8oo A MANUAL OF ANATOMY 



Intestinal Canal. — This canal is at first very short and almost straight, and, 

 as has been stated, it communicates freely with the yolk-sac. At this stage 

 there is no indication of a division into small and large intestine. When the 

 wide opening leading to the yolk-sac becomes constricted and converted into 

 the vitelline duct, the intestinal canal undergoes lengthening, and a con- 

 spicuous loop is formed, which projects into the cavity of the proximal part 

 of the umbilical cord, this cavity being a direct projongation of the coelom 

 or body-cavity. This loop is spoken of as the U"loop. The convexity of the 

 bend of the loop is directed ventralwards, and the vitelline duct is connected 

 with the convexity. The loop has two limbs, which lie at first parallel to each 

 other. One limb is cephalic, upper, or proximal, and leads from the duodenal 

 loop of the gut; the other limb is caudal, lower, or distal, and leads to the 

 caudal end of the gut. Upon the distal limb a bud makes its appearance. 

 This assumes the form of a blind diverticulum, or cul-de-sac, which is the 

 rudiment of tlie caecum. The appearance of the caecal evagination is the first 

 indication of the division of the intestinal canal into small and large intestine. 

 The primitive small intestne is the part on the proximal side of the rudimentary 

 caecum, and the primitive large intestine is the part extending from the rudi- 

 mentary caecum to the caudal end of the gut. 



The U"loop soon undergoes rotation in such a manner that the distal limb 

 bearing the caecal diverticulum crosses obliquely over, or ventral to, the proximal 

 limb, so that the original distal or lower limb now becomes proximal or upper. 

 The part of the gut which is crossed ventrally by a portion of the distal limb 

 represents the future descending part of the duodenum. This crossing accounts 

 for the fact that, in the adult, the commencement of the transverse colon lies 

 in front of the descending part of the duodenum. 



The U-loop is occupied by a mesentery, which represents the mesentery 

 proper of the adult. Close to the point of crossing of the two limbs of the 

 loop, its mesentery contains the superior mesenteric artery. The loop itself, 

 as stated, lies in the cavity of the proximal part of the umbilical cord. At 

 a later period (about the ninth week of embryonic life) it is withdrawn from 

 its original extra-embryonic position into the body-cavity (abdomen). In 

 rare cases the prolongation of the coelom into the proximal part of the um- 

 bilical cord may persist at birth, and so predispose to the occurrence of a 

 congenital umbilical hernia. 



The elongation of the jejunal and ileal portions progresses after withdrawal 

 of the U'loop. ai^d takes place more rapidly than that of the large intestine. 

 Moreover, the small intestine outgrows in length the body-cavity, in conse- 

 quence of which the jejunum and ileum are thrown into coils. These coils 

 are placed, at first, on the right side of the median line. 



The primitive large intestine is represented by (i) the caecum, which is its 

 initial part, (2) a short transverse colon, (3) a descending colon, and (4) a 

 terminal part, which subsequently forms an iliac colon, pelvic colon, rectum, 

 and upper two-thirds of the anal canal. There is no ascending colon at this 

 stage. These initial parts (caecum and colon) lie, at first, on the lejt side of 

 the median line, the caecum being underneath the left part of the liver. The 

 caecum gradually shifts to the median line, and then to the right side, still 

 lying beneath the liver, and, as it thus alters its position, the transverse colon 

 undergoes elongation. Finally, the caecum, as a rule, descends slowly to its 

 permanent abode in the right iliac jossa. During this descent the ascending 

 colon becomes formed. 



The large intestine having, like the small, outgrown the body-cavity, now 

 has the form of an arch or loop, in the concavity of which the coiled jejunum 

 and ileum lie. In the course of these changes in the position of the caecum, 

 the jejunum and ileum are also shifting their position. Originally, as stated, 

 they lie on the right side of the median line, but they, along with the lower 

 part of the duodenum, are gradually shifted (but not entirely) towards the 

 left side, so as to be brought within the loop of the large intestine. 



The foregoing transposition of parts is brought about by the twist which 

 has been referred to as occurring in the U-l"oP- ^^is twist, or rotation, 

 involves not only the loop itself, but also its mesentery, and it takes place 



