Visceral Anatomy: Cardiovascular, Respiratory, Digestive, Urogenital and Endocrine Systems

The viscera are where the spine meets the rest of the patient. Pain from the heart, gallbladder, kidney, pancreas or uterus can arrive in a chiropractic office dressed as back, shoulder or neck pain, and the board expects you to know exactly which organ sits where, what supplies it and how its sensory fibers reach the cord. This chapter walks through the cardiovascular, respiratory, digestive, urogenital and endocrine systems with an eye on relations, blood supply and the landmarks that let you find each organ on the body surface and on film.

Section 1The heart, great vessels and lymphatic system

The mediastinum

The mediastinum is the central compartment of the thorax between the two pleural cavities, extending from the superior thoracic aperture to the diaphragm and from the sternum to the bodies of the thoracic vertebrae. An imaginary transverse thoracic plane passes from the sternal angle (manubriosternal joint, the second costal cartilage) back to the T4 to T5 intervertebral disc. Above it is the superior mediastinum; below it the inferior mediastinum, which is subdivided by the pericardium into anterior, middle and posterior parts.

The sternal angle plane is one of the most heavily tested landmarks in thoracic anatomy because so many things happen there: the aortic arch begins and ends, the trachea bifurcates (the carina is at about T4 to T5 in the cadaver, a little lower in the living upright subject), the azygos vein arches over the right main bronchus into the superior vena cava, the thoracic duct crosses from right to left, the left recurrent laryngeal nerve hooks under the arch, and the ligamentum arteriosum connects the pulmonary trunk to the arch.

Pericardium

The heart sits in a double sac. The outer fibrous pericardium is tough, fuses with the central tendon of the diaphragm below and the adventitia of the great vessels above, and prevents sudden overdistension. Inside it, the serous pericardium has a parietal layer lining the fibrous sac and a visceral layer (epicardium) on the heart surface, with a thin film of fluid in the pericardial cavity between them. The fibrous and parietal layers are supplied by the phrenic nerve (C3 to C5), so pericarditis refers pain to the shoulder and supraclavicular region. A rapid pericardial effusion causes cardiac tamponade (hypotension, distended neck veins and muffled heart sounds, Beck triad).

External features and chambers

The heart is a cone lying obliquely, with its apex pointing anteriorly, inferiorly and to the left, usually at the left fifth intercostal space at the midclavicular line (about 9 cm from the midline), where the apex beat is palpated. The base faces posteriorly and is formed mostly by the left atrium. The sternocostal (anterior) surface is mainly right ventricle; the diaphragmatic (inferior) surface is mainly left ventricle with some right ventricle; the left (pulmonary) surface is left ventricle.

The coronary (atrioventricular) sulcus encircles the heart between atria and ventricles and carries the right coronary artery and the circumflex branch and the coronary sinus. The anterior and posterior interventricular sulci mark the septum and carry the anterior interventricular (left anterior descending) and posterior interventricular arteries.

Valves and the fibrous skeleton

The four valves sit in a dense fibrous skeleton of four rings and two trigones that anchors the valve leaflets, gives the myocardium attachment and, crucially, electrically insulates atria from ventricles so that the only normal electrical path between them is the AV bundle. The atrioventricular valves (tricuspid on the right, mitral on the left) have cusps tethered by chordae tendineae to papillary muscles, which contract during systole to prevent the cusps from everting into the atria. The semilunar valves (pulmonary and aortic) have three pocket-shaped cusps without chordae; the spaces behind the aortic cusps are the aortic sinuses, and the right and left coronary arteries arise from the right and left aortic sinuses respectively.

Valve sounds are heard best not over the valve itself but downstream where the turbulent flow is carried. Remember the auscultation points with the phrase "All People Enjoy Time Magazine": aortic (right second intercostal space at the sternal border), pulmonary (left second intercostal space), Erb point (left third), tricuspid (left fourth or fifth at the lower sternal border) and mitral (apex, left fifth intercostal space at the midclavicular line).

Coronary circulation

The myocardium is supplied by the right and left coronary arteries, the first branches of the aorta. Coronary flow to the left ventricle occurs mainly during diastole, because systolic contraction compresses intramural vessels; tachycardia shortens diastole and so reduces subendocardial perfusion.

Dominance is defined by which artery gives the posterior interventricular (posterior descending) artery. About 70% of people are right dominant, about 10% left dominant and the rest codominant. Venous blood returns mainly through the great cardiac vein (alongside the LAD), the middle cardiac vein (alongside the posterior interventricular artery) and the small cardiac vein, all draining into the coronary sinus in the posterior coronary sulcus, which opens into the right atrium.

Conduction system

Specialized cardiac muscle cells, not nerves, generate and spread the heartbeat. The sinoatrial (SA) node lies in the wall of the right atrium at the upper end of the crista terminalis near the SVC opening and is the normal pacemaker (about 60 to 100 beats per minute). The impulse spreads through atrial muscle to the atrioventricular (AV) node in the interatrial septum near the opening of the coronary sinus (within the triangle of Koch). The AV node delays conduction (about 0.1 second), allowing the atria to finish filling the ventricles. The impulse then passes through the fibrous skeleton by the AV bundle (bundle of His), which divides at the top of the muscular septum into right and left bundle branches; the right bundle runs partly in the moderator band to reach the anterior papillary muscle early. Terminal Purkinje fibers spread the impulse through the subendocardial ventricular myocardium from apex to base, so the ventricles contract upward and wring blood into the outflow tracts.

The heart's rate is modified by autonomic nerves through the cardiac plexus: sympathetic fibers from the cervical and upper thoracic ganglia (T1 to T4) increase rate and force; parasympathetic fibers from the vagus slow the SA node and AV conduction.

The great arteries

The ascending aorta gives only the coronary arteries. The aortic arch begins and ends at the sternal angle plane and gives three branches from right to left: the brachiocephalic trunk (which divides behind the right sternoclavicular joint into the right common carotid and right subclavian), the left common carotid and the left subclavian. The descending thoracic aorta runs in the posterior mediastinum to the left of the vertebral bodies, gives posterior intercostal arteries (3rd to 11th), subcostal, bronchial, esophageal and superior phrenic branches, and passes through the aortic hiatus at T12 behind the median arcuate ligament to become the abdominal aorta.

The abdominal aorta lies on the left anterior surface of the lumbar bodies and bifurcates at about L4 (the level of the iliac crests, the supracristal plane) into the common iliac arteries. Its branches fall into three groups.

The great veins

The brachiocephalic veins form behind each sternoclavicular joint from the union of the internal jugular and subclavian veins; the left one is longer and crosses obliquely behind the manubrium. They join behind the right first costal cartilage to form the superior vena cava, which descends to enter the right atrium at the level of the third costal cartilage and receives the azygos vein just before it enters the pericardium. The inferior vena cava forms at L5 from the common iliac veins, ascends to the right of the aorta, receives the lumbar, right gonadal, renal, right suprarenal, hepatic and inferior phrenic veins, and pierces the central tendon of the diaphragm at T8. The left gonadal and left suprarenal veins drain into the left renal vein, which crosses in front of the aorta just below the origin of the superior mesenteric artery.

The azygos system drains the posterior thoracic wall and provides an important collateral route between the IVC and SVC. The azygos ascends to the right of the thoracic vertebral bodies; the hemiazygos and accessory hemiazygos on the left cross behind the aorta at about T8 and T7 to join it. Because these veins connect with the valveless internal and external vertebral venous plexuses (Batson plexus), tumors of the prostate, breast, lung, kidney and thyroid can seed the vertebrae, a fact every chiropractor should remember when an older patient has unexplained night pain.

The lymphatic system

Lymphatic vessels collect the protein-rich interstitial fluid that escapes capillaries (about 2 to 3 liters a day), carry absorbed fat from the gut as chyle, and filter both through lymph nodes before returning them to the venous system.

• The cisterna chyli is a dilated sac anterior to L1 to L2, to the right of the aorta, under the right crus of the diaphragm. It receives the intestinal and lumbar lymph trunks.
• The thoracic duct leaves the cisterna, passes through the aortic hiatus at T12, ascends in the posterior mediastinum between the aorta and azygos vein behind the esophagus, crosses to the left at about T4 to T5, and empties into the junction of the left internal jugular and left subclavian veins (the left venous angle). It drains everything except the right upper quadrant of the body.
• The right lymphatic duct drains the right side of the head and neck, right upper limb and right side of the thorax into the right venous angle.

The testes drain to para-aortic nodes at L1 to L2 because they descended from there, while the scrotal skin drains to the superficial inguinal nodes. The spleen is the largest lymphoid organ and filters blood rather than lymph; the thymus is the site of T lymphocyte maturation and is described with the endocrine glands.

Section 2The respiratory system

Upper airway and larynx

Air enters through the nasal cavity, where three conchae (superior, middle, inferior) increase surface area to warm, humidify and filter it. The pharynx is divided into nasopharynx (pharyngeal tonsil, opening of the auditory tube), oropharynx (palatine tonsils) and laryngopharynx, which continues into the esophagus at C6.

The larynx lies at C3 to C6 and guards the lower airway. Its unpaired cartilages are the thyroid (the laryngeal prominence), the cricoid (the only complete ring, at C6) and the epiglottis; the paired cartilages are the arytenoids (on which the vocal ligaments pivot), corniculates and cuneiforms. The vocal folds (true cords) bound the rima glottidis, the narrowest part of the adult airway; the vestibular folds (false cords) lie above them.

Sensation above the vocal folds is carried by the internal laryngeal nerve (a branch of the superior laryngeal), which mediates the cough reflex; below the folds it is carried by the recurrent laryngeal. The left recurrent laryngeal nerve loops under the aortic arch at the ligamentum arteriosum and the right loops under the right subclavian artery; both ascend in the tracheoesophageal groove, where they are at risk in thyroid surgery and can be compressed by a left atrial enlargement, an aortic aneurysm or an apical (Pancoast) lung tumor, producing hoarseness.

Trachea and bronchial tree

The trachea runs from C6 to its bifurcation at the sternal angle (T4 to T5), held open by 16 to 20 C-shaped hyaline cartilage rings whose open posterior ends are bridged by the trachealis muscle, adjacent to the esophagus. The carina is the internal ridge at the bifurcation and is highly sensitive to cough. The right main bronchus is wider, shorter and more vertical than the left, so aspirated objects and a misplaced endotracheal tube usually go right, typically into the posterior segment of the right upper lobe or the superior segment of the right lower lobe when the patient is supine.

Each main bronchus divides into lobar (secondary) bronchi (three on the right, two on the left) and then segmental (tertiary) bronchi, each supplying a bronchopulmonary segment: a surgically resectable pyramidal unit with its own segmental bronchus and segmental artery centrally and intersegmental veins at its margins. There are about 10 segments on the right and 8 to 10 on the left. Branching continues to bronchioles (no cartilage, no glands), terminal bronchioles (end of the conducting zone), respiratory bronchioles, alveolar ducts and alveoli (the respiratory zone).

Lungs and pleura

The pleura is a serous membrane: the visceral pleura covers the lung surface and is insensitive to pain (autonomic supply), while the parietal pleura lines the thoracic wall (costal), the diaphragm (diaphragmatic), the mediastinum (mediastinal) and the apex (cervical pleura or cupula). The costal parietal pleura is supplied by intercostal nerves, so pleurisy produces sharp, localized chest wall pain; the mediastinal and central diaphragmatic pleura are supplied by the phrenic nerve, referring pain to the shoulder. The cupula rises about 2 to 3 cm above the medial third of the clavicle, so a needle or penetrating wound at the root of the neck can cause pneumothorax. The costodiaphragmatic recess is the lowest part of the pleural cavity and is where fluid collects first (blunting of the costophrenic angle on an upright film).

Surface projections

Use the "rule of twos" style pattern: the pleura reaches lower than the lung by about two rib spaces.

The oblique fissure runs from about the T3 spinous process posteriorly to the sixth costal cartilage anteriorly, roughly following the medial border of the abducted scapula. The horizontal fissure of the right lung follows the fourth costal cartilage from the sternum to meet the oblique fissure in the midaxillary line. As a result, the lower lobes are heard mostly from behind and the right middle lobe from the front. Because the pleura dips below the twelfth rib medially, a posterior approach to the kidney or a poorly placed needle at the thoracolumbar junction can enter the pleural cavity.

Diaphragm and the muscles of respiration

The diaphragm is the principal muscle of quiet inspiration. It arises from the xiphoid (sternal part), the inner surfaces of the lower six costal cartilages (costal part) and the lumbar vertebrae by two crura (right crus from L1 to L3, left from L1 to L2) and the medial and lateral arcuate ligaments, and inserts into its own central tendon. It is innervated by the phrenic nerve (C3, C4, C5), which "keeps the diaphragm alive", so a cord injury above C3 requires ventilatory support. The lower costal margins receive sensory fibers from the lower intercostal nerves.

Memory aid: "I 8 10 Eggs At 12" (IVC 8, esophagus 10, aorta 12). Quiet inspiration uses the diaphragm (descends, increasing vertical diameter) with the external intercostals and the interchondral part of the internal intercostals elevating the ribs (the upper ribs move in a "pump handle" fashion that increases anteroposterior diameter; the lower ribs move in a "bucket handle" fashion that increases transverse diameter). Forced inspiration recruits the scalenes (which fix and elevate ribs 1 and 2), sternocleidomastoid, pectoralis minor, serratus posterior superior and, with the arms fixed, pectoralis major and latissimus dorsi. Quiet expiration is passive, driven by elastic recoil of the lungs and chest wall. Forced expiration uses the abdominal muscles (rectus abdominis, external and internal obliques, transversus abdominis), the internal intercostals (interosseous part) and the quadratus lumborum, which anchors the twelfth rib.

Section 3The digestive system

Oral cavity to stomach

The parotid duct (Stensen) pierces the buccinator to open opposite the upper second molar; the submandibular duct (Wharton) opens on the sublingual papilla beside the frenulum. The anterior two thirds of the tongue receive general sensation from the lingual nerve (V3) and taste from the chorda tympani (VII); the posterior third receives both from the glossopharyngeal (IX). All tongue muscles are supplied by the hypoglossal nerve (XII) except palatoglossus (vagus).

The esophagus is about 25 cm long, from C6 to the cardia of the stomach at about T11. Its upper third is skeletal muscle, the middle third mixed and the lower third smooth muscle. It has three classic constrictions: at the cricopharyngeus (C6), where it is crossed by the aortic arch and left main bronchus (T4 to T5), and where it passes the diaphragm (T10). These are sites where swallowed objects lodge and strictures form.

The stomach has a cardia, fundus, body and pyloric part (antrum, canal and sphincter), with a lesser curvature facing up and right and a greater curvature down and left. The stomach lies mostly in the left upper quadrant and epigastrium and is related posteriorly to the stomach bed: the pancreas, left kidney and adrenal, spleen, splenic artery and transverse mesocolon, all behind the lesser sac.

Small and large intestine

The duodenum (about 25 cm) is C-shaped around the head of the pancreas and is mostly retroperitoneal. Its four parts are the superior (first, L1, the duodenal cap or bulb, intraperitoneal at its start and the usual site of peptic ulcers), descending (second, L1 to L3, receiving the bile and pancreatic ducts at the major duodenal papilla through the hepatopancreatic ampulla and sphincter of Oddi), horizontal (third, L3, crossed anteriorly by the superior mesenteric vessels) and ascending (fourth, L2 to L3, ending at the duodenojejunal flexure, suspended by the suspensory muscle or ligament of Treitz). The ligament of Treitz separates upper from lower GI bleeding.

The large intestine includes the cecum and appendix, ascending, transverse, descending and sigmoid colon, rectum and anal canal. The appendix arises from the posteromedial cecum where the three teniae converge; its base projects to McBurney point, one third of the way from the anterior superior iliac spine to the umbilicus. Early appendicitis pain is periumbilical (midgut visceral afferents to T10) and then shifts to the right lower quadrant once the parietal peritoneum is irritated. The rectum begins at S3, has no teniae and lies in the curve of the sacrum. The anal canal is divided by the pectinate line: above it, the epithelium is endodermal, supplied by the superior rectal artery, drained to the portal system and internal iliac nodes and innervated by painless autonomic fibers (internal hemorrhoids are painless); below it, the epithelium is ectodermal, supplied by the inferior rectal artery, drained systemically and to the superficial inguinal nodes and innervated by the somatic inferior rectal (pudendal) nerve (external hemorrhoids and fissures are painful).

The peritoneum

The parietal peritoneum lines the abdominal wall and is somatically innervated and precisely localized; the visceral peritoneum covers organs and refers poorly localized pain. Organs are either intraperitoneal (suspended by a mesentery) or retroperitoneal. The greater omentum hangs from the greater curvature of the stomach like an apron; the lesser omentum runs from the lesser curvature and duodenum to the liver and has two parts, the hepatogastric and hepatoduodenal ligaments. The free edge of the hepatoduodenal ligament contains the portal triad: the portal vein (posterior), the common bile duct (right anterior) and the proper hepatic artery (left anterior). Behind it lies the omental (epiploic) foramen of Winslow, the opening from the greater sac into the lesser sac (omental bursa) behind the stomach.

Liver, gallbladder, pancreas and spleen

The liver is the largest gland, in the right upper quadrant under the diaphragm, protected by ribs 7 to 11. Anatomically it has right, left, caudate and quadrate lobes, divided on its anterior surface by the falciform ligament (carrying the ligamentum teres, the remnant of the umbilical vein). Functionally, it is divided by the plane through the gallbladder fossa and IVC (Cantlie line) into right and left lobes and into eight Couinaud segments, each with its own portal triad branch. The bare area on its posterior surface touches the diaphragm without peritoneum, bounded by the coronary ligaments. The liver receives about 75% of its blood from the portal vein and 25% from the hepatic artery; it drains by three hepatic veins into the IVC.

The gallbladder lies in a fossa on the visceral surface of the liver; its fundus projects to the tip of the right ninth costal cartilage where the linea semilunaris meets the costal margin (the site of Murphy sign). Bile flows from the right and left hepatic ducts into the common hepatic duct, joins the cystic duct to form the common bile duct, which runs behind the first part of the duodenum and through the pancreatic head to join the main pancreatic duct at the ampulla. The cystic artery usually arises from the right hepatic artery within the hepatobiliary (Calot) triangle, bounded by the cystic duct, common hepatic duct and inferior liver surface.

The pancreas lies retroperitoneally across L1 to L2: the head in the duodenal C (with the uncinate process hooking behind the superior mesenteric vessels), then the neck, body and the tail, which reaches the splenic hilum in the splenorenal ligament (the only intraperitoneal part). The main pancreatic duct (Wirsung) joins the bile duct; the accessory duct (Santorini) opens at the minor papilla. Pancreatic pain is deep epigastric pain boring through to the back at T12 to L1, often eased by leaning forward.

The spleen sits in the left upper quadrant along ribs 9 to 11 in the midaxillary line, behind the stomach and above the left kidney and colic flexure. A normal spleen is not palpable. It is held by the gastrosplenic and splenorenal ligaments and is the most commonly injured abdominal organ in blunt trauma, especially with left lower rib fractures; referred pain to the left shoulder (Kehr sign) follows diaphragmatic irritation by blood.

Blood supply and the portal system

The marginal artery links the SMA and IMA along the colon.

The hepatic portal vein forms behind the neck of the pancreas at about L2 by the union of the superior mesenteric vein and the splenic vein (the inferior mesenteric vein usually joins the splenic vein). It carries nutrient-rich blood from the gut, spleen, pancreas and gallbladder to the liver sinusoids. When flow through the liver is obstructed (cirrhosis), portal hypertension diverts blood through portocaval anastomoses.

Section 4The urogenital system

Kidneys and ureters

The kidneys lie retroperitoneally on the posterior abdominal wall against the psoas major, quadratus lumborum and diaphragm, from about T12 to L3. The right kidney sits lower than the left because of the liver. The hilum faces medially at about L1 (the transpyloric plane) and contains, from anterior to posterior, the renal vein, renal artery and renal pelvis (VAP). Each kidney is wrapped in a fibrous capsule, then perirenal fat, then the renal (Gerota) fascia, then pararenal fat. Rib 12 crosses the posterior surface of both kidneys, and the costovertebral angle (between rib 12 and the erector spinae) is where renal tenderness is elicited by percussion (Murphy punch).

Internally, the outer cortex contains renal corpuscles and convoluted tubules; the inner medulla forms 8 to 18 renal pyramids whose apices (papillae) empty into minor calyces, which join to form two or three major calyces and the renal pelvis. Each renal artery divides into five segmental arteries that are end arteries (no anastomoses), then interlobar, arcuate, interlobular (cortical radiate) and afferent arterioles.

The ureter (about 25 cm) descends on the psoas major, crosses the pelvic brim anterior to the bifurcation of the common iliac artery, runs along the lateral pelvic wall and enters the bladder obliquely. It is crossed anteriorly by the gonadal vessels in the abdomen and, in the pelvis, passes under the uterine artery in females ("water under the bridge") and under the ductus deferens in males. Stones lodge at three constrictions: the ureteropelvic junction, the pelvic brim (crossing the iliac vessels) and the ureterovesical junction. Renal colic travels with T11 to L2 afferents and radiates from loin to groin, into the scrotum or labia and the medial thigh.

Bladder and urethra

The urinary bladder lies behind the pubic symphysis in the pelvis when empty and rises into the abdomen above the pubis when full, where it can be ruptured intraperitoneally by a blow. The trigone is a smooth triangle on the posterior wall between the two ureteric openings and the internal urethral orifice; it is mesodermal in origin and a common site of infection. The detrusor muscle is supplied by pelvic splanchnic nerves (S2 to S4, parasympathetic), which contract it for voiding; sympathetic fibers (T11 to L2) relax the detrusor and contract the internal sphincter, and the somatic pudendal nerve (S2 to S4) controls the voluntary external urethral sphincter. "S2, 3, 4 keeps the pee off the floor."

The female urethra is about 4 cm, opening in the vestibule anterior to the vagina, so ascending infection is common. The male urethra (about 20 cm) has preprostatic, prostatic (receives ejaculatory ducts and prostatic ducts), membranous (through the external sphincter, the narrowest and least distensible part, ruptured in pelvic fractures) and spongy (penile) parts, the last receiving the bulbourethral gland ducts.

Reproductive organs

The uterosacral ligaments run from the cervix to the front of the sacrum. Uterine, ovarian and cervical conditions such as endometriosis, dysmenorrhea and retroversion can therefore cause sacral and low back pain, which the chiropractor must distinguish from sacroiliac dysfunction by history (cyclic timing) and examination. The rectouterine pouch (of Douglas) is the lowest point of the female peritoneal cavity and collects blood or pus.

Pelvic floor and perineum

The pelvic diaphragm is formed by the levator ani (puborectalis, pubococcygeus and iliococcygeus) and the coccygeus (ischiococcygeus). It supports the pelvic viscera, resists increases in intra-abdominal pressure, and the puborectalis sling maintains the anorectal angle for fecal continence. It is supplied by the nerve to levator ani (S3, S4) and the inferior rectal branch of the pudendal nerve. The coccygeus overlies the sacrospinous ligament and attaches to the coccyx, so coccygeal manipulation and coccydynia involve this muscle.

The perineum is the diamond-shaped region below the pelvic diaphragm, bounded by the pubic symphysis, ischiopubic rami, ischial tuberosities, sacrotuberous ligaments and coccyx. A line between the ischial tuberosities divides it into the anterior urogenital triangle (external genitalia, perineal membrane, deep and superficial perineal pouches) and the posterior anal triangle (anal canal, external anal sphincter and the fat-filled ischioanal fossae). The perineal body is a fibromuscular node at the center where many muscles attach; tearing it in childbirth weakens pelvic floor support.

The pudendal nerve (S2 to S4) leaves the pelvis through the greater sciatic foramen below piriformis, hooks around the sacrospinous ligament near the ischial spine and re-enters through the lesser sciatic foramen to run in the pudendal (Alcock) canal on the obturator internus fascia. It supplies the external anal and urethral sphincters and the skin of the perineum.

Section 5The endocrine glands

Endocrine glands are ductless and richly vascular; their anatomy matters clinically for surgery, for the mass effects of enlargement and for the structures they lie against.

Pituitary and hypothalamus

The anterior pituitary (adenohypophysis) develops from Rathke pouch, an upgrowth of oral ectoderm, and secretes GH, prolactin, TSH, ACTH, FSH and LH under control of hypothalamic releasing and inhibiting hormones delivered by the portal system. The posterior pituitary (neurohypophysis) is a downgrowth of the diencephalon; it stores and releases ADH and oxytocin made in the supraoptic and paraventricular nuclei and carried down the hypothalamohypophyseal tract in the infundibulum. A pituitary adenoma growing upward compresses the decussating nasal retinal fibers in the optic chiasm, producing bitemporal hemianopia; lateral growth into the cavernous sinus affects the ocular motor nerves.

Thyroid and parathyroids

The thyroid develops from the floor of the pharynx at the foramen cecum of the tongue and descends along the thyroglossal duct; remnants form a midline neck cyst that moves upward when the tongue protrudes, and a pyramidal lobe may persist. Follicular cells make T3 and T4; parafollicular (C) cells, from the ultimobranchial body, make calcitonin. The relationship of the thyroid arteries to the laryngeal nerves is the key surgical fact: the superior thyroid artery is ligated close to the gland to spare the external laryngeal nerve (cricothyroid, voice pitch), and the inferior thyroid artery is ligated away from the gland to spare the recurrent laryngeal nerve. Inadvertent removal of the parathyroids during thyroidectomy causes hypocalcemia with tetany (Chvostek and Trousseau signs).

Adrenal glands

Each adrenal has two embryologically distinct parts. The cortex is mesodermal and has three zones: glomerulosa (aldosterone), fasciculata (cortisol) and reticularis (androgens), remembered as "salt, sugar, sex: the deeper you go, the sweeter it gets." The medulla is derived from neural crest; its chromaffin cells are modified postganglionic sympathetic neurons that receive preganglionic fibers directly from the thoracic splanchnic nerves (about T10 to L1) and secrete epinephrine and norepinephrine. A pheochromocytoma produces episodic headache, sweating, palpitations and hypertension.

Islets, gonads, pineal and thymus

The islets of Langerhans make up about 1 to 2% of pancreatic mass: alpha cells (glucagon) at the periphery, beta cells (insulin), the most numerous, in the core, and delta cells (somatostatin). Islet venous blood drains into the portal system, so the liver sees the highest concentration of insulin. The pineal gland secretes melatonin in darkness, regulated by a retinohypothalamic pathway through the suprachiasmatic nucleus and the superior cervical sympathetic ganglion; its calcification makes it a useful midline marker on skull films and CT. The thymus develops from the third pharyngeal pouch, educates T lymphocytes and secretes thymic hormones; thymoma is associated with myasthenia gravis.

Words worth knowing