314 THE ANATOMY OF THE HORSE. 



inferior aspect of the aorta at the 1st lumbar vertebra. It is only about 

 an inch and a half in length, but it has a large calibre, and in old 

 horses it often shows aneurismal dilatation. It divides into three ter- 

 minal branches, which from their direction are distinguished as left, 

 right, and anterior. The left distributes its branches to the whole of 

 the small intestine except a few inches at the beginning of the duo- 

 denum and about two feet at the end of the ileum ; the right supplies 

 the terminal portion of the ileum, the entire caecum, and the double 

 colon as far as the pelvic flexure ; and the anterior is distributed to 

 the double colon beyond the pelvic flexure, and to the first few inches 

 of the single colon. It is an assistance to the memory to study the 

 different branches in the order of their distribution to the intestine, 

 taking first those that supply the most anterior segment of the tube. 



1. The Left Branch of the anterior mesenteric artery is no sooner 

 detached than it splits up into about fifteen or twenty arteries, which 

 pass between the layers of the mesentery to supply the small intestine. 

 Indeed, the left branch can scarcely be said to exist, for these arteries of 

 the small intestine seem to spring from a common point of the anterior 

 mesenteric trunk. As each artery approaches the intestine it bifurcates, 

 each branch inosculating with the corresponding branch of an adjacent 

 artery to form an arch. From the convexity of these arches smaller 

 vessels pass to each side of the intestine, and anastomose round it. At 

 the anterior part of the tube two sets of superposed arches are formed 

 before the ultimate vessels to the intestine are detached. The branch 

 which is most anterior in point of distribution anastomoses with the 

 duodenal branch of the cceliac axis, while the one which is most pos- 

 terior anastomoses with the ileo-caecal artery from the right branch 

 of the anterior mesenteric. 



2. The Right Branch of the anterior mesenteric artery divides into 

 four vessels, viz., the ileo-caecal, the superior caecal, the inferior caecal, 

 and the direct colic arteries. 



a. The Ileo-ccecal Artery supplies the terminal portion of the ileum 

 (about two feet in length), and inosculates with the last of the arteries 

 from the left branch. 



b. The Superior Ccecal Artery, in the present inverted position of the 

 intestines, passes beneath the termination of the ileum to run along one 

 of the longitudinal muscular bands of the caecum. It sometimes gives 

 off the ileo-caecal artery as a collateral branch, and at the point of the 

 caecum it anastomoses with the next vessel. It gives off branches right 

 and left to the walls of the caecum. 



c. The Inferior Coecal Artery, in the present position of parts, passes 

 above the termination of the ileum to run along another of the muscular 

 bands of the caecum. Besides collateral branches to the main portion of 

 the bowel, it gives off the artery of the arch, which follows the concavity 



