THE ORGANS OF DIGESTION. 281 



226). The inferior mesenteric vein passes forwards along the 

 dorsal mesentery to the splenic vein, which is also then in the dorsal 

 mesentery (Fig. 187, p. 229). The mesentery of the hind-gut is 

 stretched as the ascending colon migrates to the right ; the 

 inferior mesenteric vein checks it and gives rise to a fold, which 

 bounds the fossa (Fig. 226). 



The Mesentery of the small gut is formed out of the primitive 

 mesentery of the U-shaped intestinal loop, chiefly from that 

 part of it (the pre-arterial) which lies between the superior 

 mesenteric artery and the anterior limb of the loop (Fig. 223 A). 

 After the rotation, the aspect of the mesentery, which was directed 

 towards the right, becomes left and anterior. During the rotation 

 of the gut the superior mesenteric artery comes to lie in front of 

 the third stage of the duodenum. At first the mesentery is 

 attached in front of the spine only at the origin of the superior 

 mesenteric artery. Its oblique attachment to the posterior 

 abdominal wall, from the duodenum to the right iliac fossa, is 

 a secondary adhesion, formed after the rotation of the gut, and 

 this extensive attachment is found only in animals adapted to 

 the upright posture. 



