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

by George S. Huntington

4. The splenic flexure develops early and is well marked. It indicates the point of transition of the original ascending limb of the umbilical loop into the remaining vertical median segment of the large intestine, from which the descending colon is formed.

In the adult the ascending and descending portions of the colon are vertical. The transverse colon is not quite horizontal since the splenic flexure is higher and placed more dorsally than the hepatic flexure. In the embryo the rapidly-growing coils of the small intestine push the descending colon to the left and dorsad into close contact with the dorsal abdominal wall.

Fig. 150.—Caudal portion of human embryo of 5 mm., with the end- and caudal gut at the highest stage of its development. × 25. (Reconstruction after His.)

A small bend which appears about the middle of the third month in the left iliac fossa indicates the rudiment of the future sigmoid flexure or omega loop.

The rest of the endgut follows the body wall in a well-marked curve, whose termination lies within the concavity of the caudal portion of the embryo (Fig. 150). From this terminal part the rectum develops after the division of the cloaca and the union of the proctodæum with the entodermal intestinal pouch has taken place as detailed above.

The early position of the colon produced by the large size of the fœtal liver, and before the descent of the cæcum has occurred, is shown in [Fig. 124]. In [Fig. 123], where the liver has regained its normal proportions with reference to the abdominal cavity and viscera, and the cæcum has descended into the right iliac fossa, the hepatic flexure is well marked and the first segment of the colon has acquired the vertical position on the right side, the single obliquely transverse segment of [Fig. 124], having become divided into an ascending and a transverse colon.

[[Fig. 124]. Early stage. Liver relatively large. Proximal portion of the colon extends obliquely between the right lumbar region and the spleen. The cæcum has not yet descended.

[Fig. 123]. Later stage. The cæcum occupies the right iliac fossa. Relative reduction in the size of the liver allows the colic segment to be divided by the hepatic flexure into an ascending colon and a transverse colon.]