194 



THE ENTODERMAL CANAL AND THE BODY CAVITIES 



out into the umbilical cord between the limbs of the intestinal loop. 

 When the intestine elongates and its loop rotates, the cascal end of the 

 large intestine comes to lie cranially and to the left, the small intestine 

 caudally and to the right, the future duodeum and colon crossing in 

 close proximity to each other (Fig. 179). On the return of the intestinal 

 loop into the abdomen from the umbilical cord, the cascal end of the colon 

 lies to the right and the transverse colon crosses the duodenum ventrally 

 and cranially (Fig. 203 A). The primary loops of the small intestine 

 lie caudal and to the left of the ascending colon (Fig. 203 B). There 

 has thus been a torsion of the mesentery about the origin of the superior 



Lesser amentum 



'lomach 



Dorsal mesogastrium 



Transverse 



mesocolori 



Ccecu; 



Mesentery, 

 Mesorectu 



Iliac mesocolon 

 A B 



PIG. 203. Diagram showing the development of the mesenteries in ventral view (modified 

 after Tourneux). *, Cut edge of greater omentum; a, area of ascending mesocolon fused to dor- 

 sal body wall; b, area of descending mesocolon fused to dorsal body wall. Arrow in omental 

 bursa. 



mesenteric artery as an axis. From this focal point the mesentery of the 

 small intestine and colon spreads out fan-like. The mesoduodenum is 

 pressed against the dorsal body wall, fuses with its peritoneal layer, and 

 is obliterated (Fig. 202). Since the transverse colon lies ventral to the 

 duodenum it cannot come into apposition with the body wall ; where its 

 mesentery crosses the duodenum it fuses at its base with the surface of 

 the latter and of the pancreas. Its fixed position now being transverse 

 instead of sagittal, the mesentery is known as the transverse mesocolon. 

 The mesentery of the ascending colon is flattened against the dorsal body 

 wall on the right and fuses with the peritoneum (Fig. 203). Similarly, 

 the descending mesocolon is applied to the body wall of the left side. 



