30G THE ANATOMY OP THE HORSE. 



colon must be spread out over the left flank after the manner of 

 Plate 42. 



The P0.STER10R Mesenteric Artery is a much smaller vessel than the 

 anterior. It is a branch of the abdominal aorta, from which it is given 

 off at the -ith lumbar vertebra. It passes in a curved direction between 

 the laj'ers of the colic mesentery and meso-rectum, and terminates near 

 the anus in vessels which supply the end of the intestinal tube. 

 From the convexity of its curve, which is directed downwards, about 

 twelve or fourteen branches pass to supply the small colon (except a few 

 inches at its beginning) and the rectum. The branches which supplj' 

 the first half of the small colon divide and form arches by anastomosis 

 in the mesentery, close to the bowel ; but the more posterior branches 

 do not anastomose until they perforate the intestinal wall. 



The Intestinal Veins. — The blood which is brought to the intestines by 

 the arteries just considered is carried away by vessels belonging to 

 the portal system. These veins for the most part i-un in close company 

 witli the arteries, and receive the same names. 



The Anterior Mesenteric Vein is a veiy large vessel having tribut- 

 aries which coiTespond almost exactly to the divisions of the artery of 

 the same name. It joins the splenic and posterior mesenteric veins to 

 constitute the vena portso. 



Tlie Posterior Mesenteric Vein has its roots in the hsemorrhoidal 

 veins around the termination of the rectum, which veins, on the other 

 hand, communicate with the internal pudic vein. After receiving blood 

 from the walls of the rectum and small colon, the posterior mesenteric 

 vein forms by union with the splenic a very short trunk which joins 

 the anterior mesenteric to form the vena portte. 



Lymphatic Vessels of the Intestine. — In an ordinary dissecting-room 

 subject the l^nnphatic vessels will not be visible imless the animal is 

 emaciated and has been killed shortly after a meal, in which case the 

 mesenteric vessels may be seen without dissection. They will be recog- 

 nised as vessels with very thin walls and milky contents, coui-sing 

 between the layers of the mesentery, from the intestine towards the 

 anterior mesenteric artery. The Ijonpathic vessels of the small intestine 

 are called lacteals. 



The Lymphatic Glands of the Intestine are very numerous. Those 

 of the small intestine are chiefly aggregated in the form of a cluster of 

 about tliirty included between the layers of the mesentery, near the 

 anterior mesenteric artery ; but a number are placed lower down in the 

 mesentery, along the course of the ileo-caecal artery. The glands of the 

 caecum are distributed in the form of two chains along the track of the 

 superior and inferior cajcal arteries, and numerous glands are similarly 

 placed on the colon along the course of the direct and retrograde colic 

 arteries. Those of the small colon and rectum are, for the most part, 



