THE MESENTERIC ARTERIES. 463 



convex or left side of the vessel. They are usually twelve or more in number, and 

 are all included between the layers of the mesentery. They run parallel to one 

 another for some distance, and then divide into two branches, each of which forms 

 an arch with the neighbouring branch. From the first set of arches other branches 

 issue, which divide and communicate in the same way, until finally, after forming 

 from three to five such tiers of arches, the smaller as they are nearer to the intestine, 

 the ultimate divisions proceed directly to the intestine, spreading upon both sides, 

 and ramifying in its coats. Small offsets are also furnished to the glands and other 

 structures between the layers of the mesentery. 



(c) The colic branches arise from the right or concave side of the artery, and are 

 usually three in number. 



1. The ileo-colic artery, the first in order from below upwards, inclines down- 

 wards and to the right side, towards the caecum, and divides into two branches : one 

 of these descends to inosculate with the termination of the mesenteric artery itself, 

 and to form an arch, from the convexity of which branches proceed to supply the 

 junction of the small with the large intestine, and the caecum and its appendix ; 

 the other division ascends and inosculates with the next mentioned branch. The 

 ileo-colic artery is not always distinct from the termination of the superior 

 mesenteric. 



2. The right colic artery passes transversely towards the right side, beneath the 

 peritoneum, to the middle of the ascending colon, opposite which it divides into two 

 branches : of these one descends to communicate with the ileo-colic artery, while the 

 other ascends to join in an arch with the middle colic. This artery and the ileo-colic 

 often arise by a common trunk. 



3. The middle colic artery runs forwards between the layers of the transverse meso- 

 colon towards the transverse colon, and divides in a manner exactly similar to that 

 of the vessels just noticed. One of its branches inclines to the right, where it inos- 

 culates with the preceding vessel ; the other passes to the left side, and forms a 

 similar communication with the left colic branch, derived from the inferior mesen- 

 teric artery. From the arches of inosculation thus formed, small branches pass to 

 the colon for the supply of its coats. 



Those branches of the superior mesenteric artery which supply the ascending 

 colon have a layer of peritoneum only on their anterior aspect ; the others lie 

 between two strata. 



Varieties. The number of the branches of the superior mesenteric artery, both intestinal 

 and colio, is by no means constant. It also frequently gives off accessory branches to the 

 neighbouring viscera ; of these that to the liver is the most common. An offset of this artery 

 may replace the gastro- duodenal or its chief branch, the right gastro-epiploic, or it may give 

 accessory pancreatic and splenic branches (Hyrtl), or the artery to the gall-bladder. It has 

 also been seen to give off the left colic artery, and in one case in which the inferior mesen- 

 teric was absent also the superior hsemorrhoidal (Fleischmann). A rare anomaly is the 

 presence of an omphalo-mesenteric artery, arising either from the main stem or from one of 

 the branches of the superior mesenteric. In one case it ran directly to the umbilicus, where it 

 gave a branch to the urachus (Haller). In another it reached the anterior wall of the 

 abdomen rather below the umbilicus, and after giving a branch to the rectus, which anasto- 

 mosed with the deep epigastric, it terminated by ascending in the round ligament, and 

 forming a capillary network in the falciform ligament of the liver (Hyrtl). 



The inferior mesenteric artery (iii-iv) arises from the front of the aorta 

 between one and two inches above its bifurcation, and supplies the lower half of the 

 large intestine. Inclining slightly to the left, it passes downwards close to the aorta, 

 gives off branches to the descending colon and the sigmoid flexure, and is continued 

 under the name of superior hcemorrhoidal artery over the left common iliac vessels 

 to the back of the rectum. 



Branches. (a) The left colic.- artery is directed to the left side behind the peri- 



