse metabolic functions.
  • Kidney Tubule Cells: Selective reabsorption/secretion, urine concentration. Podocytes (filtration).
  • Endothelial Cells: Regulation of vascular tone, permeability, angiogenesis.
  • Sertoli Cells (Testis): Support spermatogenesis.
  • Granulosa Cells (Ovary): Support oocyte development, estrogen synthesis.

IV. Molecular Level Functions

  • A. Genetic Information: Storage, Replication, Expression & Repair:
    • DNA Structure & Organization: Double Helix, Genes, Chromatin (Histones, Nucleosomes), Euchromatin, Heterochromatin.
    • DNA Replication: Origin of Replication, Helicase, SSBs, Topoisomerases, Primase, DNA Polymerases, DNA Ligase, Telomerase.
    • Gene Expression - Transcription: RNA Polymerases, Promoters, Enhancers, General Transcription Factors, mRNA Processing (Capping, Splicing, Polyadenylation).
    • Gene Expression - Translation: Ribosomes (A,P,E sites), mRNA (Codons), tRNA (Anticodon, Aminoacyl-tRNA Synthetases), Initiation/Elongation/Termination Factors.
    • Regulation of Gene Expression: Transcriptional (Transcription Factors, Chromatin Remodeling, Epigenetics - DNA methylation, histone modification), Post-Transcriptional (RNA splicing, stability, localization, RNAi - miRNA/siRNA), Translational, Post-Translational Control (Folding, modifications, degradation).
    • DNA Repair Mechanisms: Mismatch Repair, Base Excision Repair, Nucleotide Excision Repair, Homologous Recombination, Non-Homologous End Joining.
  • B. Protein Structure, Function & Dynamics:
    • Amino Acid Structure & Properties. Peptide Bond.
    • Levels of Protein Structure: Primary, Secondary (α-helix, β-sheet), Tertiary, Quaternary.
    • Protein Folding & Chaperones. Misfolding & Aggregation.
    • Enzyme Catalysis: Active Site, Lowering Activation Energy, Michaelis-Menten kinetics (Km, Vmax), Inhibition, Allosteric regulation, Coenzymes, Cofactors.
    • Receptor-Ligand Interactions: Affinity (Kd), Specificity.
    • Motor Proteins: Myosins, Kinesins, Dyneins. ATP hydrolysis for mechanical work.
    • Structural Proteins: Collagen, Elastin, Keratin.
    • Transport Proteins: Hemoglobin (Cooperative binding, Bohr effect), Channel proteins, Carrier proteins.
    • Antibodies (Immunoglobulins): Fab, Fc regions. Antigen binding.
    • Protein Degradation: Ubiquitin-Proteasome System, Lysosomal degradation.
  • C. Membrane Dynamics & Transport (Molecular Level):
    • Lipid Bilayer Structure: Phospholipids, Cholesterol, Glycolipids. Fluid mosaic model. Lipid rafts.
    • Ion Channel Gating Mechanisms: Voltage-gated, Ligand-gated, Mechanically-gated, Temperature-gated.
    • Carrier Protein Conformational Cycling.
    • Na+/K+ ATPase Pump Cycle.
    • Vesicle Formation & Fusion: Coat proteins (Clathrin, COPI, COPII), SNAREs (v-SNARE, t-SNARE), Rab GTPases.
  • D. Cellular Signaling & Communication (Molecular Pathways):
    • G Protein Cycle (GPCR signaling).
    • Second Messenger Systems: cAMP/PKA, Phosphoinositide/IP3/DAG/PKC, Ca2+/Calmodulin, NO/cGMP/PKG.
    • Receptor Tyrosine Kinase (RTK) Signaling: Dimerization, Autophosphorylation, Recruitment of SH2-domain proteins, Ras-MAPK pathway, PI3K-Akt pathway.
    • MAPK Cascades (Raf-MEK-ERK).
    • Jak-STAT Pathway.
    • Nuclear Receptor Signaling (Steroid/Thyroid hormones, HREs).
  • E. Energy Transduction & Metabolism (Molecular Reactions):
    • ATP Hydrolysis & Synthesis. Substrate-level & Oxidative phosphorylation.
    • Redox Reactions: NAD+/NADH, NADP+/NADPH, FAD/FADH2. Electron Transport Chain Components (Complex I-IV, CoQ, Cytochrome c).
    • Glycolytic Pathway Reactions & Key Enzymes (Hexokinase, PFK1, Pyruvate Kinase).
    • Citric Acid Cycle Reactions & Key Enzymes (Citrate Synthase, Isocitrate Dehydrogenase, α-KG Dehydrogenase).
    • Fatty Acid β-Oxidation.
  • F. Muscle Contraction & Cell Motility (Molecular Basis):
    • Sliding Filament Model: Actin (Tropomyosin, Troponin), Myosin. Cross-Bridge Cycle. Ca2+ Regulation (SR, RyR, SERCA).
    • Microtubule-Based Motility (Kinesin, Dynein).
  • G. Immune Recognition & Response (Molecular Interactions):
    • Antigen Recognition by Antibodies (Paratope-Epitope binding).
    • T Cell Receptor (TCR) Recognition of Peptide-MHC complexes. CD4/CD8 co-receptors.
    • Complement System Activation Cascades (Classical, Alternative, Lectin). C3/C5 convertases, MAC.
    • Cytokine Signaling (Interleukins, Interferons, TNFs) via specific receptors, Jak-STAT, NF-κB.
  • H. Hemostasis & Blood Clotting (Molecular Cascade):
    • Platelet Activation: Adhesion (vWF, GpIb), Granule release (ADP, TXA2), Aggregation (GpIIb/IIIa, Fibrinogen).
    • Coagulation Cascade (Intrinsic, Extrinsic, Common Pathways): Clotting factors, Thrombin, Fibrinogen to Fibrin, Factor XIIIa.
    • Fibrinolysis (Plasminogen, Plasmin, tPA).
  • I. Neurotransmission (Molecular Events at Synapse):
    • Neurotransmitter Synthesis, Storage (vesicles).
    • Action Potential -> Ca2+ Influx (Voltage-gated Ca2+ channels).
    • Vesicle Docking/Fusion (SNAREs) -> Neurotransmitter Release.
    • Binding to Postsynaptic Receptors (Ionotropic, Metabotropic).
    • Generation of EPSPs/IPSPs.
    • Neurotransmitter Inactivation/Removal (Enzymatic degradation, Reuptake transporters, Diffusion).

An overview of the intricate human body.

May this knowledge deepen our wonder and gratitude for the miraculous creation that is the human body.