1. The great omentum is free, hanging down from the greater curvature of the stomach over the coils of intestine. Turning the omentum up it will be observed that the body of the pancreas is included between the two dorsal layers of the membrane.

2. The omentum, containing the pancreas, can be lifted up, exposing the next succeeding structure, viz., the transverse colon and mesocolon. In the cat the large intestine has been brought over, by the manipulations above indicated, into a transverse position so as to represent the human transverse colon and its mesocolon. It is therefore necessary to remember that in this mammal the fixation of the transverse mesocolon in the position indicated, by adhesion of ascending and descending mesocola to the parietal peritoneum of the abdominal background, has not yet occurred. Consequently the membrane must be held in the transverse position in order to represent the human arrangement.

Fig. 224.—Schematic sagittal section of abdominal viscera of cat, after the intestines have been rotated to correspond to the adult human disposition, to show lines of peritoneal reflection before adhesion.
Fig. 225.—The same figure indicating the areas of adhesion and peritoneal obliteration (shaded) which produce the arrangement of the adult human peritoneum. 1. Area of adhesion between opposed surfaces of great omentum and transverse mesocolon and colon. 2. Area of adhesion between parietal peritoneum, duodenum, and caudal layer of transverse mesocolon. 3. Adhesion of opposed walls of omental bursa leading to obliteration of distal portion of pouch and producing “gastro-colic” ligament of adult human subject.

It will of course be observed that both surfaces of the transverse mesocolon established in this way are free, not adherent to either omentum or pancreas on the one hand, nor to the transverse duodenum on the other.

3. The third or transverse portion of the duodenum is seen to be attached by the distal part of the mesoduodenum, both of the serous surfaces of the membrane being free. The duodenum having been brought from right to left transversely across vertebral column and aorta, underneath the superior mesenteric artery, the mesoduodenum, in the segment corresponding to the transverse duodenum, exchanges its original sagittal position for one in a horizontal plane, with cephalic (primitive left) and caudal (primitive right) surfaces.

Now compare the above arrangement of the intestines and peritoneum in the cat at once with the conditions presented in the adult human subject, reserving certain intermediate stages, as exhibited by some of the lower monkeys, for subsequent study.

The examination of a similar sagittal section representing schematically the adult human arrangement of the parts (Fig. 225) will reveal the following points of difference as compared with the cat:

1. The peritoneum covering the dorsal surface of the pancreas, derived from the primitive dorsal mesogastrium, has become adherent to the parietal peritoneum, as previously described.

2. The cephalic surfaces of the transverse colon and mesocolon fuse with the corresponding area of the dorsal (4th) layer of the great omentum (dorsal mesogastrium).