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

by George S. Huntington

[Fig. 297] shows the primitive condition at a time when the connection with the transverse colon and mesocolon has not yet taken place.

The omental bag or bursa epiploica develops in the region of the dorsal mesogastrium and the viscera included between its layers, by changes in the position and extent of the membrane which finally result in placing a part of the right half of the primitive cœlom cavity behind the stomach. Up to the sixth week the line of origin of the dorsal mesogastrium is from the mid-dorsal line of the abdomen. It deviates from this origin to the left because the great curvature of the stomach to which it is attached turns in this direction. On this account, and because of the rapid growth of this portion of the mesogastrium, a bag or space is formed behind the stomach. The entrance into this space is situated to the right of the lesser curvature, behind the peritoneal layers connecting the same with the liver (lesser or gastro-hepatic omentum and hepato-duodenal ligament). The ventral wall of this space is formed by the dorsal surface of the stomach itself, the dorsal wall by the mesogastrium, turning to the left and presenting its original right surface, now directed ventrad. The caudal limit of the retro-gastric space is given by the turn of the mesogastrium to reach its attachment along the greater curvature of the stomach (rudiment of great omentum).

The stomach, in contributing to produce these changes, passes from the vertical to the oblique and finally into the transverse position. The pylorus, formerly directed caudad, passes up and to the right. The fundus develops and the original left side of the stomach becomes the ventral, the right side the dorsal. The original dorsal border, now the greater curvature, moving caudad, carries the attached dorsal mesogastrium with it into its new position. The mesogastrium now pouches to form the great omentum and rapidly enlarges. At first hardly projecting beyond the greater curvature, it increases in length until it forms a four-layered apron which hangs down as a loose sac over the transverse colon and the coils of the small intestine ([Fig. 177]). In the fœtus of six months the cavity of the omental bag extends caudad as far as the lower edge of the omentum. Later adhesions between the peritoneal surfaces lining the interior of the bursa limit this extension.

The omental bursa is therefore formed by a ventral lamella, consisting of two peritoneal layers, which hangs down from the greater curvature of the stomach and passes around the caudal free edge of the omentum into the double-layered dorsal lamella, which ascends, over the transverse colon, to the original starting point of the dorsal mesogastrium along the front of the vertebral column and aorta. Hence the “great omentum” is originally composed of four layers of peritoneum.

The dorsal double lamella becomes adherent over a considerable area to the parietal peritoneum of the dorsal abdominal wall. In this way the organs developed between the two layers of the lamella obtain their final fixed position. The pancreas becomes anchored and appears in the adult as a “retro-peritoneal” structure, while the spleen is attached by the “phrenico-lienal ligament” to the diaphragm.

In addition the dorsal omental lamella adheres in the fourth month to the cephalic layer of the transverse mesocolon and to the transverse colon.

Important illustrations of some of the intermediate stages in the human development of this portion of the peritoneal tract are afforded by the permanent adult conditions found in the abdominal cavity of some of the lower primates, notably certain of the cynomorphous monkeys.

Fig. 227.—Abdominal cavity of Macacus rhesus, Rhesus monkey, with the small intestine removed. (Columbia University Museum, No. 63/1831.)

Fig. 227 shows the abdominal cavity and disposition of the peritoneum in a macaque monkey (Macacus rhesus, ♂) in the ventral view, with the coils of small intestines removed and the omentum lifted up and reflected upon the ventral body wall. The following important points of difference from the arrangement in the cat on the one hand, and in man on the other, are to be noted: