DISSECTION OP THE ABDOMEN. 315 



of the caecal crook and terminates on the beginning of the double 

 colon. 



d. The Direct or Right Colic Artery. — This is a large vessel, receiving 

 the first of these designations because the course of its blood stream is 

 the same as that of the alimentary matters in the bowel. It supplies, 

 by right and left collateral branches, the 1st and 2nd portions of the 

 double colon, and anastomoses at the pelvic flexure with the retrograde 

 colic artery. 



3. The Anterior Branch of the anterior mesenteric artery divides after 

 a very short course into two vessels of unequal size, viz., the retrograde 

 colic artery and the first artery of the small colon. 



a. The Retrograde or Left Colic Artery, much the larger of the two, 

 supplies successively the 4th and 3rd portions of the double colon, 

 running parallel to the direct colic artery, but carrying its blood in a 

 direction counter to the course of the alimentary matters in the 

 intestine. 



b. The First Artery of the Small Colon supplies a short piece at the 

 beginning of that bowel. It is included between the layers of the colic 

 mesentery, and anastomoses with the first branch of the posterior 

 mesenteric artery. 



Directions. — To display the posterior mesenteric artery, the small 

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

 Plate 42. 



The PosTERioE Mesenteric Aetery is a much smaller vessel than the 

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

 ofl" at the 4th lumbar vertebra. It passes in a curved direction between 

 the layers 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 supply 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 2Mrtal system. These veins for the most part run in close company 

 with the arteries, and receive the same names. 



The Anteeior Mesenteric Vein is a very large vessel having tribut- 

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

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

 constitute the vena portse. 



The Posterior Mesenteric Vein has its roots in the hemorrhoidal 

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



