THE IXTFSTIXAL CAXAL 



995 



As the coils of bowel become more fully developed, it will be seen that all the 

 small intestines lie to the right hand side, and the eiecum and the connnencement 

 of the colon to the left. The vasa intestini tenuis of the superior mesenteric artery 

 consi'(|uently arise from the right side of the vessel, while the ileo-colic, right colic, 

 and middle colic all arise from the left (fig. 59o). 



Further devel()j)ment in this direction may not proceed, and the condition al)()ve 

 described is met witli in some of the lower mammals as a permanent arrangement 

 (tig. ')9(;). 



The rotation of the intestinal canal. — As development proceeds, the jjiates 

 which are forming the anterior abdominal parietes approach one another, and the 

 closure of the cavit}'^ of the abdomen is immment. The loops which have up to 

 tliis time jirojected beyond the site of tlie future umbilicus are now withdrawn into 

 the abdomen. 



About this time a remarkable and (juite characteristic rotation of the general 

 mesenteric fold takes place. The rotation is from left to right, and is around an 

 axis whit'h may be represented by a line drawn from the neck or pedicle of the 

 common mesentery to the site of the vitelline duct. This is in fact the line of the 

 superior mesenteric artery (fig. 593). The rotation is therefore at the neck of the 

 great and complex loop of intestine, which neck, as has been already said, is 

 bounded by the duodenum and the right end of the transverse colon. 



Fig. 595. — Alimentary Canal of Salamaajjua maculosa. 



As a result of this rotation, the cajcum and the ascending colon turn over to the 

 right, while the great mass of the small intestines is brought to the left (fig. 597). 



The lower end of the duodenum will consequently be carried to the left; the 

 transverse colon will lie in front of it, instead of below it; and the superior mesen- 

 teric artery (2, tig. 597), which must still run between these two segments of the 

 1 towel, will now be descrilx'd as passing over the duodenum, or as crossing its 

 anterior surface. It is needless to say that in the fully-developed l)ody this artery 

 crosses the left end of the third ])art of the duodenum, and is l)ehind the right end 

 of the transverse colon. It will be seen that, in spite of this revolution, the 

 duodenum and the transverse colon still retain their relations to one another, and 

 still mark the neck or pedicle of attachment of the common mesenteric fold. 



The effect of this rotation of the intestinal tube is shown in the diagram 

 (fig. 597). 



The caecum and ascending colon are now moved to the right; the transverse 

 colon is more distinctly transverse; the small intestine has ))asse(l over to the left 

 side. The right side of the mesentery has become the left side, and rice verm ; and 

 the vasa intestini tenuis now ai)i)ear to arise from the left border of the superior 

 mesenteric artery instead of from the right (tig. 597). The duodenum is almost 

 hidden from view, and has been brought into close contact with the vertebral 

 column. The descending colon remains unchanged, and is supported l)y the still 

 undisturbed median fold of the jteritoneum. 



