The upper limb skeleton consists of the clavicle, scapula, humerus, radius, ulna, carpals (8), metacarpals (5), and phalanges (14). The clavicle is the first bone to ossify and the most commonly fractured. The scapula has three angles, three borders, and processes including the acromion, coracoid, and glenoid cavity. The humeral head articulates with the glenoid at the shoulder joint — a ball-and-socket joint with remarkable mobility but inherent instability. The elbow is a hinge joint (humeroulnar) plus pivot joint (proximal radioulnar). The wrist (radiocarpal) is a condyloid joint. The interphalangeal joints are hinge joints. Important joint stabilizers include the glenoid labrum (deepens the glenoid), the ulnar collateral ligament (elbow), and the scapholunate ligament (wrist). The shoulder joint relies on the rotator cuff muscles for dynamic stability.

The arm has two compartments: anterior (flexors: biceps brachii, brachialis, coracobrachialis, all supplied by musculocutaneous nerve) and posterior (triceps brachii, supplied by radial nerve). The forearm has three compartments: anterior (flexor-pronator group, median/ulnar nerves), posterior (extensor-supinator group, radial nerve), and lateral (brachioradialis, extensor carpi radialis longus). The hand contains three groups: thenar (abductor pollicis brevis, opponens pollicis, flexor pollicis brevis, adductor pollicis), hypothenar (abductor digiti minimi, flexor digiti minimi, opponens digiti minimi), and central (lumbricals, interossei, palmaris brevis). The rotator cuff consists of supraspinatus (abduction 0–15°), infraspinatus (external rotation), teres minor (external rotation), and subscapularis (internal rotation). The deltoid is the prime abductor beyond 15°. The pronator teres and pronator quadratus produce forearm pronation; supinator and biceps produce supination.

The brachial plexus is formed by C5–T1 ventral rami, organized as roots → trunks (upper, middle, lower) → divisions (anterior, posterior) → cords (lateral, posterior, medial) → branches. The axillary nerve (C5–C6, posterior cord) supplies deltoid and teres minor. The musculocutaneous nerve (C5–C7, lateral cord) supplies the anterior arm. The radial nerve (C5–T1, posterior cord) supplies triceps, forearm extensors, and brachioradialis; injury produces wrist drop. The median nerve (C5–T1, lateral and medial cords) supplies most forearm flexors plus thenar muscles and lateral 2 lumbricals; injury produces ape hand and carpal tunnel syndrome. The ulnar nerve (C8–T1, medial cord) supplies hypothenar muscles, interossei, medial 2 lumbricals, and adductor pollicis; injury produces claw hand. Erb-Duchenne palsy (C5–C6) results from excessive neck-shoulder traction, causing waiter’s tip deformity (adducted, internally rotated arm). Klumpke palsy (C8–T1) involves hyperabduction, causing claw hand with Horner syndrome if T1 is involved.

The axillary artery (continuation of subclavian) has three parts relative to pectoralis minor. The brachial artery (continuation of axillary at teres major) bifurcates into radial and ulnar arteries in the cubital fossa. The radial artery courses superficially (used for pulse and arterial blood gas sampling), while the ulnar artery supplies the superficial palmar arch. The deep palmar arch is formed mainly by the radial artery, the superficial arch by the ulnar artery. Important anastomoses include the scapular anastomosis (around the scapula) and the cubital anastomosis (around the elbow). Venous drainage: superficial (cephalic, basilic, median cubital) and deep (venae comitantes). The cephalic vein ascends lateral to biceps and pierces the clavipectoral fascia to drain into the axillary vein. Lymphatics drain via axillary nodes (5 groups: pectoral, subscapular, humeral, central, apical), then the subclavian trunk. The deltopectoral node is a clinically palpable sentinel node.

Clavicle fracture (middle third most common) risks subclavian vessel injury. Surgical neck of humerus fracture risks axillary nerve injury. Midshaft humeral fracture risks radial nerve injury (wrist drop). Supracondylar fracture of the humerus (common in children) risks brachial artery and median nerve injury. Colles fracture (distal radius, dinner fork deformity) results from fall on outstretched hand. Scaphoid fracture (snuffbox tenderness) risks avascular necrosis of the proximal pole. Carpal tunnel syndrome (median nerve compression at wrist) presents with thenar wasting and pain/numbness in the lateral 3.5 digits, worse at night. Ulnar nerve entrapment at the elbow (cubital tunnel syndrome) causes clawing of the ring and little fingers. Rotator cuff tears present with painful arc (60–120° abduction) and drop arm test. Shoulder dislocation (most common anterior/inferior) risks axillary nerve injury with deltoid weakness and loss of shoulder contour.

The lower limb skeleton includes the pelvis (ilium, ischium, pubis), femur, patella, tibia, fibula, tarsals (7), metatarsals (5), and phalanges (14). The hip joint is a ball-and-socket joint between the femoral head and acetabulum, stabilized by the labrum, capsule, and strong ligaments (iliofemoral, pubofemoral, ischiofemoral — the iliofemoral ligament is the strongest). The knee is a complex hinge joint with cruciate ligaments (ACL, PCL), collateral ligaments (MCL, LCL), and menisci (medial C-shaped, lateral O-shaped). The ankle (talocrural) is a mortise joint between tibia, fibula, and talus, stabilized by syndesmosis and collateral ligaments (medial deltoid, lateral anterior talofibular/calcaneofibular/posterior talofibular). The tibia bears approximately 85% of body weight; the fibula is non-weight-bearing but provides muscle attachment and forms part of the ankle mortise. The patella is the largest sesamoid bone, embedded in the quadriceps tendon, improving the mechanical advantage of the quadriceps. The foot has three arches: medial longitudinal, lateral longitudinal, and transverse, maintained by bony configuration, ligaments (spring, plantar fascia), and muscles.

The gluteal region includes gluteus maximus (hip extension, external rotation), medius and minimus (abduction — Trendelenburg test). The thigh has three compartments: anterior (quadriceps femoris, sartorius, iliopsoas, femoral nerve), medial (adductors, gracilis, obturator nerve), and posterior (hamstrings: semitendinosus, semimembranosus, biceps femoris, sciatic nerve). The leg has four compartments: anterior (tibialis anterior, extensor digitorum longus, extensor hallucis longus, peroneus tertius, deep peroneal nerve), lateral (peroneus longus and brevis, superficial peroneal nerve), superficial posterior (gastrocnemius, soleus, plantaris, tibial nerve), and deep posterior (tibialis posterior, flexor digitorum longus, flexor hallucis longus, tibial nerve). The plantar foot muscles are arranged in four layers. The iliopsoas is the principal hip flexor, innervated by the femoral nerve. The quadriceps femoris extends the knee. The hamstrings flex the knee and extend the hip. The popliteus unlocks the knee by rotating the femur laterally on the tibia.

The lumbar plexus (L1–L4) gives the femoral nerve (L2–L4, anterior thigh muscles and hip flexors, sensation to anterior thigh and medial leg via saphenous) and the obturator nerve (L2–L4, medial thigh adductors, sensation to medial thigh). The sacral plexus (L4–S4) gives the sciatic nerve (L4–S3, the largest nerve in the body), which divides into the tibial nerve (all leg/foot muscles below the knee except those supplied by the common peroneal) and the common peroneal nerve (short head of biceps, then divides into deep and superficial peroneal branches). The superior gluteal nerve (L4–S1) supplies gluteus medius, minimus, and tensor fascia lata; injury produces Trendelenburg gait. The inferior gluteal nerve (L5–S2) supplies gluteus maximus. The posterior femoral cutaneous nerve (S1–S3) supplies sensation to the posterior thigh. The pudendal nerve (S2–S4) supplies the perineum. The tibial nerve at the ankle divides into medial and lateral plantar nerves. Common peroneal nerve injury at the fibular neck causes foot drop (loss of dorsiflexion) and loss of sensation over the dorsum of the foot.

The femoral artery (continuation of external iliac artery after the inguinal ligament) enters the femoral triangle, bounded by the inguinal ligament, sartorius, and adductor longus; contents from lateral to medial are femoral nerve, artery, vein, and canal (NAVEL mnemonic). The profunda femoris (deep artery of the thigh) is the main branch supplying the thigh. The femoral artery passes through the adductor hiatus to become the popliteal artery, which divides into anterior tibial artery (continuing as dorsalis pedis) and posterior tibial artery (giving off peroneal branch and dividing into medial and lateral plantar arteries). Cruciate anastomosis around the knee and trochanteric anastomosis around the hip provide collateral circulation. Venous drainage: the great saphenous vein (longest vein in the body) ascends from the medial foot to the femoral vein at the saphenous opening; the small saphenous vein ascends from the lateral foot to the popliteal vein. Perforating veins with valves connect the superficial and deep systems. Incompetent valves lead to varicose veins. The dorsal venous arch of the foot drains both saphenous systems.

Hip fractures are classified as intracapsular (femoral neck, risking avascular necrosis) or extracapsular (intertrochanteric, subtrochanteric, with better blood supply). ACL tear presents with anterior drawer sign, Lachman test, and pivot shift; frequently associated with meniscal tear and MCL injury (O’Donoghue triad). PCL injury results from a dashboard impact. Ankle sprain most commonly involves the anterior talofibular ligament from inversion injury; deltoid ligament injury occurs with eversion. Compartment syndrome of the leg presents with pain with passive stretch, paresthesia, and paralysis (late) and requires emergency fasciotomy. Deep vein thrombosis typically involves popliteal, femoral, or iliac veins with Virchow triad (stasis, hypercoagulability, endothelial injury). Baker cyst in the popliteal fossa may mimic DVT. Femoral hernia (below the inguinal ligament and medial to the femoral vein) is more common in women and has a high risk of strangulation. Plantar fasciitis causes heel pain with the first steps in the morning. Patellar dislocation (usually lateral) reduces with knee extension.

The mediastinum is the central compartment of the thoracic cavity, divided into the superior mediastinum (above the sternal angle at T4/T5) and the inferior mediastinum (below), which is further divided into anterior (thymus, lymph nodes, internal thoracic vessels), middle (heart, pericardium, phrenic nerves, great vessel roots), and posterior (esophagus, thoracic duct, descending aorta, azygos/hemiazygos veins, sympathetic trunk) compartments. The sternal angle (angle of Louis) is a key landmark for the bifurcation of the trachea, the beginning and end of the aortic arch, and the level of the second rib articulation. The thoracic inlet is bounded by T1, the first ribs, and the manubrium. The thoracic outlet is bounded by T12, the 12th ribs, the costal margin, and the xiphoid. The diaphragm (innervated by the phrenic nerve, C3–C5) has three openings: the caval opening (T8, IVC, right phrenic nerve), esophageal hiatus (T10, esophagus, vagus nerves), and aortic hiatus (T12, aorta, thoracic duct, azygos/hemiazygos veins).

The pericardium is a fibroserous sac with an outer fibrous layer and an inner serous pericardium (parietal and visceral layers), separated by the pericardial cavity containing 15–50 mL of serous fluid. The heart has four chambers: the right atrium receives the SVC, IVC, and coronary sinus; the right ventricle pumps to the pulmonary trunk; the left atrium receives four pulmonary veins; the left ventricle pumps to the aorta. The interatrial septum contains the fossa ovalis (remnant of the foramen ovale). The interventricular septum has membranous and muscular parts. The tricuspid valve has three leaflets (anterior, posterior, septal), while the mitral valve has two (anterior and posterior). Chordae tendineae connect the valve leaflets to the papillary muscles. The coronary arteries: the left main divides into the left anterior descending (LAD, supplying the anterior wall, septum, and apex) and the left circumflex (LCx, supplying the lateral wall). The right coronary artery (RCA) supplies the right ventricle and inferior wall; it gives off the SA nodal artery (60%) and the AV nodal artery (90%). Dominance is right-sided in approximately 85% of individuals (posterior descending artery from the RCA). The coronary sinus drains cardiac veins into the right atrium.

The right lung has three lobes (superior, middle, inferior) and 10 bronchopulmonary segments. The left lung has two lobes (superior and inferior) plus the lingula (homologous to the right middle lobe) and 8–10 segments. Each bronchopulmonary segment is supplied by a segmental bronchus and a branch of the pulmonary artery, making each segment a surgically resectable unit. The hilum contains the main bronchus, pulmonary artery and veins, bronchial vessels, lymphatics, and nerves. The pleura consists of a visceral layer (adherent to the lung) and a parietal layer (lining the thoracic wall, mediastinum, and diaphragm). The pleural cavity contains a thin film of serous fluid. Important recesses include the costodiaphragmatic and costomediastinal recesses. The trachea bifurcates at the carina (T4/T5) into right and left main bronchi. The right main bronchus is shorter, wider, and more vertical, making it the most common site for aspiration. The bronchial arteries (from the descending aorta) supply the lung parenchyma, while the pulmonary arteries supply the alveoli for gas exchange.

The aorta ascends from the left ventricle, arches posteriorly (giving off the brachiocephalic trunk, left common carotid, and left subclavian arteries), then descends through the posterior mediastinum to the aortic hiatus at T12. The SVC is formed by the brachiocephalic veins at the level of the right first costal cartilage and receives the azygos vein before entering the right atrium. The IVC pierces the diaphragm at T8. The pulmonary trunk bifurcates into the right and left pulmonary arteries. The ligamentum arteriosum (remnant of the ductus arteriosus) connects the left pulmonary artery to the aortic arch. The esophagus descends from the pharynx (C6) through the superior and posterior mediastinum, passing through the diaphragm at T10. It has three constrictions: cervical (cricopharyngeus), broncho-aortic (T4/T5, where the left main bronchus and aorta cross), and diaphragmatic (T10). The thoracic duct begins at the cisterna chyli (L1/L2), ascends through the aortic hiatus, courses posterior to the esophagus, and drains into the left venous angle. It drains lymph from the lower body, left thorax, left arm, and left head and neck. The right lymphatic duct drains the right upper quadrant of the body.

Cardiac tamponade presents with Beck triad (hypotension, JVD, muffled heart sounds), pulsus paradoxus, and electrical alternans on ECG; treatment is urgent pericardiocentesis via the subxiphoid approach. Tension pneumothorax causes tracheal deviation away from the affected side, hyperresonance, absent breath sounds, and hypotension; immediate needle decompression at the 2nd intercostal space in the midclavicular line is required. Pleural effusion is accessed by thoracentesis at the 8th–9th intercostal space in the midscapular line. Chest tube insertion is performed at the 5th intercostal space just anterior to the midaxillary line, staying along the superior border of the rib to avoid the intercostal neurovascular bundle. Aortic dissection presents with sudden tearing chest pain radiating to the back, pulse deficits, and mediastinal widening on CXR; Type A (ascending) requires emergent surgery, while Type B (descending) is managed medically. Pancoast tumor at the superior sulcus may involve the brachial plexus (C8–T1) and cause Horner syndrome (ptosis, miosis, anhidrosis). Pericarditis causes pleuritic chest pain with diffuse ST elevation and PR depression.

The anterolateral abdominal wall consists of layers: skin, superficial fascia (Camper fatty and Scarpa membranous layers), external oblique, internal oblique, transversus abdominis, transversalis fascia, extraperitoneal fat, and parietal peritoneum. The rectus sheath encloses the rectus abdominis muscle and is formed by the aponeuroses of the three lateral abdominal muscles. The arcuate line (midway between the umbilicus and pubis) marks the transition where the posterior rectus sheath disappears; below this line, all aponeuroses pass anterior to the rectus. The inguinal canal (4–5 cm) runs from the deep inguinal ring (lateral to the inferior epigastric vessels) to the superficial inguinal ring (medial and superior to the pubic tubercle). It contains the spermatic cord in males and the round ligament of the uterus in females. Hesselbach triangle (bounded by the inferior epigastric artery, lateral border of the rectus abdominis, and inguinal ligament) is the site of direct inguinal hernias. Indirect inguinal hernias pass through the deep ring, lateral to the inferior epigastric vessels. Direct hernias bulge through Hesselbach triangle, medial to the inferior epigastric vessels.

The peritoneum is a serous membrane with parietal (lining the abdominal wall) and visceral (covering organs) layers. The peritoneal cavity is the potential space between them, containing serous fluid. Intraperitoneal organs include the stomach, liver, spleen, jejunum, ileum, cecum, appendix, transverse colon, and sigmoid colon. Retroperitoneal organs include the kidneys, ureters, suprarenal glands, aorta, IVC, duodenum (2nd–4th parts), pancreas (except the tail), ascending colon, descending colon, and rectum. The greater omentum (four-layered peritoneal fold) hangs from the greater curvature of the stomach. The lesser omentum runs from the liver to the lesser curvature and duodenal bulb. The omental bursa (lesser sac) communicates with the greater sac via the epiploic foramen of Winslow, bounded by the IVC posteriorly, the portal triad anteriorly, the duodenum inferiorly, and the caudate lobe of the liver superiorly. The pouch of Douglas (rectouterine pouch) is the most dependent part of the peritoneal cavity in the supine position and is a common site for fluid collection.

The foregut (supplied by the celiac artery at T12–L1) includes the distal esophagus, stomach, proximal duodenum (to the ampulla of Vater), liver, gallbladder, pancreas (head, neck, body), and spleen. The celiac trunk trifurcates into the left gastric, splenic, and common hepatic arteries. The stomach is divided into cardia, fundus, body, and pylorus. The duodenum is C-shaped and mostly retroperitoneal. The midgut (supplied by the superior mesenteric artery at L1) includes the distal duodenum, jejunum, ileum, cecum, appendix, ascending colon, and proximal two-thirds of the transverse colon. The SMA gives off jejunal and ileal branches, the ileocolic, right colic, and middle colic arteries. The hindgut (supplied by the inferior mesenteric artery at L3) includes the distal third of the transverse colon, descending colon, sigmoid colon, and rectum. The IMA branches into the left colic, sigmoid, and superior rectal arteries. The marginal artery of Drummond provides an anastomotic connection between the SMA and IMA along the mesenteric border of the colon. The portal vein is formed by the union of the SMV and splenic vein behind the neck of the pancreas.

The hepatic portal vein carries nutrient-rich, toxin-laden blood from the GI tract, spleen, pancreas, and gallbladder to the liver for metabolic processing. Portal hypertension (from cirrhosis, portal vein thrombosis, or Budd-Chiari syndrome) leads to portacaval anastomoses at four major sites. (1) Esophageal: left gastric vein (portal) communicates with azygos/hemiazygos veins (systemic), forming esophageal varices. (2) Rectal: superior rectal vein (portal) connects with middle and inferior rectal veins (systemic), causing hemorrhoids. (3) Paraumbilical: paraumbilical veins (portal) connect with epigastric veins (systemic), producing caput medusae. (4) Retroperitoneal: colic veins (portal) connect with retroperitoneal veins via the veins of Retzius. The liver has four lobes (left, right, caudate, quadrate) and is divided into eight functionally independent segments based on portal and hepatic venous anatomy. The porta hepatis contains the portal vein, hepatic artery, and common hepatic duct (the biliary triad). The gallbladder lies in the fossa between the right and quadrate lobes of the liver.

The retroperitoneal space contains the kidneys (T12–L3, right lower than left), ureters, adrenal glands, aorta, IVC, duodenum (2nd–4th parts), pancreas (head and body), ascending colon, descending colon, and rectum. The renal hilum contains the renal vein (anterior), renal artery (middle), and renal pelvis (posterior). The ureters course along the psoas major and cross the pelvic brim at the bifurcation of the common iliac arteries. Clinically, acute appendicitis presents with periumbilical pain (visceral, T10) localizing to McBurney point (one-third of the distance from ASIS to umbilicus). Rovsing sign, psoas sign, and obturator sign are helpful adjuncts. AAA may present as a pulsatile mass and can rupture catastrophically. Intestinal obstruction may be mechanical (adhesions, hernia, tumor, volvulus) or functional (ileus). Acute mesenteric ischemia (SMA occlusion) presents with abdominal pain out of proportion to physical findings, metabolic acidosis, and rapid deterioration, requiring emergency revascularization. Paracentesis and liver biopsy require careful planning to avoid injury to the inferior epigastric vessels and colon.

The bony pelvis is formed by the two hip bones (ilium, ischium, pubis), sacrum, and coccyx. The pelvis is divided into the greater (false) pelvis above the pelvic brim and the lesser (true) pelvis below. The pelvic inlet is bounded by the sacral promontory, arcuate and pectineal lines, and the pubic symphysis. The pelvic outlet is bounded by the coccyx, ischial tuberosities, and the pubic arch. The female pelvis is broader, shallower, with a wider subpubic angle (>80°), a wider sciatic notch, and a more oval pelvic inlet, all adaptations for childbirth. The male pelvis is narrower and deeper with a subpubic angle of less than 70°. The sacrotuberous and sacrospinous ligaments connect the sacrum to the ischial tuberosity and spine, converting the greater and lesser sciatic notches into foramina. The obturator foramen is closed by the obturator membrane except for the obturator canal, through which the obturator nerve and vessels pass. The pubic symphysis is a cartilaginous joint with limited movement that increases during pregnancy.

The pelvic floor is formed by the levator ani and coccygeus muscles, collectively the pelvic diaphragm. The levator ani has three parts: pubococcygeus (including puboprostaticus/pubovaginalis and puborectalis), iliococcygeus, and ischiococcygeus. It supports the pelvic viscera, maintains urinary and fecal continence, and assists in parturition. The puborectalis sling maintains the anorectal angle (80–90°), critical for fecal continence. The perineal body (central tendon) lies between the anus and external genitalia, serving as an attachment point for several perineal muscles. The urogenital diaphragm (perineal membrane) separates the urogenital triangle from the pelvic cavity. The perineum is diamond-shaped, bounded by the pubic symphysis, ischial spines, and coccyx, divided into the urogenital triangle (anterior) and anal triangle (posterior). The pudendal nerve (S2–S4) is the principal nerve of the perineum, supplying motor innervation to perineal muscles and sensory innervation to the external genitalia. The internal pudendal artery, a branch of the internal iliac artery, is the main arterial supply.

In males, the testes are suspended by the spermatic cord within the scrotum and descend through the inguinal canal during fetal development. The ductus deferens courses through the spermatic cord, joins the seminal vesicle to form the ejaculatory duct, which passes through the prostate to open into the prostatic urethra. The prostate is a walnut-shaped gland surrounding the prostatic urethra with three zones: peripheral (most common site of carcinoma), central, and transitional (site of BPH). The bulbourethral glands (Cowper) lie in the deep perineal pouch. In females, the ovaries lie on the lateral pelvic wall, suspended by the mesovarium. The uterine tubes are divided into the infundibulum (with fimbriae), ampulla (most common fertilization site), isthmus, and intramural portion. The uterus is a pear-shaped organ with three layers (perimetrium, myometrium, endometrium) and is normally anteverted and anteflexed. The broad ligament covers the uterus, tubes, and ovaries. The round ligament extends from the uterine cornu through the inguinal canal to the labium majus. The vagina is a fibromuscular tube with rugae; the posterior fornix is the deepest.

The rectum (12–15 cm) begins at the rectosigmoid junction (S3) and follows the sacral curvature with three lateral flexures and three transverse rectal folds (valves of Houston). The anal canal (4 cm) extends from the anorectal junction (puborectalis sling) to the anus. The pectinate line divides the anal canal: above (columnar epithelium, internal hemorrhoids, visceral innervation) and below (non-keratinized squamous epithelium, external hemorrhoids, somatic innervation). The internal anal sphincter (smooth muscle, involuntary) and external anal sphincter (skeletal muscle, voluntary) control continence. The urinary bladder lies posterior to the pubic symphysis. The trigone is a smooth triangular region between the two ureteric orifices and the internal urethral orifice. The male urethra (18–22 cm) has four parts: pre-prostatic, prostatic (passes through the prostate, receives ejaculatory ducts), membranous (narrowest, external sphincter, deep perineal pouch), and spongy (penile, through corpus spongiosum). The female urethra is short (4 cm), opening into the vestibule between the labia minora.

BPH arises from the transitional zone causing urinary frequency, hesitancy, nocturia, and incomplete emptying. Prostate cancer arises from the peripheral zone (hard nodule on DRE). Pelvic organ prolapse includes cystocele (anterior vaginal wall with bladder), rectocele (posterior wall with rectum), and uterine prolapse. Episiotomy is a mediolateral incision to enlarge the vaginal introitus during childbirth, avoiding the external anal sphincter. Ectopic pregnancy most commonly occurs in the ampulla, presenting with adnexal pain and mass; rupture is a surgical emergency. DRE evaluates the prostate (male), cervix (female), rectal wall, and pelvic floor. Anal fissures are tears at the posterior midline causing painful defecation with bright red blood. Ischiorectal abscess presents with perianal pain, swelling, and fever, requiring incision and drainage. The pudendal nerve block (at the ischial spine) provides anesthesia for perineal procedures and operative vaginal deliveries. Obstetric lacerations are classified as 1st through 4th degree; 4th degree involves the external anal sphincter and rectal mucosa.

The skull is divided into the calvaria (skullcap) and the facial skeleton. Important foramina and their contents include: the cribriform plate (CN I, anterior ethmoidal vessels, emissary veins), optic canal (CN II, ophthalmic artery), superior orbital fissure (CN III, IV, V1, VI, ophthalmic veins), foramen rotundum (V2), foramen ovale (V3, accessory meningeal artery, lesser petrosal nerve), foramen spinosum (middle meningeal artery), foramen lacerum (carotid canal above, cartilage fills the foramen), internal acoustic meatus (CN VII, VIII, labyrinthine artery), jugular foramen (CN IX, X, XI, internal jugular vein, sigmoid sinus), hypoglossal canal (CN XII), and foramen magnum (medulla oblongata, vertebral arteries, spinal roots of CN XI, anterior/posterior spinal arteries). The pterion is the H-shaped suture junction between the frontal, parietal, temporal, and sphenoid bones. The middle meningeal artery runs in a bony canal deep to the pterion; trauma here causes an extradural (epidural) hematoma. The cavernous sinus contains CN III, IV, V1, V2, VI, and the internal carotid artery; thrombosis causes ophthalmoplegia and cranial nerve palsies.

CN I (Olfactory): sensory, smell, cribriform plate. CN II (Optic): sensory, vision, optic canal. CN III (Oculomotor): motor to all extraocular muscles except LR (CN VI) and SO (CN IV); parasympathetic to sphincter pupillae and ciliary muscle; runs through the cavernous sinus and superior orbital fissure. CN IV (Trochlear): motor to superior oblique; only nerve emerging from the dorsal brainstem; long, thin course through the cavernous sinus. CN V (Trigeminal): three divisions — V1 ophthalmic (SOF), V2 maxillary (foramen rotundum), V3 mandibular (foramen ovale). Sensory to face, motor to muscles of mastication. CN VI (Abducens): motor to lateral rectus; long intracranial course, vulnerable to raised ICP; runs through the cavernous sinus and SOF. CN VII (Facial): motor to muscles of facial expression; parasympathetic to lacrimal, submandibular, and sublingual glands; taste to anterior 2/3 of tongue; exits via internal acoustic meatus, through the facial canal, and stylomastoid foramen. CN VIII (Vestibulocochlear): sensory, hearing and balance, internal acoustic meatus. CN IX (Glossopharyngeal): motor to stylopharyngeus; parasympathetic to parotid; taste/sensation to posterior 1/3 of tongue; jugular foramen. CN X (Vagus): motor to palate, pharynx, larynx; parasympathetic to thoracic and abdominal viscera; jugular foramen. CN XI (Accessory): spinal root to SCM and trapezius; jugular foramen. CN XII (Hypoglossal): motor to tongue muscles; hypoglossal canal.

The neck is divided by the sternocleidomastoid muscle into the anterior and posterior triangles. The anterior triangle is bounded by the mandible, the midline, and the SCM; it contains the submandibular triangle (submandibular gland, facial artery, hypoglossal nerve), submental triangle, carotid triangle (carotid sheath with common carotid artery, internal jugular vein, vagus nerve, carotid bifurcation, carotid body/sinus, ansa cervicalis), and muscular triangle (strap muscles). The posterior triangle is bounded by the trapezius, SCM, and clavicle; it contains the accessory nerve (CN XI), trunks of the brachial plexus (emerging between the scalene muscles), the phrenic nerve (C3–C5, descending on the anterior scalene), and the supraclavicular lymph nodes. The scalene triangle (between the anterior and middle scalene muscles) contains the subclavian artery and the roots of the brachial plexus. The prevertebral fascia surrounds the deep neck structures. The strap muscles (infrahyoid: sternohyoid, omohyoid, sternothyroid, thyrohyoid) stabilize the hyoid bone and larynx during swallowing and phonation.

The pharynx is divided into the nasopharynx (posterior to the nasal cavity, contains the pharyngeal tonsil, torus tubarius around the Eustachian tube orifice), oropharynx (between the soft palate and hyoid, contains the palatine tonsils), and laryngopharynx (between the hyoid and cricoid, leading to the esophagus). The pharyngeal constrictors (superior, middle, inferior) are innervated by the pharyngeal plexus (CN IX, X, sympathetic fibers). The larynx extends from the epiglottis (C3) to the inferior border of the cricoid cartilage (C6). Cartilages: thyroid (largest, forming the laryngeal prominence), cricoid (signet ring, the only complete cartilaginous ring), epiglottis (leaf-shaped, closes during swallowing), and arytenoids (paired, with vocal processes). The true vocal cords are adducted by the lateral cricoarytenoids and interarytenoids (recurrent laryngeal nerve) and abducted by the posterior cricoarytenoids. The recurrent laryngeal nerve (right loops around the subclavian, left loops around the aortic arch) supplies all intrinsic laryngeal muscles except cricothyroid (external branch of the superior laryngeal nerve). The parotid gland (largest salivary gland) lies superficial to the masseter; Stensen duct pierces the buccinator to open opposite the second maxillary molar. The submandibular gland lies in the submandibular triangle; Wharton duct opens at the sublingual caruncle. The thyroid gland has an isthmus at the level of C2–C4 and lateral lobes extending to the 5th–6th tracheal rings. The parathyroid glands (usually 4) lie on the posterior aspect of the thyroid lobes.

Cervical lymphadenopathy is divided into levels I–VI; the jugulodigastric (tonsillar) node is commonly enlarged in tonsillitis. Thyroidectomy risks include recurrent laryngeal nerve injury (hoarseness, airway compromise), superior laryngeal nerve injury (loss of cricothyroid function with reduced vocal pitch), hypoparathyroidism (hypocalcemia with perioral numbness, Chvostek/Trousseau signs), and post-operative hematoma with airway compression requiring emergency decompression. Facial nerve palsy (Bell palsy) presents with sudden unilateral weakness of all muscles of facial expression. UMN lesions spare the forehead (bilateral cortical innervation); LMN lesions involve the entire half of the face. Parotid tumors (most commonly pleomorphic adenoma) present as a painless preauricular mass; malignancy is suggested by pain, fixation, or facial nerve involvement. Pharyngeal pouches (Zenker diverticulum) occur in Killian triangle between the thyropharyngeus and cricopharyngeus, causing dysphagia, regurgitation, and aspiration. The retropharyngeal space is a potential space that can develop abscesses, threatening airway patency. Branchial cleft anomalies (most commonly from the second branchial cleft) present as lateral neck cysts or fistulas along the anterior border of the SCM.

The central nervous system comprises the cerebrum (telencephalon), brainstem (midbrain, pons, medulla), cerebellum, and spinal cord. The cerebrum has two hemispheres separated by the falx cerebri, each with four lobes: frontal (motor cortex, Broca area, executive function), parietal (primary sensory cortex, spatial processing, integration), temporal (primary auditory cortex, Wernicke area, memory via hippocampus, emotion via amygdala), and occipital (primary visual cortex). The insula is the fifth lobe, deep within the lateral sulcus, involved in visceral sensation and autonomic control. The brainstem consists of the midbrain (cerebral peduncles, substantia nigra, red nuclei, oculomotor and trochlear nuclei), pons (abducens, facial, and trigeminal nuclei, cerebellar peduncles), and medulla oblongata (hypoglossal, glossopharyngeal, vagus, and spinal accessory nuclei, with vital cardiorespiratory centers). The cerebellum has the flocculonodular lobe (vestibular), anterior lobe (spinocerebellum), and posterior lobe (cerebrocerebellum), with three pairs of peduncles (superior, middle, inferior) and four deep nuclei (fastigial, globose, emboliform, dentate). The spinal cord extends from the foramen magnum (C1) to L1/L2, with cervical and lumbar enlargements and the conus medullaris, cauda equina, and filum terminale.

The ventricular system consists of two lateral ventricles (C-shaped, within the cerebral hemispheres), the third ventricle (diencephalon, between the thalami), and the fourth ventricle (between the brainstem and cerebellum). Each lateral ventricle has five parts: frontal horn, body, atrium (trigone), occipital horn, and temporal horn. The lateral ventricles communicate with the third ventricle via the interventricular foramina of Monro. The third ventricle communicates with the fourth ventricle via the cerebral aqueduct of Sylvius. The fourth ventricle communicates with the subarachnoid space through the median aperture of Magendie and the two lateral apertures of Luschka. CSF is produced by the choroid plexus (modified ependymal cells) in the lateral, third, and fourth ventricles at approximately 500–600 mL/day, with a total CSF volume of 140–150 mL. CSF circulates from the ventricles into the subarachnoid space and is absorbed into the dural venous sinuses (primarily the superior sagittal sinus) via the arachnoid granulations. Normal CSF is clear and colorless with low protein and glucose approximately 60% of serum. Obstructive hydrocephalus can occur at the aqueduct, foramina, or at the level of arachnoid granulations (communicating hydrocephalus).

The brain receives blood from the internal carotid arteries (anterior circulation) and the vertebral arteries (posterior circulation). The internal carotid artery gives off the ophthalmic artery, posterior communicating artery, anterior choroidal artery, and then bifurcates into the anterior and middle cerebral arteries. The vertebral arteries unite to form the basilar artery, which gives off the anterior inferior cerebellar (AICA), pontine, superior cerebellar (SCA), and posterior cerebral arteries. The Circle of Willis is a vascular anastomosis at the base of the brain, formed by the anterior communicating artery, anterior cerebral arteries, internal carotid arteries, posterior communicating arteries, and posterior cerebral arteries. It provides collateral circulation in case of arterial occlusion, but it is complete in only about 20–30% of individuals. The middle cerebral artery (MCA) is the most common site of ischemic stroke, supplying the lateral cerebral cortex, internal capsule, and basal ganglia. The anterior cerebral artery (ACA) supplies the medial frontal and parietal lobes. The posterior cerebral artery (PCA) supplies the occipital lobe and medial temporal lobe. The lenticulostriate arteries (from the MCA) are small penetrating vessels that supply the internal capsule and basal ganglia and are commonly involved in hypertensive hemorrhage and lacunar infarcts.

The basal ganglia are a group of subcortical nuclei involved in motor control, cognition, and emotion. They include the caudate nucleus, putamen, globus pallidus (internal and external segments), subthalamic nucleus, and substantia nigra (pars compacta and pars reticulata). The striatum (caudate + putamen) is the main input nucleus, receiving cortical and thalamic afferents. The direct pathway (cortex → striatum → GPi/SNr → thalamus → cortex) facilitates movement. The indirect pathway (cortex → striatum → GPe → subthalamic nucleus → GPi/SNr → thalamus → cortex) suppresses movement. Dopamine from the substantia nigra pars compacta modulates the striatum: D1 receptors facilitate the direct pathway, D2 receptors inhibit the indirect pathway. Dopamine deficiency in Parkinson disease produces rigidity, bradykinesia, tremor, and postural instability. Huntington disease (loss of indirect pathway neurons in the striatum) produces chorea. Hemiballismus (contralateral flinging movements) results from a lesion of the subthalamic nucleus. The extrapyramidal system also includes the red nucleus, reticular formation, and vestibular nuclei that modulate motor activity through descending pathways.

The dorsal column-medial lemniscal pathway carries fine touch, vibration, and proprioception. First-order neurons enter the spinal cord and ascend ipsilaterally in the fasciculus gracilis (lower body, medial) and fasciculus cuneatus (upper body, lateral) to synapse in the nucleus gracilis and cuneatus in the medulla. Second-order neurons decussate as the internal arcuate fibers and form the medial lemniscus to the ventral posterolateral (VPL) nucleus of the thalamus. Third-order neurons project to the primary sensory cortex. The spinothalamic tract carries pain and temperature. First-order neurons synapse in the substantia gelatinosa, second-order neurons decussate in the anterior white commissure and ascend contralaterally. The anterior spinothalamic tract carries crude touch. The corticospinal tract (pyramidal tract) originates from the primary motor cortex (Betz cells and other pyramidal neurons). Fibers descend through the corona radiata, internal capsule (posterior limb), cerebral peduncle, medullary pyramids, and decussate at the spinomedullary junction. The lateral corticospinal tract (crossed, 80–90%) controls limb muscles; the anterior corticospinal tract (uncrossed, 10–20%) controls axial muscles. The corticobulbar tract projects to cranial nerve motor nuclei bilaterally (except the lower face of CN VII and CN XII, which are contralateral).

Epithelium is avascular tissue covering surfaces, lining cavities, and forming glands. Simple squamous epithelium (endothelium, mesothelium, lung alveoli) permits diffusion and filtration. Simple cuboidal epithelium (kidney tubules, thyroid follicles) functions in secretion and absorption. Simple columnar epithelium (stomach, intestines, gallbladder) is specialized for absorption and secretion; it may contain microvilli (brush border in intestines), cilia (respiratory tract), or goblet cells. Stratified squamous epithelium (keratinized = epidermis; non-keratinized = oral cavity, esophagus, vagina) protects against abrasion. Stratified cuboidal/columnar (ducts of sweat glands, salivary glands) is rare. Pseudostratified columnar epithelium (respiratory tract) appears stratified but all cells contact the basement membrane; ciliated with goblet cells. Transitional epithelium (urothelium, lining the urinary bladder, ureters, and renal pelvis) is specialized to stretch and accommodate varying volumes, appearing cuboidal when relaxed and squamous when distended. Glandular epithelium is classified as exocrine (ducts, secretions to surface: merocrine, apocrine, holocrine) or endocrine (ductless, hormones into blood). The basement membrane is a specialized extracellular matrix separating epithelium from connective tissue, containing the lamina lucida, lamina densa (type IV collagen), and lamina reticularis.

Connective tissue consists of cells (fibroblasts, chondroblasts, osteoblasts, adipocytes, macrophages, mast cells, plasma cells, leukocytes) embedded in an extracellular matrix (ECM) composed of protein fibers (collagen, elastin, reticular fibers) and ground substance (proteoglycans, GAGs, glycoproteins). Loose (areolar) connective tissue has abundant ground substance with fewer fibers, found in the papillary dermis, around vessels and organs. Dense regular connective tissue (tendons, ligaments) has parallel collagen fibers for tensile strength. Dense irregular connective tissue (dermis, organ capsules) has randomly oriented collagen fibers resisting multidirectional forces. Reticular connective tissue (lymph nodes, spleen, bone marrow) contains reticular fibers (type III collagen) forming a supportive stroma. Adipose tissue: white (unilocular, energy storage, insulation, endocrine via leptin and adipokines) and brown (multilocular, thermogenesis via UCP1, abundant mitochondria). Cartilage is avascular, receiving nutrition by diffusion from the perichondrium (except fibrocartilage). Bone is a specialized mineralized connective tissue with osteocytes, osteoblasts (bone formation), and osteoclasts (bone resorption) embedded in a matrix of type I collagen and hydroxyapatite crystals (calcium phosphate).

Skeletal muscle: long, cylindrical, multinucleated fibers with peripheral nuclei and striations (actin and myosin organized into sarcomeres between Z-discs). Voluntary, innervated by somatic motor neurons. Each muscle fiber is surrounded by endomysium; fascicles are surrounded by perimysium; the entire muscle is surrounded by epimysium. Satellite cells are muscle stem cells involved in regeneration. Types: type I (slow oxidative, red, fatigue-resistant, marathon running) and type II (fast glycolytic, white, powerful but fatigable, sprinting). Cardiac muscle: branched, striated, uninucleated (central) fibers connected by intercalated discs (containing desmosomes and gap junctions). Involuntary, innervated by the autonomic nervous system. Self-depolarizing pacemaker cells (SA node, AV node) generate intrinsic rhythm. Purkinje fibers are specialized conducting cells. Cardiac muscle has a longer refractory period than skeletal muscle, preventing tetanus. Smooth muscle: spindle-shaped, uninucleated, non-striated (actin and myosin are arranged in a lattice). Involuntary, innervated by the autonomic nervous system. Visceral (single-unit) smooth muscle contracts as a syncytium via gap junctions; multi-unit smooth muscle (iris, vas deferens) has independent innervation. Dense bodies anchor actin filaments (analogous to Z-discs). Smooth muscle can undergo prolonged contraction with minimal energy (latch state).

Nervous tissue consists of neurons (excitable cells transmitting electrical signals) and glial cells (supporting, insulating, and nourishing neurons). The neuron has a cell body (soma), dendrites (receiving signals), and an axon (conducting signals away). Axons may be myelinated (by oligodendrocytes in the CNS and Schwann cells in the PNS) or unmyelinated. Myelin increases conduction velocity via saltatory conduction at the nodes of Ranvier. Synapses are specialized junctions where neurotransmitters are released from the presynaptic terminal to bind receptors on the postsynaptic membrane. CNS glia: astrocytes (maintain BBB, regulate ions/neurotransmitters, provide structural support), oligodendrocytes (myelinate multiple axons), microglia (immune cells, phagocytic, derived from mesoderm), and ependymal cells (line the ventricles, produce CSF). PNS glia: Schwann cells (myelinate a single axon, aid regeneration) and satellite cells (surround cell bodies in ganglia). The blood-brain barrier is formed by tight junctions between cerebral capillary endothelial cells, reinforced by astrocyte foot processes. The peripheral nerve is organized into endoneurium (surrounds individual axons), perineurium (surrounds fascicles, contributes to blood-nerve barrier), and epineurium (surrounds the entire nerve).

Cartilage has three types. Hyaline cartilage (most abundant) has type II collagen and abundant ground substance; found in articular surfaces, nose, trachea, bronchi, and fetal skeleton (provides a model for endochondral ossification). Elastic cartilage (type II collagen + elastin) provides flexibility in the external ear, epiglottis, and auditory tube. Fibrocartilage (type I collagen + type II) resists compression and tension; found in the intervertebral discs, pubic symphysis, and menisci. Chondrocytes reside in lacunae within the ECM; cartilage grows by apposition (from perichondrium) and interstitial (chondrocyte division within the matrix) growth. Bone develops by two mechanisms: intramembranous ossification (directly within mesenchymal condensations, forming flat bones of the skull, mandible, clavicle) and endochondral ossification (replacing a hyaline cartilage model, forming long bones, vertebrae, and pelvis). The growth plate (physis) has zones: resting, proliferating (chondrocyte division), hypertrophic (chondrocyte enlargement), calcified, and ossification. Long bones have a diaphysis (shaft, cortical bone), metaphysis (growth plate region), and epiphysis (ends, cancellous bone). Osteons (Haversian systems) are cylindrical functional units of compact bone, with concentric lamellae surrounding a central Haversian canal (containing blood vessels and nerves).

Fertilization occurs in the ampulla of the fallopian tube within 12–24 hours of ovulation. The zygote undergoes cleavage divisions (2-cell, 4-cell, morula at day 3) as it travels through the fallopian tube. The blastocyst forms at day 5, consisting of an inner cell mass (embryoblast, which becomes the fetus) and an outer trophoblast (which becomes the placenta). Implantation in the endometrium occurs at day 6–7, mediated by the trophoblast differentiating into syncytiotrophoblast (invasion, hCG production) and cytotrophoblast. At week 2, the bilaminar disc forms: epiblast (dorsal, gives rise to all three germ layers through gastrulation) and hypoblast (ventral, contributes to extraembryonic structures). The amniotic cavity forms dorsal to the epiblast, and the yolk sac ventral to the hypoblast. At week 3, gastrulation begins with the formation of the primitive streak. Epiblast cells invaginate at the primitive streak to form the three germ layers: ectoderm (gives rise to the nervous system, epidermis), mesoderm (gives rise to muscles, bones, cardiovascular system, kidneys, reproductive organs), and endoderm (gives rise to the epithelial lining of the GI tract, respiratory tract, and various organs). The notochord develops from the prechordal process and forms the basis for the neural axis and vertebral column. The primitive node functions as the organizer, directing axial development.

The embryonic period (weeks 4–8) is when most major organs develop (organogenesis). The neural tube forms via neurulation: the notochord induces the overlying ectoderm to form the neural plate, which folds and fuses to create the neural tube (which becomes the brain and spinal cord). Neural crest cells migrate from the neural folds to form peripheral nerves, melanocytes, chromaffin cells, and facial skeleton components. The somites (paraxial mesoderm) differentiate into sclerotome (vertebrae, ribs), myotome (muscle), and dermatome (dermis). The intraembryonic coelom forms and divides into the pericardial, pleural, and peritoneal cavities. The heart begins beating at week 4. The limb buds appear at week 4 (upper) and week 5 (lower). By week 8, all major organ systems are established and the embryo is approximately 3 cm in length. The fetal period (week 9 to birth) is characterized by growth and maturation of existing structures. Teratogens during organogenesis (weeks 4–8) produce major congenital malformations. The critical periods for each organ vary: neural tube (week 3–4), heart (week 3–6), limbs (week 4–7), palate (week 6–9), external genitalia (week 7–12). Folic acid deficiency during the first 4 weeks prevents neural tube defects.

Pharyngeal (branchial) arches appear in weeks 4–5 and contribute to the formation of the head and neck. Each arch contains an aortic arch artery, a nerve, a cartilage rod, and mesoderm that forms muscles. The first pharyngeal arch (mandibular) forms the mandible, maxilla, malleus, incus, muscles of mastication (CN V3). The second arch (hyoid) forms the stapes, styloid process, hyoid bone (lesser horn and upper body), muscles of facial expression (CN VII). The third arch forms the hyoid bone (greater horn and lower body) and the stylopharyngeus muscle (CN IX). The fourth and sixth arches form the laryngeal cartilages and muscles (CN X, superior and recurrent laryngeal branches). The pharyngeal pouches (endodermal outpocketings between the arches) form: pouch 1 → middle ear cavity and Eustachian tube; pouch 2 → palatine tonsil; pouch 3 → thymus (dorsal) and inferior parathyroid (ventral); pouch 4 → superior parathyroid; pouch 5 → ultimobranchial body (parafollicular/C cells of the thyroid, which produce calcitonin). The thyroid gland develops from the foramen cecum at the base of the tongue and descends through the thyroglossal duct to its final position. Remnants of the thyroglossal duct can form thyroglossal duct cysts in the midline of the neck.

The heart begins as a linear heart tube formed by the fusion of two endocardial tubes at week 3. The heart tube undergoes looping (day 23–28) to create the four-chambered configuration. Atrial septation begins at week 4 with the formation of the septum primum, which grows toward the endocardial cushions. The foramen primum is the opening between the septum primum and the cushions; it closes as the septum primum fuses with the cushions. The foramen secundum develops as perforations in the septum primum before the foramen primum completely closes. The septum secundum grows to the right of the septum primum, leaving the foramen ovale (an opening that shunts blood from the right atrium to the left atrium in fetal life). The fossa ovalis is the remnant of the foramen ovale after closure at birth. Ventricular septation involves the muscular interventricular septum (growing upward from the apex) and the membranous interventricular septum (derived from the endocardial cushions and conotruncal ridges). The truncus arteriosus divides into the aorta and pulmonary trunk via the spiral (aorticopulmonary) septum. The conotruncal ridges help form the outflow tracts of the ventricles. Endocardial cushions contribute to the atrial and ventricular septa, and the atrioventricular valves. The sinus venosus gives rise to the coronary sinus, and the smooth parts of the right atrium and left atrium (from the pulmonary veins).

Neural tube defects (NTDs) result from failure of neural tube closure by day 28. Anencephaly (failure of cranial neuropore closure) is incompatible with life. Spina bifida occulta (failure of caudal neuropore closure without herniation) is usually asymptomatic. Spina bifida cystica includes meningocele (meninges herniate) and myelomeningocele (neural tissue herniates, with neurologic deficits). Folic acid (400–800 mcg/day) reduces NTD risk by up to 70%. Congenital heart defects: ventricular septal defect (most common), atrial septal defect (ostium secundum type most common), tetralogy of Fallot (VSD, overriding aorta, RV hypertrophy, pulmonary stenosis), transposition of the great arteries (TGA), and coarctation of the aorta. Cleft lip results from failure of the maxillary and medial nasal processes to fuse (week 6–7); cleft palate results from failure of the palatal shelves to fuse (week 8–9). Teratogens: ethanol (fetal alcohol syndrome: growth retardation, facial dysmorphism, CNS abnormalities), isotretinoin (retinoic acid syndrome: CNS, ear, heart defects), valproic acid (NTDs), thalidomide (limb defects), warfarin (nasal hypoplasia, stippled epiphyses), ACE inhibitors/ARBs (renal tubular dysplasia, oligohydramnios, fetal hypotension), tetracycline (tooth discoloration), varicella (limb hypoplasia, cutaneous scars), and cytomegalovirus (microcephaly, sensorineural hearing loss).

Surface anatomy landmarks are critical for physical examination and procedural planning. The sternal angle (angle of Louis, T4/T5) marks the tracheal bifurcation, aortic arch start/end, and the level of the 2nd rib. The jugular notch (suprasternal notch) is at T2/T3. The xiphoid process is at T9/T10. The costal margin runs from T9 to L1. The umbilicus is at L3/L4. The iliac crests are at L4 (intercristal line, landmark for lumbar puncture). The pubic symphysis is at the level of the coccyx. The spine of the scapula is at T3, and the inferior angle of the scapula is at T7/T8. Important vertebral levels for spinal cord anatomy: the spinal cord ends at L1/L2 (conus medullaris) in adults; the subarachnoid space extends to S2. The cauda equina lies below L2. The cervical enlargement (C3–T1) contains the brachial plexus outflow. The lumbar enlargement (L1–S3) contains the lumbosacral plexus outflow. The kidneys lie at T12–L3, with the right kidney slightly lower due to the liver. The spleen is positioned posterior to the 9th–11th ribs on the left side. The liver is below the right diaphragm at the level of T7–T11. The appendix is located at McBurney point (one-third of the distance from the ASIS to the umbilicus).

Chest tube insertion (tube thoracostomy) is performed for pneumothorax, hemothorax, or pleural effusion. The standard insertion site is the 5th intercostal space, just anterior to the midaxillary line. The incision is made through the skin and subcutaneous tissue, followed by blunt dissection of the intercostal muscles into the pleural cavity. The tube is directed posteriorly and superiorly (for air) or posteriorly and inferiorly (for fluid). Important: the incision should be made along the superior border of the rib to avoid the intercostal neurovascular bundle (which runs in the costal groove along the inferior border of the rib, arranged as vein, artery, nerve from superior to inferior). For tension pneumothorax, immediate needle decompression is performed at the 2nd intercostal space in the midclavicular line using a large-bore needle (14G or 16G). Thoracentesis (diagnostic or therapeutic pleural fluid aspiration) is performed at the 8th–9th intercostal space, midscapular line, along the superior border of the rib. Ultrasound guidance reduces complications (pneumothorax, hemothorax, laceration of the liver, spleen, or diaphragm). The pleural recess (costodiaphragmatic) is the most dependent part of the pleural cavity in the upright position and is the initial site of fluid accumulation.

Lumbar puncture (LP) is performed to obtain CSF for analysis (meningitis, subarachnoid hemorrhage, demyelinating disease). The patient is positioned in the lateral decubitus position (fetal position) or sitting upright, flexed to increase the interspinous space. The needle is inserted at the L3–L4 or L4–L5 interspace (at or below the intercristal line connecting the iliac crests, which crosses at L4). The needle is angled cephalad and advanced through the skin, supraspinous ligament, interspinous ligament, ligamentum flavum (which provides a characteristic “pop”), epidural space, dura mater, and arachnoid mater into the subarachnoid space. The distance from the skin to the subarachnoid space is approximately 4–6 cm in adults. Opening pressure (normal 10–20 cm H2O) is measured. Contraindications: increased ICP with mass effect, coagulopathy (platelets <50,000, INR >1.5), infection over the puncture site. Central venous catheterization (central line) is commonly performed at the internal jugular vein (in the triangle between the sternal and clavicular heads of the SCM), the subclavian vein (1 cm inferior to the midpoint of the clavicle, directed toward the suprasternal notch), or the femoral vein (medial to the femoral artery, 1–2 cm below the inguinal ligament). Anatomic landmarks: the IJ vein lies anterolateral to the carotid artery. The subclavian vein lies posterior to the medial third of the clavicle, separated from the subclavian artery by the anterior scalene muscle. The apex of the lung extends above the clavicle and is at risk during subclavian puncture.

McBurney incision (gridiron incision): oblique incision in the right lower quadrant, one-third of the way from the ASIS to the umbilicus, splitting the external oblique, internal oblique, and transversus abdominis muscles in the direction of their fibers. Used for open appendectomy. Kocher incision (right subcostal): 2–3 cm below the right costal margin, used for cholecystectomy and hepatobiliary surgery. Midline incision: through the linea alba (no muscle transection, relatively avascular, provides excellent access for abdominal exploratory laparotomy). Pfannenstiel incision: transverse suprapubic incision for cesarean section and gynecologic surgery; it heals well with a good cosmetic result. Thoracotomy: posterolateral thoracotomy (through the 4th–5th intercostal space for lung surgery) or median sternotomy (for cardiac surgery, splitting the sternum). Inguinal hernia repair: the incision is made in the inguinal crease, and the external oblique aponeurosis is opened through the superficial ring. Thyroidectomy: a collar (Kocher) incision in a skin crease approximately 2 cm above the sternal notch, through the platysma and strap muscles. Craniotomy: the pterional (frontotemporal) approach is commonly used for aneurysm clipping and frontal lobe lesion access; a burr hole for epidural hematoma evacuation is placed at the pterion.

Advanced Trauma Life Support (ATLS) protocols emphasize the primary survey (ABCDE: Airway with cervical spine protection, Breathing, Circulation, Disability, Exposure). Airway management includes chin lift/jaw thrust, oropharyngeal/nasopharyngeal airway, and endotracheal intubation. Surgical airway (cricothyroidotomy) is performed in a “cannot intubate, cannot ventilate” scenario: the cricothyroid membrane is palpated between the thyroid and cricoid cartilages, and a horizontal incision is made to insert a tracheostomy tube or large-bore IV catheter. Needle decompression of tension pneumothorax: 2nd intercostal space, midclavicular line, using a 14G angiocatheter. Pericardiocentesis for cardiac tamponade: the subxiphoid approach uses a long needle inserted at a 45° angle to the skin, directed toward the left shoulder, targeting the pericardial space. Focused Assessment with Sonography in Trauma (FAST) evaluates four windows: perihepatic (Morison pouch), perisplenic, pelvic (rectovesical/rectouterine), and pericardial. Diagnostic peritoneal lavage (DPL) is used in unstable patients with equivocal FAST findings. Pelvic fracture stabilization: a sheet or pelvic binder is wrapped around the greater trochanters to reduce pelvic volume and tamponade bleeding. Extremity trauma: hemorrhage control with tourniquet, wound packing, and pressure. The anatomic snuffbox (dorsal radial region) is the site for scaphoid fracture tenderness. The anatomical layers of the scalp (SCALP: Skin, Connective tissue, Aponeurosis, Loose connective tissue, Pericranium) must be respected during scalp laceration repair.