304 TUE AX ATOM V OF THE HORSE. 



and pull upon the terminal part of the jejunum. On each side of the 

 point at which the latter perforates the csecal crook, there will be found 

 a recess, the posterior (in this position) being the deeper. 



4. Another considerable pocket will readily be found on the concave 

 side of the Cfscal crook, being formed by the peritoneum in passing 

 between the caecum and the beginning of the double colon. 



5. Another but much smaller pocket will be found in the cavity of 

 Winslow, above and in front of the first few inches of the single colon. 



Directions. — For the display' of the mesenteric vessels and the sympa- 

 thetic nerve, the intestines should first be disposed after the manner of 

 Plate 41. When well injected, the arteries require but little dissection, 

 and they are closely accompanied by the veins and nerves. The arteries 

 of the caecum and colon should be taken where most conspicuous, and 

 traced in both directions. Each of these vessels must be carefully dis- 

 sected up to its point of origin, but only two or three of the arteries of 

 the small intestine need be fully dissected. The whole intestinal tube 

 with the exception of a shoi't piece of the duodenum next the stomach, 

 is supplied by the anterior and posterior mesenteric arteries, which are 

 branches of the abdominal aorta. The first supplies the whole of the 

 small intestine except the piece of duodenum specified; and it also sup- 

 plies the caecum, the large colon, and a few inches of the beginning of 

 the small colon. The remainder of the small colon, and the rectum are 

 supplied by the posterior mesentei'ic artery. 



The Anterior Mesenteric Artery (Plate 41) comes off from the 

 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 shews aneurismal dilatation. It divides into three terminal 

 branches, which from their direction ai'e distinguished as left, fight, 

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

 intestine except a few inches at the beginning of the duodenum and 

 about two feet at the end of the ileum; the right su])plies the terminal 

 portion of the ileum, the entire cfccum, and the double colon as f\xr as 

 the pelvic flexure; and the anterior is distributed to the double colon 

 Ijeyond 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 distril)ution 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 laj'ers 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 tlie anterior 

 mesenteric trunk. As each artery appi'oaches the intestine, it bifurcates, 

 each branch inosculating with the corresponding branch of an adjacent 



