Peritoneum & Endocrine Glands

The peritoneum is a single continuous serous sheet that lines the entire abdomen — think of it as cling film draped over and around every organ, with the visceral layer kissing organ surfaces and the parietal layer hugging the walls. The critical distinction is that parietal peritoneum has somatic innervation (sharp, localisable pain), while visceral peritoneum has only autonomic supply (dull, midline, colicky pain) — which is why appendicitis starts as central periumbilical discomfort before somatic peritoneal irritation produces the classic right-iliac-fossa localisation. Organs classified as retroperitoneal sit behind this membrane; they were either always posterior (kidneys, aorta) or started intraperitoneal and fused to the posterior wall during gut rotation — a developmental trick that stabilises structures like the duodenum and ascending colon.

9.1.2 — Gut Development, Rotation & Peritoneal Fixation

Fig. 4.13 — Development of the gut and mesenteries (A–H): rotation of the midgut and fixation of parts of the dorsal mesentery explain which organs become retroperitoneal.

Gray's Anatomy for Students, 4e

Understanding embryological gut rotation explains the adult layout of the peritoneum, mesenteries, and retroperitoneal organs.

Stage	Weeks	Events
Herniation	6–10	Midgut elongates rapidly → herniates through umbilical ring (physiological umbilical hernia); rotates 270° anticlockwise around the superior mesenteric artery (SMA) axis as it returns
First 90° rotation	6–8 (outside)	Pre-arterial (small bowel) limb rotates to the left; post-arterial (large bowel) limb rotates to the right
Return + next 180°	10–11	Small bowel re-enters abdomen first (displaced to right then final left position); caecum starts high, descends to right iliac fossa; total = 270° anticlockwise rotation
Peritoneal fixation	After return	Ascending colon + descending colon fuse to posterior abdominal wall → secondarily retroperitoneal; transverse colon + sigmoid remain intraperitoneal; mesentery of small bowel = 15 cm diagonal root (duodenojejunal flexure L2 left → right iliac fossa)

⚠ Clinical — Malrotation & Midgut Volvulus

Malrotation: failure of complete 270° rotation → duodenojejunal flexure stays to the right of midline (should be left); caecum stays subhepatic rather than descending to right iliac fossa. Ladd's bands: fibrous peritoneal bands from abnormally positioned caecum → cross and compress duodenum → neonatal/infant bilious vomiting. Upper GI contrast = diagnostic (DJ flexure position). Midgut volvulus: the unrotated midgut has a narrow mesenteric pedicle around the SMA → prone to axial twist → catastrophic ischaemia of almost all small bowel (SMA occlusion). Presents neonates as bilious vomiting + abdominal distension → emergency Ladd's procedure (detorsion, division of Ladd's bands, appendicectomy, caecum placed left, duodenum right).

Fig. 4.55 — Transverse section showing the continuity between the greater and lesser sacs through the omental (epiploic) foramen of Winslow.

Gray's Anatomy for Students, 4e

The peritoneal cavity is not one uniform space — it has a hidden compartment called the lesser sac (omental bursa), tucked behind the stomach and connected to the greater sac through only one narrow door: the epiploic foramen of Winslow. Knowing the four boundaries of this foramen is surgically vital, because the hepatoduodenal ligament — carrying the portal triad of portal vein, hepatic artery, and bile duct — forms its anterior wall, and squeezing it between finger and thumb (the Pringle manoeuvre) instantly controls haemorrhage from the liver. The greater omentum, a four-layered fat-rich apron hanging from the stomach, earns its nickname "abdominal policeman" by migrating toward inflamed or perforated viscera to wall off infection and prevent generalised peritonitis.

Gravity and posture determine which peritoneal pouch fills first with blood or pus, making knowledge of these spaces directly actionable in resuscitation. In the supine patient, Morison's pouch — the hepatorenal recess between the right liver lobe and right kidney — is the most dependent point of the entire peritoneal cavity, so the FAST ultrasound exam targets it first in abdominal trauma. When the patient sits or stands, the rectouterine pouch of Douglas becomes the lowest point in females (accessible by needle culdocentesis for a ruptured ectopic), while the rectovesical pouch serves the same role in males, and the subphrenic spaces communicate diaphragmatic irritation as referred right-shoulder-tip pain via the C3–C5 phrenic nerve.

The adrenal glands sit atop both kidneys but are anatomically and functionally distinct from them — the right is pyramidal and drains via an extremely short vein directly into the IVC (a hazard in right adrenalectomy), while the left is crescent-shaped and drains more forgivingly into the left renal vein. Each gland is really two organs fused together: the outer cortex (mesodermal, zoned GFR — producing aldosterone, cortisol, and androgens) and the inner medulla (neural crest–derived chromaffin cells releasing adrenaline and noradrenaline as the body's emergency catecholamine reservoir). When the cortex fails, as in Addison's disease, the absence of cortisol feedback drives ACTH sky-high, and because ACTH shares sequence with melanocyte-stimulating hormone (both cleaved from POMC), the hallmark sign is hyperpigmentation — the patient's skin and mucous membranes darkening even as they collapse from hypotension and hypoglycaemia.

9.4.1 — Anatomy & Zones

Fig. 4.163 — Suprarenal (adrenal) glands on the superomedial aspect of each kidney, with their triple arterial supply (superior, middle, inferior suprarenal arteries).

Gray's Anatomy for Students, 4e

Feature	Right	Left
Shape	Pyramidal	Crescent/semilunar
Position	Anteromedial to right kidney; posterior to IVC	Anteromedial to left kidney; related to pancreas + splenic vessels
Vein drainage	Short right suprarenal vein → directly into IVC (IMPORTANT — very short!)	Left suprarenal vein → left renal vein → IVC (longer)
Arterial supply	Superior (inferior phrenic a.), middle (abdominal aorta at T12), inferior (renal artery) — all three on each side

Zone (outer → inner)	Mnemonic layer	Hormone
Zona glomerulosa (outer)	GFR (like kidney): Glomerulosa → Fasciculata → Reticularis	Aldosterone (mineralocorticoid) — regulated by RAAS + K⁺
Zona fasciculata (middle)		Cortisol (glucocorticoid) — regulated by ACTH
Zona reticularis (inner)		Androgens (DHEA, androstenedione) — regulated by ACTH
Medulla	Chromaffin cells	Adrenaline (80%) + noradrenaline — sympathetic preganglionic → chromaffin cells (modified postganglionic neurones)

Fig. 8.178 — Thyroid gland in the anterior triangle of the neck. A. Anterior view (lobes + isthmus + pyramidal lobe). B. Transverse view. C–E. Ultrasound and nuclear medicine scans.

Gray's Anatomy for Students, 4e

The thyroid gland wraps around the trachea at C5–T1, anchored to the cricoid by Berry's ligament — which is why it bobs up on swallowing, a clinical sign that differentiates a thyroid mass from every other neck lump. Two nerves run in dangerous proximity during thyroid surgery: the recurrent laryngeal nerve (RLN), which passes posterior to Berry's ligament and supplies all intrinsic laryngeal muscles except cricothyroid, and the external branch of the superior laryngeal nerve (EBSLN), the "singer's nerve" that tunes pitch via cricothyroid — injuring either causes hoarseness or loss of high notes respectively. The gland originates embryologically from the foramen caecum at the tongue base, descends through the thyroglossal duct, and leaves C cells (from neural crest via the ultimobranchial body) scattered among its follicles — these parafollicular cells make calcitonin and are the cell of origin for medullary thyroid carcinoma.

Fig. 8.180 — Superior and inferior thyroid arteries, the recurrent laryngeal nerves and the four parathyroid glands on the posterior surface of the thyroid.

Gray's Anatomy for Students, 4e

Most people have four parathyroid glands, but their embryological origins explain why they are so hard to find: the superior pair descend from the 4th pharyngeal pouch and settle reliably behind the upper thyroid poles, while the inferior pair migrate with the thymus from the 3rd pouch and may end up anywhere from the carotid bifurcation to the anterior mediastinum. All four glands usually share their blood supply from the inferior thyroid artery — which means that preserving this vessel close to the thyroid capsule during thyroidectomy is the single most important manoeuvre for avoiding post-operative hypocalcaemia. PTH from chief cells is the master regulator of serum calcium: it mobilises bone, enhances renal calcium reabsorption, and activates vitamin D to promote gut absorption — three levers that primary hyperparathyroidism pulls simultaneously, producing the classic "Bones, Stones, Groans, and Moans" syndrome.

The pituitary sits in the bony sella turcica of the sphenoid bone, directly beneath the optic chiasm, which means that an expanding macroadenoma compresses the crossing nasal fibres and destroys temporal vision in both eyes — bitemporal hemianopia — before any other symptom develops. The anterior lobe (adenohypophysis) is an island of gut-derived epithelium that receives no direct nerve supply but is controlled entirely through the hypothalamo-hypophyseal portal system, while the posterior lobe (neurohypophysis) is literally a downward extension of the hypothalamus, storing and releasing ADH and oxytocin made in supraoptic and paraventricular nuclei. Prolactin is unique among anterior pituitary hormones in that it is under tonic dopamine inhibition — meaning any lesion that compresses the pituitary stalk disconnects the inhibition and paradoxically raises prolactin, a "disconnection hyperprolactinaemia" that mimics a prolactinoma.

The thymus is the primary lymphoid organ that programs T-cell identity: in its cortex, T cells are tested for MHC recognition (positive selection), then shuffled to the medulla where those that attack self-antigens are deleted (negative selection), with Hassall's corpuscles — whorls of keratinised epithelium — marking the medulla uniquely on histology. Because the thymus shares its 3rd pharyngeal pouch origin with the inferior parathyroid glands, a failure of that pouch to develop (DiGeorge syndrome) simultaneously abolishes T-cell immunity, wipes out the inferior parathyroids causing hypocalcaemia, and disrupts pharyngeal arch heart development — a trio of deficits linked by one embryological event. The pineal gland, tucked behind the superior colliculi in the epithalamus, translates light/dark cycles into melatonin pulses; its clinical relevance is that a germinoma compressing the colliculi produces Parinaud's syndrome (paralysis of upgaze), and its post-pubertal calcification on skull X-ray serves as a midline reference — a shift suggests a space-occupying lesion.