Reading

Body Surface

The primary surface landmarks of the abdomen are associated with the sternum (xyphoid), ribs (costal margin), innominate (iliac crest, anterior superior iliac spine, pubic tubercle and pubic symphysis) and inguinal ligament. The skin overlying the abdomen is innervated by the anterior and lateral cutaneous branches of the lower (T7-11) intercostal nerves, and subcostal, iliohypogastric and ilioinguinal nerves.

Skeleton and Joints

The bones associated with the abdomen are the sternum (xyphoid process), costal cartilages, lower ribs and thoracic vertebrae, lumbar vertebrae, sacrum and innominate. The sacrum and innominate bones articulate (gliding or plane) at the sacroiliac joint and are supported by the sacroiliac and sacrolumbar ligaments.

Organization

The abdomen is divided into the anterior abdominal wall, inguinal canal and associated spermatic cord, the peritoneal cavity and its associated viscera, and the posterior abdominal wall and its associated retroperitoneal structures.

The anterior abdominal wall is covered by a layer of superficial (subcutaneous) fascia. The superficial layer (Camper's fascia) of this fascia is typically laden with adipose tissue and is continuous with the superficial fascia of the thigh. The deep membranous layer (Scarpa's fascia) is distinct over the lower portion of the anterior abdominal wall and fuses with the fascia lata at the inguinal ligament. The deep fascia and aponeuroses of the exernal oblique, internal oblique and transversus abdominis muscles contribute to the rectus sheath that splits around and encases the rectus abdominis muscle. A thin layer of fibrous deep fascia, the transversalis fascia, is positioned between the transversus abdominis and the fatty extraperitoneal fascia (immediately superficial to the peritoneum). The muscles of the anterior abdominal wall, external oblique, internal oblique, transversus abdominis and rectus abdominis, are active during flexion and rotation of the trunk and compression of the abdominal contents (increasing intra-abdominal pressure).

The inguinal canal is a passage through the anterior abdominal wall extending from the deep ring in the transversalis fascia to the superficial ring in the aponeurosis of the external oblique. In the male, each layer of the anterior abdominal wall contributes a concentric layer of the spermatic cord surrounding the ductus deferens and testicular vessels as these structures traverse the inguinal canal.

The peritoneum is a serous membrane lining the abdominopelvic cavity. Parietal peritoneum lines the body wall, visceral peritoneum covers the surface of organs, and mesenteries (double layers of peritoneum) suspend intraperitoneal organs from the body wall. Double layers of peritoneum passing between two structures are referred to as ligaments. Retroperitoneal structures (organs or large vessels) are covered by peritoneum on at least one surface, but are not freely suspended from the body wall by a mesentery. The falciform ligament is a mesentery-like membrane extending from the anterior abdominal wall to the liver. The lesser omentum is a double layer of peritoneum extending between the liver and stomach (lesser curvature) and first part of the duodenum. The greater omentum is a large double layer of mesentery folded upon itself suspended from the stomach and draping over the transverse colon and small intestine. The peritoneal cavity is subdivided into two compartments. The lesser sac is the space posterior to the lesser omentum and stomach that extends between the folds of the greater omentum. The greater sac is the remainder of the peritoneal cavity. The greater and lesser sacs communicate through the epiploic foramen positioned directly posterior to the free margin of the lesser omentum. Paracolic gutters are present on either side (although more prominent laterally) of the ascending and descending colon.

The posterior abdominal wall is formed by the psoas, iliacus, quadratus lumborum and transversus abdominis muscles and their associated deep fascia. The muscles of the posterior abdominal wall are active during flexion of the of the thigh (psoas and iliacus), extension and lateral flexion of the trunk (quadratus lumborum), fixation of the 12th rib during respiration (quadratus lumborum), and compression of the abdominal contents (transversus abdominis). The diaphragm overlaps the superior aspect of the posterior abdominal wall. The diaphragm passes from the xiphoid process, lower ribs and costal cartilages, medial and lateral arcuate ligaments and lumbar vertebrae to the central tendon. The medial arcuate ligament passes anterior to the psoas and the lateral arcuate ligament passes anterior to the quadratus lumborum. The diaphragm contracts (moves inferiorly) during inspiration.

Viscera

The abdomen can be divided into four quadrants radiating from the umbilicus. The upper right quadrant contains the liver, gallbladder, duodenum, pancreas, right colic flexure and pylorus of the stomach. The upper left quadrant contains the stomach, body and tail of the pancreas, spleen, left colic flexure and jejunum. The lower right quadrant contains the cecum, ascending colon and ileum. The lower left quadrant contains the descending and sigmoid colon, jejunum and ileum.

The esophagus traverses the diaphragm through the esophageal aperature to join the stomach at its cardiac orifice. The stomach, an intraperitoneal structure, has a fundus, body and pylorus, and greater and lesser curvatures. The lesser omentum (hepatogastric ligament) extends from the lesser curvature and the greater omentum hangs from the greater curvature. A pyloric sphincter is present where the stomach is continuous with the duodenum. The duodenum has four divisions or sections. The first (free) is intraperitoneal and associated with the lesser omentum (hepatoduodenal ligament). The second (descending), third (horizontal) and fourth (ascending) divisions are retroperitoneal. The duodenum is continuous with the jejunum and ileum at the duodenojejunal junction. The jejunum and ileum are intraperitoneal and suspended from the posterior abdominal wall by the mesentery proper. The ileum is continuous with the cecum at the ileocecal junction. The cecum is the dilation at the origin of the ascending colon. The ascending colon, including the cecum, is (secondarily) retroperitoneal. The ascending colon is continuous with the transverse colon at the right colic flexure. The transverse colon, an intraperitoneal structure is suspended from the posterior abdominal wall by the transverse mesocolon. The transverse colon is continuous with the descending colon, a (secondarily) retroperitoneal structure, at the left colic flexure. The descending colon is continuous with the sigmoid colon, an intraperitoneal structure suspended by the sigmoid mesocolon. The digestive tract ends with the rectum, a retroperitoneal structure.

The liver and pancreas are associated with the 2nd division of the duodenum. The liver has four lobes, right, left, quadrate and caudate. The impression for the gallbladder and fissure for the ligamentum teres separate the quadrate lobe from the right and left lobes. The caval groove and fissure for ligamentum venosum separate the caudate lobe from the right and left lobes. The portahepatis separates the quadrate and caudate lobes. The hepatic arteries and hepatic portal vein enter the liver at the portahepatis. Hepatic bile ducts exit the liver at the portahepatis and are joined by the cystic duct to form the bile (common) duct. The bile duct empties into the 2nd division of the duodenum at the major duodenal papilla. The gallbladder has a fundus, body and neck. The cystic duct arises from the neck.

The pancreatic duct also drains into the 2nd division of the duodenum at the major duodenal papilla. The pancreatic head, neck, body and uncinate process are retroperitoneal. The tail of the pancreas is intraperitoneal where it meets the spleen. The tail of the pancreas and spleen are suspended from the posterior abdominal wall by the lienorenal ligament.

The kidneys and suprarenal glands are retroperitoneal structures associated with the posterior abdominal wall. Urine drains from the kidney into the ureter at the renal pelvis. The ureters stay retroperitoneal until they reach the bladder in the pelvis. The kidneys are surrounded by perirenal fat, renal fascia and pararenal fat.

Innervation

The muscles of the anterior abdominal wall and adjacent parietal peritoneum are innervated (sensory, motor and postganglionic sympathetic) by the lower six intercostal nerves, and ilioinguinal and iliohypogastric nerves. The muscles of the posterior abdominal wall are innervated (motor, sensory and postganglionic sympathetic) by branches of the ventral rami of lower thoracic and upper lumbar spinal nerves.

The abdominal viscera are innervated (sensory, sympathetic and parasympathetic) by splanchnic nerves, ganglia and plexuses associated with the branches of the abdominal aorta. The viscera supplied by the celiac artery are innervated by postganglionic sympathetic axons arising from the celiac ganglion (greater splanchnic nerve, T5-9), and preganglionic parasympathetic axons of the vagus nerve. The viscera supplied by the superior mesenteric artery are innervated by postganglionic sympathetic axons arising from the superior mesenteric ganglion (lesser splanchnic nerve, T10-11), and preganglionic parasympathetic axons of the vagus nerve. The viscera supplied by the renal and suprarenal arteries are innervated by postganglionic sympathetic axons arising from the aorticorenal ganglion (least splanchnic nerve, T12) and preganglionic parasympathetic axons of the vagus nerve. The viscera supplied by the inferior mesenteric artery are innervated by postganglionic sympathetic axons arising from the inferior mesenteric ganglion (lumbar splanchnic nerves), and preganglionic parasympathetic axons arising from ventral rami S2-4. The preganglionic sympathetic splanchnic nerves (greater, lesser, least and lumbar) branch from the sympathetic chain. The postganglionic parasympathetic neurons are in the wall of the viscera. Visceral sensory axons travel with the pre- and postganglionic sympathetic and preganglionic parasympathetic axons.

Blood Supply

Branches of the superior epigastric, musculophrenic and inferior epigastric arteries are the primary blood supply to the muscles of the anterior abdominal wall. Tributaries of the superior epigastric, musculophrenic and inferior epigastric veins drain the muscles of the anterior abdominal wall. Branches of the iliolumbar and lumbar arteries are the primary blood supply of the posterior abdominal wall. Tributaries of the iliolumbar and lumbar veins drain the posterior abdominal wall. Branches of the musculophrenic and inferior phrenic arteries and veins supply and drain the diaphragm.

Branches (left gastric, splenic and common hepatic [proper hepatic, gastroduodenal and right gastric]) of the celiac trunk supply the viscera (esophagus, stomach, duodenum, liver, gall bladder, pancreas and spleen) derived from the embryonic foregut. Branches (inferior pancreaticoduodenal, jejunal, ileal, right colic and middle colic) of the superior mesenteric artery supply the viscera (duodenum, pancreas, jejunum, ileum, ascending colon and transverse colon) derived from the embryonic midgut. Branches (left colic, sigmoid and superior rectal arteries) of the inferior mesenteric artery supply the viscera (descending and sigmoid colon, and rectum) derived from the embryonic hindgut. Tributaries of the hepatic portal vein drain all the abdominal viscera.

Prev Next