Mesenteries and Omenta: How the Gut Hangs and Still Gets Fed
Seven metres of intestine have to be stored inside a space no bigger than a washbasin. They must not tangle. They must slide freely against one another through every meal, every breath, every turn in bed. And at every single point along their length they must receive an artery, a vein, a lymphatic and a nerve — because a loop of bowel starved of blood for six hours is a dead loop of bowel. Four impossible requirements, one elegant answer: hang the gut from the back wall on double sheets of peritoneum, and run the plumbing inside the sheets. That is a mesentery, and once you see it as a suspension bridge that also carries the cables, the entire abdomen stops being a bag of organs and becomes a designed space.
A surgeon opens an abdomen for what the scan called a bowel obstruction and finds, first, nothing dramatic at all — just distended loops of small intestine, shining and pink. She runs the bowel through her fingers, hand over hand, from the duodenojejunal flexure downwards, and eighty centimetres from the end she finds it: a single white thread of scar, no thicker than a shoelace, left behind by an appendicectomy performed when the patient was nineteen. A loop of ileum has fallen over it like a rope over a peg and kinked. She divides the band with scissors — the whole operation takes four seconds — and the bowel beyond it fills and softens in front of her. Then she keeps looking, because the real question is never the band. She lifts the small bowel to the right and studies the fan of tissue holding it: its blood vessels, its fat, its narrow root running obliquely across the back wall. Everything the intestine will ever need arrives through that fan. Cut it in the wrong place and no clever repair afterwards can save the bowel it was feeding.
What a mesentery actually is
Two sheets of peritoneum, back to back, with a neurovascular bundle sandwiched between them. A MESENTERY is a double layer of visceral peritoneum that connects an organ to the posterior abdominal wall and carries that organ's vessels, nerves and lymphatics between its two leaves. The definition matters because it explains everything else. Since it is a fold of the same continuous membrane described in the peritoneum, both of its outer surfaces are slippery serosa, which is why a loop of bowel on a mesentery glides against its neighbours instead of gripping them. Since the vessels run BETWEEN the leaves, they are protected, suspended in fat, and reachable by a surgeon who only has to open one thin layer to find them. And since the mesentery is attached along a line — its ROOT — rather than over a broad area, every organ that has one is mobile: it can be lifted out of the wound, rotated, examined and returned. An organ with a mesentery is INTRAPERITONEAL; an organ plastered to the back wall with peritoneum on its front surface only is RETROPERITONEAL. That single distinction decides how an organ moves, how it hurts, how it spreads infection, and how it is approached at operation. Between the leaves of a mesentery you will find, invariably: the artery and its arcades, the accompanying veins draining to the portal system, lymphatic vessels running to nodes that lie along the artery, autonomic nerve fibres travelling on the arterial adventitia, and a variable amount of fat that anchors and cushions the whole assembly.
The mesentery proper: fifteen centimetres carrying six metres
The most extraordinary ratio in the abdomen. When anatomists say "THE mesentery" without qualification they mean the mesentery of the small intestine. Its free border is enormous — some six metres, gathered into pleats and frills so that the whole length of jejunum and ileum described in the small intestine can be attached to it. Its ROOT, by contrast, is astonishingly short: about fifteen centimetres. That root begins at the duodenojejunal flexure, just to the LEFT of the body of the second lumbar vertebra, and runs obliquely downwards and to the right, crossing in turn the third part of the duodenum, the abdominal aorta, the inferior vena cava, the right ureter and the right gonadal vessels, to end at the RIGHT SACROILIAC JOINT. Between its leaves lie the superior mesenteric artery and vein, the jejunal and ileal branches with their characteristic arcades — few, large arcades and long vasa recta in the jejunum; many, short arcades and short vasa recta in the ileum — together with roughly a hundred and fifty lymph nodes in three tiers and the nerve fibres of the superior mesenteric plexus. The mesentery is thin and translucent near the duodenum and becomes progressively fatter towards the ileum, where the fat creeps around the bowel wall as "fat encroachment", a sign the surgeon reads as inflammation. And the ratio itself — six metres of gut on a fifteen-centimetre stalk — is precisely why the whole small intestine can swing and twist about that narrow axis. This is VOLVULUS: the bowel rotates about its own mesenteric root, kinks at both ends, and strangles the artery that keeps it alive.
Think of a folding hand fan. The pleated paper is vast when you open it, yet the whole thing pivots on a single rivet at the base, and every rib of the fan radiates from that one point. The mesentery is a fan whose rivet is the origin of the superior mesenteric artery, whose ribs are the jejunal and ileal branches, and whose pleated edge is six metres of bowel. Two consequences follow immediately, and both are clinical. First, damage at the rivet is catastrophic in a way that damage at the edge never is: an embolus lodged in the superior mesenteric artery kills the entire midgut at once, whereas a single divided vasa recta at the free border kills nothing, because the arcades find a way around. Second, a fan on a rivet can SPIN. Grip six metres of loaded bowel and turn it once about a fifteen-centimetre root, and you have twisted the rivet shut.
The other named mesenteries
The TRANSVERSE MESOCOLON suspends the transverse colon from a root that runs horizontally across the anterior surface of the head and body of the pancreas. Between its leaves run the middle colic vessels, and its position makes it the great internal partition of the abdomen: everything above it — liver, stomach, spleen — is the SUPRACOLIC compartment, and everything below it — small bowel, ascending and descending colon — is the INFRACOLIC compartment, itself divided into right and left by the oblique root of the mesentery proper. Pus and blood respect these boundaries, which is why a subphrenic abscess collects where it does. The SIGMOID MESOCOLON has the most distinctive root of all: an inverted V, whose apex sits over the bifurcation of the left common iliac artery near the point where the left ureter crosses the pelvic brim, with one limb running medially along the brim and the other descending into the pelvis. It carries the sigmoid and superior rectal vessels, and in the elderly — especially with a chronically loaded, elongated sigmoid — that long loop on a narrow V-shaped root twists on itself: SIGMOID VOLVULUS, the classic massively distended "coffee-bean" loop on the abdominal film. Finally the MESOAPPENDIX, a small triangular fold passing behind the terminal ileum from the lower end of the mesentery to the appendix, carrying the appendicular artery — an END ARTERY, which is exactly why an inflamed appendix becomes gangrenous and perforates rather than simply settling down.
The lesser omentum and the doorway it guards
A small apron with the most important free edge in the abdomen. An OMENTUM is a peritoneal fold passing from the stomach to another organ. The LESSER OMENTUM runs from the lesser curvature of the stomach and the first two centimetres of the duodenum upwards to the liver, where it attaches along the fissure for the ligamentum venosum and the porta hepatis. It has two named parts: the broad HEPATOGASTRIC LIGAMENT, thin and often translucent, through which an accessory left hepatic artery may run; and the thicker HEPATODUODENAL LIGAMENT on the right, which is nothing less than the pedicle of the liver. Its FREE RIGHT EDGE contains the PORTAL TRIAD, and the arrangement is worth committing to memory exactly: the BILE DUCT lies anteriorly on the RIGHT, the HEPATIC ARTERY PROPER anteriorly on the LEFT, and the PORTAL VEIN BEHIND them both — the vein of the portal system carrying three quarters of the liver's blood supply. That free edge forms the ANTERIOR boundary of the EPIPLOIC FORAMEN (the foramen of Winslow), the single natural doorway between the greater sac and the lesser sac. Behind the foramen lies the inferior vena cava, above it the caudate lobe of the liver, and below it the first part of the duodenum. A finger placed through that foramen and a thumb in front of the free edge can compress the hepatic artery and portal vein together — the Pringle manoeuvre — and stop torrential bleeding from a torn liver in a single second.
The free edge of the lesser omentum is the only place in the human body where you can lay a finger BEHIND the great vessels of an organ and a thumb in FRONT of them without cutting anything. That is an accident of embryology — the ventral mesentery of the foregut simply stopped there, leaving a free border and, behind it, a gap — and generations of surgeons have been grateful for it. It is also why the bile duct, the hepatic artery and the portal vein are injured together or not at all: they travel as a single bundle, wrapped in one fibrous sheath, for the last few centimetres of their journey into the liver. A clamp placed carelessly on that edge to control bleeding has, more than once in the history of surgery, also occluded the duct — and the patient who leaves theatre alive returns weeks later, yellow.
The greater omentum: the abdominal policeman
A fatty apron that moves towards trouble. The GREATER OMENTUM hangs from the greater curvature of the stomach like an apron, descends in front of the transverse colon and the coils of small intestine — sometimes as far as the pelvis — then folds back on itself and ascends to attach to the transverse colon and its mesocolon. Because it descends and returns, it is FOUR layers of peritoneum thick, and the potential space between the descending and ascending pairs is the inferior recess of the lesser sac, obliterated in the adult. Between its anterior two layers run the right and left gastro-omental (gastroepiploic) arteries, which anastomose along the greater curvature and give the omentum a generous blood supply of its own, along with a rich lymphatic network and clusters of immune cells called milky spots. Those features make it more than fat. When any abdominal organ becomes inflamed, the omentum drifts towards it — carried by peristalsis, gravity and the movement of the abdominal wall — adheres to it, and wraps it. It walls off a perforated ulcer, plugs a leaking appendix, seals a small anastomotic leak, and confines pus into an abscess instead of letting it become generalised peritonitis. This is why it earned its nickname, coined by the surgeon Rutherford Morison: the abdominal policeman. The same stickiness has a price. The omentum is the commonest site of postoperative ADHESIONS; it is where tuberculosis and, far more often, metastatic carcinoma of the ovary, stomach or colon settle to form the thickened, nodular sheet called OMENTAL CAKING on a CT scan; and it is often the first structure to appear in the neck of an incisional or umbilical hernia.
The plug: a man with a perforated duodenal ulcer arrives with a board-rigid abdomen and free gas under the diaphragm. At operation the surgeon does not attempt to close the hole with sutures alone; she draws up a tongue of greater omentum and ties it over the defect — a Graham patch. Living, vascularised tissue seals what stitches in oedematous bowel would cut through. The wall: a woman with a five-day history of appendicitis has no free pus at all, but a firm mass in the right iliac fossa. The omentum and adjacent loops have built a wall around the diseased appendix — an appendix mass — and she is treated with antibiotics rather than an urgent operation, because her own peritoneum has already done the surgery. The warning: an elderly woman with vague bloating has a CT reported as "omental caking with ascites". No one has yet found a primary tumour, but the radiologist and the gynaecologist both know where to look, because the omentum collects malignant cells from the peritoneal fluid the way a filter collects silt.
The peritoneal ligaments
A peritoneal "ligament" is not a ligament at all — it is a fold with a job and, usually, a vessel inside it. The FALCIFORM LIGAMENT is a sickle-shaped double fold running from the umbilicus up the anterior abdominal wall and onto the diaphragm, attaching to the anterior and superior surfaces of the liver and marking the surface boundary between its right and left anatomical lobes. In its FREE INFERIOR EDGE lies the LIGAMENTUM TERES — the obliterated left umbilical vein of the fetus, which carried oxygenated placental blood. It is never entirely obliterated: small paraumbilical veins persist alongside it, connecting the left branch of the portal vein to the superficial veins of the anterior abdominal wall. In portal hypertension those veins reopen and engorge, producing the radiating cluster of dilated periumbilical veins called CAPUT MEDUSAE — one of the four classic portosystemic anastomoses, the others being oesophageal, rectal and retroperitoneal. Superiorly, the two layers of the falciform ligament separate and sweep apart to become the anterior layer of the CORONARY LIGAMENT, and the right and left ends of the coronary ligament close as the RIGHT and LEFT TRIANGULAR LIGAMENTS. Between the widely separated layers of the coronary ligament the liver has no peritoneal covering at all: this is the BARE AREA, in direct contact with the diaphragm, and it is here that the inferior vena cava lies in its groove. A bare area is a route: infection and tumour can spread from liver to diaphragm across it without ever crossing the peritoneal cavity.
The remaining ligaments are named for the two structures they join, and each one is a warning label for the vessel it hides. The GASTROSPLENIC LIGAMENT runs from the greater curvature of the stomach to the hilum of the spleen and carries the SHORT GASTRIC and LEFT GASTRO-OMENTAL vessels — which is why an over-enthusiastic pull on the stomach during upper abdominal surgery tears the splenic capsule. The SPLENORENAL (lienorenal) LIGAMENT runs from the hilum of the spleen to the left kidney and the posterior abdominal wall, and contains the SPLENIC ARTERY and VEIN and the TAIL OF THE PANCREAS — the reason a splenectomy risks a pancreatic fistula, as covered in the spleen. These two ligaments together form the left-hand boundary of the lesser sac. The GASTROPHRENIC LIGAMENT passes from the upper greater curvature to the undersurface of the diaphragm, and the PHRENICOCOLIC LIGAMENT runs from the splenic flexure of the colon to the diaphragm, forming a shelf that literally supports the lower pole of the spleen and closes the upper end of the left paracolic gutter — which is exactly why fluid from a perforated appendix tracks freely up the RIGHT gutter to the subphrenic space but is halted on the left.
The embryological logic, in plain words
Every oddity in this chapter is the fossil of a movement that happened before you were eight weeks old. The gut tube begins as a straight midline pipe slung between two mesenteries: a DORSAL mesentery running its whole length, and a VENTRAL mesentery that exists only from the lower oesophagus to the upper duodenum — the part of the foregut described in foregut, midgut and hindgut. That short ventral mesentery is derived from the septum transversum, and the liver bud grows forward into it, splitting it in two: the part between liver and anterior abdominal wall becomes the FALCIFORM LIGAMENT, and the part between liver and stomach becomes the LESSER OMENTUM. This is the whole answer to a question students rarely think to ask — why does the liver have a ligament running to the front of the body while no other abdominal organ does? Because only the foregut ever had a ventral mesentery, and the liver grew into it. Meanwhile the stomach rotates about ninety degrees clockwise around its long axis, dragging its dorsal mesogastrium to the LEFT and behind itself; the space that opens behind the stomach as a result is the LESSER SAC (omental bursa), and the residual passage on the right is the epiploic foramen. The same dorsal mesogastrium grows downwards as the double flap that folds back on itself to become the four-layered greater omentum, and its posterior wall carries the developing spleen out to the left, which is why the spleen ends up slung between two ligaments made from a single mesentery.
The final act is FUSION. As the midgut herniates into the umbilical cord, rotates a total of two hundred and seventy degrees anticlockwise around the axis of the superior mesenteric artery and returns to the abdomen, several stretches of gut come to lie against the posterior abdominal wall. Where a mesentery is pressed against that wall it fuses with it: the two apposed serosal layers break down and are replaced by a thin plane of areolar tissue — the fusion fascia, named after Toldt. The organs this happens to lose their mesentery and become SECONDARILY RETROPERITONEAL: the second, third and fourth parts of the DUODENUM, the head and body of the PANCREAS described in the duodenum and pancreas, and the ASCENDING and DESCENDING COLON. They were never truly behind the peritoneum — they were pushed there. And because the fusion plane is a real, avascular tissue layer rather than a scar, a surgeon can find it again and open it. That is what the KOCHER MANOEUVRE does: incise the peritoneum just lateral to the second part of the duodenum and sweep medially, and the duodenum and pancreatic head lift off the inferior vena cava exactly as they lay in the embryo, bloodlessly. Extend the same plane along the whole right colon and small bowel mesentery and you have the CATTELL–BRAASCH manoeuvre, which delivers the entire right side of the abdomen into view. The embryo left a set of doors, and surgery simply learned where they are.
Imagine wallpapering a room, then pressing a wide sheet of the same paper flat against a wall while the paste is still wet. Weeks later the sheet looks like part of the wall — but it is not. Slide a blade under one edge and you can peel it away cleanly along the seam of dried paste, without tearing the plaster and without cutting a single wire behind it. That seam is the fusion fascia, the wall is the posterior abdominal wall, and the pasted sheet is the mesentery of the duodenum or of the ascending colon. It is also why a surgeon can mobilise the duodenum without dividing any peritoneum that matters and without meeting a bleeding vessel: she is not cutting new ground, she is un-sticking an old join.
Adhesions: a woman who had a caesarean section eleven years ago presents with colicky central abdominal pain, vomiting and absolute constipation. She has adhesional small bowel obstruction — worldwide the commonest cause of small bowel obstruction after previous surgery — because peritoneum that was cut healed by sticking to whatever it was touching. Internal herniation: a young man some months after gastric bypass surgery develops intermittent severe pain; a loop of small bowel has slipped through a defect deliberately created in his mesentery at operation and become trapped, an emergency that a normal blood test will not reveal. Midgut volvulus: an infant with malrotation vomits bile on the third day of life; because rotation was incomplete the mesenteric root never broadened, and the whole midgut has twisted about the superior mesenteric artery — bilious vomiting in a neonate is a surgical emergency until proven otherwise. Mesenteric ischaemia: an eighty-year-old in atrial fibrillation has agonising abdominal pain with a soft, almost normal abdomen — the classic mismatch of pain out of proportion to the signs — because an embolus has occluded the artery at the rivet of the fan, and the bowel is dying before it is tender.
- A MESENTERY is a double layer of peritoneum connecting an organ to the body wall and carrying its artery, veins, lymphatics and autonomic nerves between its two leaves. An organ with a mesentery is intraperitoneal and mobile; one plastered to the back wall is retroperitoneal and fixed.
- The MESENTERY PROPER has a root only ~15 cm long running obliquely from the duodenojejunal flexure (left of L2) to the right sacroiliac joint, crossing the third part of the duodenum, aorta, IVC, right ureter and right gonadal vessels — yet it fans out to carry six metres of jejunum and ileum.
- That short root is why the small bowel can twist about it (VOLVULUS), and why an embolus at the origin of the superior mesenteric artery kills the whole midgut at once.
- TRANSVERSE MESOCOLON: root across the front of the pancreas, carries the middle colic vessels, and divides the abdomen into supracolic and infracolic compartments. SIGMOID MESOCOLON: inverted-V root over the left common iliac bifurcation — the site of sigmoid volvulus in the elderly. MESOAPPENDIX: carries the appendicular END artery.
- LESSER OMENTUM: from the lesser curvature and first part of the duodenum to the liver, in two parts — hepatogastric and hepatoduodenal ligaments.
- Its FREE RIGHT EDGE holds the PORTAL TRIAD — bile duct anterior right, hepatic artery anterior left, portal vein behind — and forms the ANTERIOR boundary of the epiploic foramen (IVC behind, caudate lobe above, first part of duodenum below).
- GREATER OMENTUM: a FOUR-layered fatty apron from the greater curvature, carrying the gastro-omental vessels; it migrates to inflamed organs, wraps and walls them off — "the abdominal policeman" — and is the commonest site of adhesions and of metastatic "omental caking".
- FALCIFORM LIGAMENT: carries the LIGAMENTUM TERES (the obliterated left umbilical vein) in its free edge, alongside paraumbilical veins — the route of the paraumbilical portosystemic anastomosis and of caput medusae.
- CORONARY and TRIANGULAR ligaments attach the liver to the diaphragm; between the widely separated layers of the coronary ligament lies the BARE AREA, where the liver touches the diaphragm directly and the IVC sits in its groove.
- GASTROSPLENIC ligament carries the short gastric and left gastro-omental vessels; SPLENORENAL carries the splenic vessels and the tail of the pancreas; the PHRENICOCOLIC ligament supports the spleen and closes the top of the LEFT paracolic gutter.
- EMBRYOLOGY: a dorsal mesentery along the whole gut, a ventral mesentery only around the lower oesophagus, stomach and upper duodenum — split by the growing liver into the falciform ligament and the lesser omentum. Stomach rotation creates the lesser sac; the epiploic foramen is what is left of the way in.
- FUSION makes organs SECONDARILY RETROPERITONEAL — duodenum (parts 2–4), pancreas, ascending and descending colon — and the fusion plane can be reopened surgically: the Kocher and Cattell–Braasch manoeuvres.
- Calling every peritoneal fold a "mesentery". A mesentery attaches an organ to the BODY WALL and carries its neurovascular bundle; a ligament or omentum connects two ORGANS (or an organ to the wall) and may or may not carry vessels. The greater omentum is an omentum, not a mesentery.
- Placing the portal vein in front in the free edge of the lesser omentum. The vein is the POSTERIOR structure; in front of it the bile duct lies to the RIGHT and the hepatic artery to the LEFT. Getting this backwards is how ducts are clamped instead of arteries.
- Believing that "retroperitoneal" means an organ was always there. The duodenum, pancreas and ascending and descending colon began with mesenteries and were pressed backwards — they are SECONDARILY retroperitoneal, and the fused plane can be reopened, which is the whole basis of the Kocher manoeuvre.
During an emergency laparotomy for torrential haemorrhage from a lacerated liver, the surgeon passes an index finger through a natural opening behind a free peritoneal edge and compresses that edge between finger and thumb. Bleeding slows dramatically. Which three structures has she just compressed, and what is the opening called?
- A mesentery is a double layer of peritoneum joining an organ to the body wall with the organ's artery, veins, lymphatics and nerves running between its leaves — which is why an organ with a mesentery is mobile, slippery and fed from one narrow line of attachment.
- The mesentery proper hangs six metres of jejunum and ileum from a root barely 15 cm long (duodenojejunal flexure to right sacroiliac joint); the transverse and sigmoid mesocolons and the mesoappendix complete the set, and each narrow root is a potential axis of volvulus.
- The lesser omentum runs stomach-and-duodenum to liver in two parts, and its free right edge carries the portal triad (duct right, artery left, vein behind) as the front wall of the epiploic foramen; the greater omentum is a four-layered apron that migrates to inflammation, walls it off, and later causes adhesions and collects metastases.
- The named ligaments — falciform with the ligamentum teres, coronary and triangular with the bare area between them, gastrosplenic, splenorenal, gastrophrenic and phrenicocolic — are all remnants of the dorsal and ventral mesenteries; where those mesenteries fused to the back wall the duodenum, pancreas and ascending and descending colon became secondarily retroperitoneal, and that fusion plane is what surgery reopens in the Kocher and Cattell–Braasch manoeuvres.
- Drake RL, Vogl AW, Mitchell AWM. Gray's Anatomy for Students — Abdomen: peritoneum, mesenteries and omenta.
- Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy — The peritoneal cavity: omenta, mesenteries and peritoneal ligaments; omental bursa and epiploic foramen.
- Netter FH. Atlas of Human Anatomy — Greater and lesser omentum; mesenteric relations of the intestines.
- Last RJ. Last's Anatomy: Regional and Applied — The peritoneum and the development of the gut mesenteries.
- Snell RS. Clinical Anatomy by Regions — Volvulus, internal herniation and the surgical anatomy of the omentum.
- TeachMeAnatomy — The Peritoneum; The Greater and Lesser Omentum; Mesentery of the Small Intestine.
