The Anatomy of the Human Peritoneum and Abdominal Cavity / Considered from the Standpoint of Development and Comparative Anatomy

by George S. Huntington

The secondary parietal peritoneum derived from the ventral layer of the ascending mesocolon covers the lower and inner portion of the ventral surface of the right kidney, the infra-colic division of the descending and the dextro-mesenteric segment of the transverse duodenum, while along the root of the jejuno-ileal mesentery it becomes continuous with the right layer of that membrane. The secondary parietal peritoneum derived from the ventral layer of the descending colon covers the lower part of the ventral surface of the left kidney and the sinistro-mesenteric segment of the transverse duodenum and becomes continuous along the mesenteric radix with the left layer of the jejuno-ileal mesentery.

Caudad the adhesion of the descending colon and mesocolon to the parietal peritoneum proceeds only to the point C, following the dotted line mesad and resulting in the formation of the free mesocolon of the sigmoid flexure.

Résumé of the Adult Arrangement of the Human Peritoneum in the Lower Compartment of the Abdomen, Below the Level of the Transverse Colon and Mesocolon.—We should now consider the arrangement of the human peritoneum in the adult below the dorsal attachment of the transverse mesocolon in the light of the embryological and comparative anatomical facts just stated. In doing this it will be advisable to study both the actual conditions encountered and their significance in the sense of determining the derivation of the peritoneal layers from the primitive dorsal mesentery. Open the abdominal cavity in the usual manner by a cruciform incision.

Turn the great omentum up on the chest wall, exposing the underlying intestines. This manipulation, as already stated, will cause the omentum to carry the transverse colon with it, on account of the adhesion, in the adult, of the gut to the dorsal layer of the omentum. Hence the cephalic or upper layer of the transverse mesocolon will not be seen at this stage because the omental adhesion just referred to prevents us from passing between the greater curvature of the stomach and the transverse colon without tearing peritoneal layers. It will, however, be possible to trace on the right side the duodenum from the pylorus down ventrad of the right kidney until the descending portion disappears behind the hepatic flexure of the colon. With the omentum and transverse mesocolon turned up, as stated, and the transverse mesocolon put upon the stretch, it will be seen that the abdominal space now overlooked is bounded cephalad by the lower layer of the transverse mesocolon and its attachment to the dorsal abdominal wall. The lateral limits of the space are given by the ascending and descending colon respectively. The attachment of the mesentery of the small intestine to the oblique line extending from the left of the vertebral column at about the level of the second lumbar vertebra to the right iliac fossa subdivides the entire space into a secondary right and left compartment.

Begin by following the caudal layer of the transverse mesocolon dorsad on the right side. In the angle between ascending and transverse colon (hepatic flexure) pressure will locate the caudal portion of the ventral surface of the right kidney. Remember that the peritoneum touched in these procedures appears in the adult as parietal prerenal peritoneum, but that in reality it is the left leaf of the originally free ascending mesocolon, whereas the original right leaf of this membrane and the primitive parietal peritoneum have, by adhesion of their serous surfaces, been converted into the loose subserous connective tissue covering the ventral aspect of the kidney beneath what now appears as parietal peritoneum.

Mesad of the resistance offered to the finger by the right kidney the caudal (infra-colic) portion of the descending duodenum and the angle of transition between it and the third or transverse portion will be found, invested in the same way by secondary (mesocolic) parietal peritoneum. It will be seen, especially if the duodenum is injected or inflated, that the hepatic flexure of the colon lies ventrad of the vertical descending second portion of the duodenum, so that one part of this intestine is situated cephalad the other caudad of the colon. (Supra- and infra-colic segments of descending duodenum.)

Individual differences are observed in the area of colic attachment to the duodenum. Usually the two intestines are in contact with each other and adherent over a considerable surface. Exceptionally the transverse mesocolon extends across to the right so as to include the hepatic flexure. In this latter case the uncovered non-peritoneal surface of the descending duodenum is small, represented by the interval between the layers of the transverse mesocolon, and the hepatic flexure is then not directly adherent to the gut.

If we now trace the transverse duodenum from right to left we will encounter the right layer of the root of the jejuno-ileal mesentery. The caudal layer of the transverse mesocolon, the right leaf of the mesentery and the secondary parietal peritoneum investing the ventral surface of the transverse duodenum all meet at this point. Surround the mesentery of the free small intestine with the fingers of one hand so that the entire mass of intestinal coils can be swung alternately from side to side.

Turning them over to the left, as already stated, the proximal portion of the transverse duodenum can be traced from right to left as far as the root of the mesentery. Here the peritoneum investing the ventral surface of the duodenum becomes continuous with the right leaf of the mesentery. Now swing the whole mass of small intestines over to the right, exposing the parietal peritoneum in the space to the left of the vertebral column, between the attachment of the mesentery to the median side, the root of transverse mesocolon cephalad and the descending colon to the left. Remember that the same significance attaches to this secondary parietal peritoneum as on the right side. It appears in the adult as parietal peritoneum, but is in its derivation the original right leaf of the descending mesocolon. Close to the root of the mesentery the continuation from the right side of the transverse duodenum will be seen, crossing the median line from right to left ventrad of aorta and vertebral column and usually turning cephalad on the left side of the lumbar vertebræ, as the fourth or ascending duodenum, to reach the caudal surface of the transverse mesocolon near its attachment, where the gut turns ventrad to form the duodeno-jejunal angle and become continuous with the free small intestine.

From the fact that the transverse duodenum is thus seen on each side of the root of the mesentery it will be recalled that after rotation of the primitive intestine the superior mesenteric artery crosses the transverse portion of the duodenum to reach its distribution between the leaves of the mesentery. Hence this portion of the small intestine consists of a dextro- and sinistro-mesenteric segment. This intersection of mesentery and duodenum marks the site of the primitive duodeno-colic isthmus through which the superior mesenteric artery passed to reach its distribution to the gut composing the embryonic umbilical loop.