I704 HUMAN ANATOMY. 



anterior and the posterior fold of which each consists of two serous surfaces enclosing 

 a thin stratum of intervening tissue ; there are, therefore, four serous layers included 

 within the original omental curtain. Tracing the posterior fold of the latter upward, 

 it is seen to pass over the transverse colon and the mesocolon, without attachment, 

 to reach the posterior body -wall. On gaining the latter, the anterior or inner serous 

 layer may be followed in front of the pancreas as the posterior wall of the lesser 

 peritoneal sac^ being continued over the under surface of the liver. The outer or 

 posterior serous layer passes behind the pancreas to reach the body- wall, from which 

 it is reflected to become continuous with the upper layer of the transverse meso- 

 colon. For a time these original foetal relations persist, the greater omentum being 

 unattached to and removable from the transverse colon and its mesentery. Later this 

 separation is no longer possible, since the posterior layer of the greater omentum and 

 the transverse mesocolon and colon become fused, the intervening serous surfaces and 

 space being obliterated in consequence. Thereafter the peritoneal layers of the 

 greater omentum are attached to and apparently enclose the large gut, one passing 

 as the upper, the other as the lower serous layer of the transverse mesocolon. In 

 consequence of these fusions the serous surfaces originally behind the pancreas also 

 disappear, and the gland thenceforth assumes its permanent, although secondary, 

 retroperitoneal relation. Subsequently the originally distinct folds constituting the 

 greater omentum fuse, and after birth usually appear as a single sheet attached above 

 to the greater curvature of the stomach and behind and below to the transverse colon. 



The excessive volume of the right half of the liver not only induces the ob- 

 liquity and rotation of the stomach, but likewise influences the disposition of the in- 

 testinal coils on their return from the umbilical ccelom into the peritoneal cavity. 

 The duodenal segment necessarily follows the migration of the pylorus ; its begin- 

 ning, therefore, lies to the right, while the lower end passes to the left with the 

 jejunum. Since the most available space within the abdomen, to the left and below, 

 is appropriated by the coils of the small intestine which first return to the peritoneal 

 cavity, the most movable portion of the elongating large intestine, the transverse 

 colon, is displaced upward and assumes an obliquely transverse position beneath the 

 stomach and liver, above the rapidly increasing volume of the coils of the small gut. 

 The latter tend to displace the descending, later also the ascending, colon later- 

 ally and backward. In consequence of these influences and changes the transverse 

 colon crosses and lies in front of the duodenum, which is thus pushed against the 

 abdominal wall. The serous investment of the duodenum undergoes obliteration 

 where such contact is maintained, and later occurs chiefly on the anterior surface of 

 this part of the gut (Fig. 1403). 



Reference to the original relation of the primitive mesentery (Fig. 1432) in- 

 cluded between the limits of the U-loop shows the principal dorsal attachment of the 

 mesentery to be the comparatively limited area along the body-wall opposite the um- 

 bilical loop. The intestinal margin of the mesentery, on the contrary, rapidly expands 

 to keep pace with the increasing length of the gut-coils, the result being that the 

 mesentery attached to the upper — soon right — limb of the umbilical loop assumes 

 more and more the form of a ruffle, towards the edge of which ramify the branches 

 of the superior mesenteric artery supplying the small intestine, — the later vasa intes- 

 tini tenuis. The branches distributed to the left or colic limb of the U-loop pass to 

 the large gut through a mesentery only slightly wavy. When the arrangement of 

 the intestinal coils takes place, the small gut occupying the left and lower parts 

 of the peritoneal cavity and the large intestine being reflected upward and across the 

 duodenum, twisting or "rotation" takes place around a fixed point marking the 

 duodeno-jejunal junction. This location also corresponds in general to the early 

 position of the superior mesenteric artery, the relations of the branches of which are 

 also affected by the rotation of the mesentery, since thereafter the vessels passing to 

 the coils of the small intestine lie on the left and those to the large gut on the 

 right side, — the opposite of their original situation. 



On assuming its position in front of the duodenum, the attachment of the trans- 

 verse colon is at first a limited sagittal one. With the backward displacement of the 

 duodenum, the mesentery of the transverse colon also comes into relation with the 

 posterior parietal peritoneum and acquires a secondary attachment extending cross- 



