The Acute Abdomen: Reading Pain as Anatomy
There is no window into the abdomen. The chest gives you breath sounds and a heartbeat; the limbs let you see the swelling and feel the pulse; the skin shows you everything at a glance. The abdomen shows you almost nothing — a smooth wall of muscle stretched over nine organs, forty feet of bowel, three great vessels and a peritoneal cavity that can hold three litres of blood without changing shape. So the abdomen must be interrogated rather than inspected, and every question you ask it is an anatomical question. Where does it hurt, and where did it hurt first? Is the pain dull and central or sharp and fixed? Does the patient writhe, or lie perfectly still? Each answer narrows the field not by guesswork but by anatomy — by which nerve carried the message, which segment of cord received it, and which structure lies beneath the point of the finger. This is the chapter where every article before it is put to work at three in the morning, with no scan yet available.
Three in the morning. A nineteen-year-old lies on a trolley with his knees drawn up, and the story he tells is unremarkable until you listen to its geography. Yesterday evening he felt vaguely unwell — a dull ache around his navel that he could not point to with one finger, only circle with a whole hand. He was not hungry at dinner. He was nauseated by midnight. And an hour ago the pain changed character entirely: it left the middle of his abdomen and settled low on the right, small and sharp and exact, so precise that he can now put a fingertip on it. The taxi ride hurt every time the car went over a bump. There is no scan available until morning. The registrar does not need one. She has just watched a pain migrate from a midline autonomic wire to a segmental somatic one, which is what happens when an inflamed organ stops complaining to itself and starts touching the wall of the room it lives in. She books the theatre.
Two nerve supplies, two completely different pains
The whole of abdominal diagnosis begins with one fact: the abdomen is wired twice. The organs and the visceral peritoneum that covers them are supplied by AUTONOMIC afferents that run back alongside the sympathetic nerves, through the splanchnic nerves and the sympathetic chain, into the spinal cord at a small number of segments. These fibres are few, slow, unmyelinated or thinly myelinated, and they converge heavily — dozens of them onto one second-order neurone — so the brain receives a message that says something is wrong in the region of a spinal level, and no more than that. The wall, by contrast, and the PARIETAL peritoneum lining it, are supplied by ordinary SOMATIC segmental nerves: the lower six intercostal nerves, the subcostal nerve and the first lumbar nerve for most of the abdomen, the phrenic nerve for the central diaphragmatic peritoneum, and the obturator nerve for part of the pelvic peritoneum. These are fast, richly represented on the cortical map, and precise to within a centimetre. The clinical consequence, laid out in detail in the peritoneum, is that the same disease produces two entirely different pains at two different times — and the moment it changes from one to the other is the single most useful sentence in the history.
The first pain: an organ complaining to itself
VISCERAL pain is produced by only a few things: stretch of a hollow organ or its capsule, powerful contraction against an obstruction, and ischaemia. Cutting, burning and crushing a viscus produce nothing — a surgeon can cauterise bowel in an awake patient without a flicker. Because the afferents enter at fixed segments and the two sides of the gut tube are innervated symmetrically, the pain is felt in the MIDLINE, and its level is dictated by the embryological origin of the organ rather than by where that organ has ended up. The foregut, supplied by the coeliac trunk and reporting through the greater splanchnic nerve at T5–T9, hurts in the EPIGASTRIUM. The midgut, supplied by the superior mesenteric artery and reporting at T10–T11, hurts around the UMBILICUS. The hindgut, supplied by the inferior mesenteric artery and reporting at L1–L2, hurts in the SUPRAPUBIC region. That is why a stomach ulcer, a gallbladder and a pancreas — three organs sitting in three different places — all announce themselves in the same square of skin: they were all built from the same tube. The rule and its embryology are set out in foregut, midgut and hindgut. The pain itself is dull, gnawing, wave-like or colicky, poorly localised, and it comes with the autonomic company you would expect from a sympathetic reflex — nausea, vomiting, pallor, sweating and restlessness. The patient with pure visceral pain cannot keep still, because no position makes a stretched tube comfortable.
The second pain: the lining is touched
The moment inflammation reaches the wall, the abdomen finally tells you where it lives. PARIETAL or somatic pain begins when the inflamed organ, or the fluid leaking from it, irritates the parietal peritoneum lining the wall next to it. Now the message travels on a named segmental nerve to a single spinal segment, and the brain localises it as precisely as it would localise a splinter in the finger. The pain is SHARP, constant, exactly pointed to, and made worse by anything that moves one layer of peritoneum against another: coughing, sneezing, deep breathing, the trolley going over a doorway, the examining hand releasing suddenly, a single percussing finger. It is accompanied by two motor phenomena that are pure spinal reflex. GUARDING is involuntary contraction of the overlying abdominal muscles supplied by the SAME segments — the cord protecting the inflamed area by splinting it, which is why guarding is localised early and cannot be talked away, unlike voluntary tensing. RIGIDITY is guarding taken to its limit: a board-like, generalised, unremitting contraction of the whole anterior wall, and it means the whole peritoneal cavity is contaminated. A patient with parietal pain lies absolutely still, breathes shallowly with the chest rather than the abdomen, and will not let you move the trolley — the exact opposite of the writhing colic patient in the next bay.
Think of a rented room inside a building. The contents of the room — the furniture, the plumbing — have no address of their own; if something goes wrong inside them, the only alarm that sounds is a general one on the floor they happen to be on, and the caretaker downstairs knows only "something is wrong on the second floor". That is visceral pain: an alarm reporting a LEVEL, not a location. But the walls of the room are wired individually, every square metre with its own sensor and its own light on the panel. The moment a leak from the furniture reaches a wall, the panel lights up one exact square, and the caretaker walks straight to it. That is parietal pain. Appendicitis is simply a leak that begins in the furniture and reaches the wall a few hours later — which is why the alarm first sounds vaguely at the second floor, then suddenly and exactly at the corner room on the right.
The third pain: a message on a shared wire
REFERRED pain is felt in a dermatome remote from the diseased organ because the two share a spinal segment, and the brain — which has spent a lifetime receiving messages from skin on those fibres — misreads the return address. The most famous example belongs to the diaphragm: blood, pus or gas irritating its central peritoneum travels on the phrenic nerve into C3–C5, and the patient grips the tip of the SHOULDER. A ruptured spleen (Kehr's sign), a perforated ulcer whose gas has risen to the highest point, a ruptured ectopic pregnancy and a subphrenic abscess all do it. Ureteric colic is the second great example: the ureter reports at T11–L2, and the pain sweeps from the LOIN TO THE GROIN and into the scrotum or labium as the stone descends, following the pattern described in the kidneys and ureters. Gallbladder pain refers to the inferior angle of the RIGHT SCAPULA and the right shoulder; pancreatic pain bores THROUGH TO THE BACK because the gland lies retroperitoneally against the vertebral column. And the traffic runs both ways: a lower lobe pneumonia irritating the diaphragmatic pleura, or an inferior myocardial infarction reporting at the same thoracic segments as the upper gut, can present as upper abdominal pain with vomiting — which is why every acute abdomen deserves a chest examination and, in the older patient, an ECG.
Watch the patient before you touch the patient. Ask them to cough, and then to blow out their abdomen and suck it in; ask them to lift their head off the pillow. Peritonitis will declare itself across the room, without a hand being laid on the abdomen, because coughing shears one inflamed peritoneal surface against another and the patient winces and grabs one exact spot with one finger. That single observation is as sensitive as rebound tenderness and infinitely kinder — the traditional deep press-and-release is painful, frightens the patient, and destroys your ability to examine them again. Percussion tenderness, tapping gently over each region, gives you the same information one square at a time. A patient who can cough hard without flinching almost never has generalised peritonitis.
Pain that moves — and why the movement is the diagnosis
Three migrations are worth knowing by heart. APPENDICITIS begins as midgut visceral pain around the umbilicus — the appendix is a midgut structure whose afferents enter at T10 — with anorexia and nausea; then, as the inflammation reaches the serosa and the adjacent parietal peritoneum, it shifts to the RIGHT ILIAC FOSSA and becomes sharp and localised, classically at McBurney's point, two-thirds of the way along a line from the umbilicus to the right anterior superior iliac spine. The full picture and the appendix's many positions are covered in the large intestine and appendix. CHOLECYSTITIS begins as foregut visceral pain in the epigastrium and moves to the RIGHT UPPER QUADRANT when the inflamed gallbladder touches the parietal peritoneum of the anterior abdominal wall — and often refers on to the right scapula at the same time.
The third is the most beautiful and the most dangerous. A PERFORATED DUODENAL ULCER begins with sudden, agonising epigastric pain as acid and gas escape into the peritoneal cavity — patients often name the minute it started. Within an hour the pain generalises as chemical peritonitis spreads. And then, in a proportion of cases, it settles into the RIGHT ILIAC FOSSA, because the escaped fluid has drained downhill along the RIGHT PARACOLIC GUTTER, the groove between the ascending colon and the lateral abdominal wall, and is now irritating the peritoneum over the caecum. A perforated ulcer can therefore present as appendicitis, with a right iliac fossa that is exquisitely tender and a story that started in the epigastrium — an anatomical impostor produced by nothing more than gravity and a gutter. The clue is the abrupt, board-rigid onset and the free gas on an erect film; the mechanism is the same reason the left gutter misleads less often, being shallower and blocked above by the phrenicocolic ligament.
The signs, and the anatomy hiding behind each one
Every eponymous sign is a way of moving one structure against another and watching the face. MURPHY'S SIGN: place the fingers under the right costal margin and ask the patient to breathe in. The descending diaphragm pushes the liver and the inflamed gallbladder down onto your hand, and the breath stops mid-inspiration — inspiratory arrest, and it must be absent on the left to count. ROVSING'S SIGN: press in the LEFT iliac fossa and the pain is felt in the RIGHT, because the pressure shifts gas and loops within the peritoneal cavity and drags the inflamed parietal peritoneum on the right. The PSOAS SIGN — pain on passive extension of the right hip, or on active hip flexion against resistance — implies the inflamed appendix is lying on psoas major behind the caecum, the commonest RETROCAECAL position. The OBTURATOR SIGN — pain on passive internal rotation of the flexed right hip — implies a PELVIC appendix lying against obturator internus. Each of these is not a trick to memorise but a small anatomical experiment: move the muscle the organ is resting on, and if the organ is inflamed, it will tell you.
Listening and looking add two more layers. BOWEL SOUNDS are absent in paralytic ileus and in established peritonitis, because inflamed bowel stops moving — a silent, rigid abdomen is one of the most sinister findings in medicine. In mechanical OBSTRUCTION the opposite happens: bowel above the block contracts furiously against it, and the sounds become loud, frequent, high-pitched and tinkling as fluid drips through distended, air-filled loops. Bruising is the slowest sign of all and the most specific: CULLEN'S SIGN is periumbilical discolouration and GREY TURNER'S SIGN is flank discolouration, appearing one to three days after retroperitoneal blood has tracked forward — along the falciform ligament and round ligament remnants to the umbilicus, and laterally through the retroperitoneal fat to the flanks. They are classically taught with severe acute pancreatitis but occur equally with a ruptured abdominal aortic aneurysm or a ruptured ectopic pregnancy: they simply mean blood is loose behind the peritoneum, and it has had time to travel.
A twenty-two-year-old woman and a fifty-five-year-old man are both tender in the right iliac fossa. Her pain began around the umbilicus twelve hours ago, crept downwards over the evening, and is now sharp and pointed to; she is mildly febrile, her tongue is furred, and she jumps when she coughs. His pain began forty minutes ago, in the epigastrium, so suddenly that he remembers dropping his cup; it spread across the whole abdomen within the hour and has now pooled low on the right; his abdomen is board-rigid everywhere, silent to the stethoscope, and he takes ibuprofen daily for his knees. Two identical points of maximum tenderness, two entirely different diseases — and the difference is written in the history, not the hand. She has appendicitis. He has a perforated duodenal ulcer whose gastric contents ran down the right paracolic gutter, and the erect chest film that shows a crescent of free gas under the right dome of the diaphragm will confirm it before any surgeon touches him. The role of anti-inflammatory drugs in that perforation is a story told in NSAIDs and their big risks.
Quadrant by quadrant: what lies beneath the finger
The differential diagnosis of abdominal pain is nothing more than a list of what is anatomically present at that point. Divide the abdomen with the two horizontal and two vertical planes of abdominal regions and surface anatomy and the list writes itself. RIGHT UPPER QUADRANT: gallbladder and biliary tree (cholecystitis, biliary colic, cholangitis), liver (hepatitis, abscess, congestion stretching Glisson's capsule), the duodenum, the hepatic flexure of the colon, the right kidney above and behind, and the lower lobe of the right lung just above the diaphragm. EPIGASTRIUM: stomach and duodenum (ulcer, gastritis, perforation), the pancreas behind, the abdominal aorta deeper still — and, crucially, the heart above it, because an inferior myocardial infarction is a classic mimic. LEFT UPPER QUADRANT: the spleen (rupture, infarct, massive splenomegaly), the fundus and body of the stomach, the tail of the pancreas, and the splenic flexure. THE FLANKS: kidneys and ureters (pyelonephritis, ureteric colic), the ascending colon on the right and the descending colon on the left, and the retroperitoneal structures behind them all.
PERIUMBILICAL: the small bowel and its mesentery, the appendix in its early hours, and — the one that must never be missed — an abdominal aortic aneurysm, which produces central pain radiating to the back with a pulsatile expansile mass, as described in the abdominal aorta. RIGHT ILIAC FOSSA: appendix, caecum and terminal ileum (Crohn's, mesenteric adenitis in children), the right ovary and fallopian tube, an ectopic pregnancy, the right ureter, and a strangulated inguinal or femoral hernia at its lower border. SUPRAPUBIC: bladder (retention, cystitis), uterus, and the sigmoid colon dipping into the pelvis. LEFT ILIAC FOSSA: sigmoid diverticular disease above all, colitis, the left ovary and tube, the left ureter, and again a hernia in the groin. Two rules protect you here. First, in any woman of reproductive age with lower abdominal pain, a pregnancy test is part of the examination, not an investigation — a ruptured ectopic can empty a patient into her own peritoneal cavity in minutes. Second, always examine the groins and the external genitalia: an obstructed abdomen with an irreducible lump in the groin has told you both the diagnosis and the operation.
The five surgical emergencies
PERFORATION: a hollow viscus opens into the peritoneal cavity, spilling acid, enzymes, bile or faeces, and produces sudden generalised peritonitis with board-like rigidity. Gas escapes with the fluid, and gas rises — so on an ERECT chest film it collects at the highest point of the peritoneal cavity, the space between the dome of the liver and the undersurface of the right hemidiaphragm, as a thin dark crescent. Take the same film with the patient lying flat and the gas spreads out invisibly; the erect posture is the whole test. OBSTRUCTION: colicky pain in waves at the visceral level of the blocked segment, distension, vomiting (early in high obstruction, late and faeculent in low), and absolute constipation. In large bowel obstruction with a COMPETENT ileocaecal valve the segment becomes a closed loop that can decompress in neither direction, and by Laplace's law the widest part — the caecum — carries the greatest wall tension and perforates first. ISCHAEMIA: acute mesenteric ischaemia from an embolus lodged in the superior mesenteric artery gives the most treacherous presentation of all, severe central pain with an almost normal abdomen — pain out of proportion to the signs — because the mucosa dies long before the serosa inflames enough to reach the parietal peritoneum. By the time there is rigidity, the bowel is dead.
HAEMORRHAGE: a ruptured abdominal aortic aneurysm, a ruptured spleen after blunt trauma, a ruptured ectopic pregnancy or a bleeding liver laceration fill the peritoneal or retroperitoneal space with blood. The abdomen may be surprisingly soft, because blood is a far weaker peritoneal irritant than acid or faeces; what dominates is hypovolaemia — tachycardia, a narrowing pulse pressure, cold peripheries, air hunger — often with SHOULDER-TIP pain as blood reaches the diaphragm, and later with Cullen's or Grey Turner's bruising. TORSION completes the list: an ovary, a testis, the sigmoid colon in a long redundant mesentery, or a loop of small bowel around an adhesive band twists on its own vascular pedicle and strangles itself. Torsion begins as pure visceral pain — sudden, severe, colicky, with vomiting — and becomes parietal only when the organ has already infarcted, which is why torsion is a diagnosis made on the story and the clock, not on the abdominal wall.
Where fluid goes when it is free
Free fluid in the peritoneal cavity does not stay where it was born; it obeys gravity and the shape of the room. The peritoneal cavity is not a smooth bag but a landscape of grooves and recesses, and knowing its low points tells you where pus will collect, where an abscess will form and where the ultrasound probe should be placed. On each side, the PARACOLIC GUTTER runs between the ascending or descending colon and the lateral abdominal wall. The right gutter is deeper and continuous from the subhepatic region all the way down into the pelvis, so fluid from a perforated ulcer or a leaking gallbladder runs freely to the right iliac fossa; the left gutter is shallower and is dammed superiorly by the phrenicocolic ligament, so contamination from the left upper quadrant is slower to descend. In the SUPINE patient — which is to say, in every patient on a trolley — the lowest point of the whole upper peritoneal cavity is the HEPATORENAL RECESS, Morison's pouch, between the posteroinferior surface of the liver and the right kidney. That is why the FAST scan looks there first: a few millilitres of free blood collect in Morison's pouch before they show up anywhere else.
Stand the patient up, or nurse them sitting, and the low point moves. The most dependent part of the whole peritoneal cavity in the ERECT position is the pelvis — the rectouterine pouch of Douglas in the female, the rectovesical pouch in the male — which is why pelvic abscesses are common after generalised peritonitis, why they present with diarrhoea and mucus as they irritate the rectum in front of them, and why they can be felt as a boggy tender mass on rectal examination and drained through the rectal or vaginal wall without opening the abdomen. Between these two extremes sit the SUBPHRENIC spaces above the liver, where the classic postoperative abscess forms with hiccups, shoulder-tip pain, a swinging fever and a raised, poorly moving hemidiaphragm on the chest film. Fluid, in other words, writes its own itinerary through the anatomy: it starts where the organ leaked, runs along the gutter that gravity offers, and stops in whichever recess is lowest for the position the patient has been kept in.
Posture is a diagnosis you can make from the doorway. The patient with COLIC — ureteric, biliary, or the wave of an obstructed bowel — cannot lie still: they roll, they get up, they pace, they curl and stretch, because a contracting hollow tube is not relieved by any position and movement gives at least the illusion of doing something. The patient with PERITONITIS lies flat and motionless with their knees slightly bent to slacken the abdominal wall, breathes with the top of the chest only, refuses to be moved, and speaks in short sentences. The first patient looks dramatic and is often less ill; the second looks calm and may be dying. A renal colic patient who suddenly goes quiet and still has usually not improved — something has ruptured.
Three groups of patients can have catastrophic intra-abdominal disease with a deceptively gentle abdomen. THE ELDERLY: thin, weak abdominal muscles cannot generate convincing guarding or rigidity; the febrile response is blunted; comorbidity and analgesia mask the story; and vascular catastrophes — mesenteric ischaemia, a leaking aneurysm — are commonest precisely in this group. THE IMMUNOSUPPRESSED: corticosteroids, chemotherapy and transplant immunosuppression suppress the inflammatory response that produces peritonism, so a patient with a frank perforation may have only mild tenderness and a normal white cell count. THE PREGNANT: from the second trimester the enlarging uterus lifts and rotates the appendix upwards and laterally, so appendicitis presents higher in the right flank; the stretched abdominal wall is held away from the inflamed viscus so guarding is muted; nausea and leucocytosis are physiological; and delay is dangerous for two patients rather than one. In all three, a normal-feeling abdomen with an abnormal story should be believed on the story.
- The abdomen is wired twice: viscera and visceral peritoneum by AUTONOMIC afferents (dull, midline, poorly localised), the wall and parietal peritoneum by SOMATIC segmental nerves (sharp, exactly localised).
- Visceral pain is caused only by stretch, forceful contraction and ischaemia — not by cutting or burning — and its midline LEVEL follows embryology: foregut/coeliac/T5–T9 epigastric, midgut/SMA/T10–T11 periumbilical, hindgut/IMA/L1–L2 suprapubic.
- Guarding is a reflex contraction of muscles sharing the segment of the irritated peritoneum; generalised board-like rigidity means the whole cavity is contaminated. Coughing and percussion tenderness give the same information as rebound, far more kindly.
- Key referrals: diaphragm → shoulder tip (C3–C5, phrenic); ureter → loin to groin (T11–L2); gallbladder → right scapula; pancreas → straight through to the back; and lower lobe pneumonia or inferior MI → upper abdomen.
- Three migrations: appendicitis periumbilical → right iliac fossa; cholecystitis epigastric → right upper quadrant; perforated duodenal ulcer epigastric → generalised → right iliac fossa as fluid runs down the right paracolic gutter.
- McBurney's point is two-thirds along a line from the umbilicus to the right anterior superior iliac spine; the commonest appendix position is RETROCAECAL, which is why the psoas sign matters.
- Murphy's sign = inspiratory arrest under the right costal margin (gallbladder pushed onto the hand by the descending diaphragm). Rovsing's = left-sided pressure felt on the right. Psoas sign = retrocaecal appendix. Obturator sign = pelvic appendix.
- Bowel sounds: ABSENT in paralytic ileus and established peritonitis (a silent rigid abdomen is ominous); LOUD, high-pitched and tinkling in mechanical obstruction.
- Cullen's (periumbilical) and Grey Turner's (flank) bruising appear 1–3 days after retroperitoneal blood tracks forward — severe pancreatitis, ruptured aneurysm or ruptured ectopic.
- The five emergencies: perforation (free gas under the diaphragm on an ERECT film, because gas rises), obstruction (closed loop with a competent ileocaecal valve; the caecum perforates first by Laplace), ischaemia (pain out of proportion to signs), haemorrhage (hypovolaemia, soft abdomen, shoulder-tip pain) and torsion.
- Free fluid runs down the right paracolic gutter (deeper, continuous) more readily than the left (shallower, dammed by the phrenicocolic ligament); Morison's pouch is the lowest point when SUPINE, the pelvic pouch when ERECT — hence FAST scanning and pelvic abscess.
- Posture and pitfalls: colic makes patients writhe, peritonitis makes them lie perfectly still. The elderly, the immunosuppressed and the pregnant can perforate with a soft abdomen — believe the history over the hand.
- Treating the site of maximum tenderness as the diagnosis. The site tells you which parietal peritoneum is inflamed NOW; where the pain STARTED tells you which organ started it. A perforated ulcer draining down the right paracolic gutter is tender exactly where an appendix would be.
- Assuming a soft abdomen excludes a surgical emergency. Blood is a weak peritoneal irritant, mesenteric ischaemia produces pain out of proportion to signs, and steroids, age and pregnancy all mute peritonism — the softest abdomens include some of the sickest patients.
- Forgetting that the abdomen has a lid and a floor. A lower lobe pneumonia, an inferior myocardial infarction and a lower rib fracture all cause upper abdominal pain; a strangulated femoral hernia and a torted testis both cause abdominal pain that will never be found without examining the groin and scrotum.
A 55-year-old man develops sudden severe epigastric pain, which becomes generalised within an hour and then settles predominantly in the right iliac fossa, where he is markedly tender. His abdomen is rigid and silent. Which anatomical mechanism best explains the final location of his pain?
- Abdominal pain comes in three anatomical flavours: VISCERAL (autonomic afferents, midline, dull and colicky, level set by embryological origin), PARIETAL (somatic segmental nerves, sharp and exactly localised, with guarding and rigidity) and REFERRED (a distant dermatome sharing a spinal segment).
- Midline levels follow the gut tube: foregut epigastric (T5–T9), midgut periumbilical (T10–T11), hindgut suprapubic (L1–L2) — which is why appendicitis starts at the navel and only later points to McBurney's point when the parietal peritoneum is reached.
- The classical signs are anatomical experiments: Murphy's (diaphragm pushing the gallbladder onto your hand), Rovsing's, psoas (retrocaecal appendix), obturator (pelvic appendix), silent abdomen in ileus versus tinkling in obstruction, and Cullen's and Grey Turner's bruising from retroperitoneal blood.
- Free fluid obeys gravity: down the deeper right paracolic gutter, into Morison's pouch when supine and the pelvic pouch when erect, and free gas rises under the right hemidiaphragm on an erect film — which is why a perforated ulcer can perfectly imitate appendicitis.
- Drake RL, Vogl AW, Mitchell AWM. Gray's Anatomy for Students — Abdomen: peritoneal cavity, referred pain and the paracolic gutters.
- Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy — Visceral versus parietal pain, dermatomes of referred abdominal pain.
- Last RJ. Last's Anatomy: Regional and Applied — The peritoneum, subphrenic spaces and the pelvic recesses.
- Snell RS. Clinical Anatomy by Regions — The acute abdomen: clinical signs and their anatomical basis.
- Netter FH. Atlas of Human Anatomy — Peritoneal recesses, greater sac and the appendix in its varied positions.
- TeachMeAnatomy — The Peritoneum; The Appendix; Referred Pain.
