This artery [a. mesenterica inf. French : Artère petite mésentérique], much smaller than the superior mesenteric, supplies the left side of the colon, and the greater part of the rectum. It arises from the aorta, between an inch and two inches above the bifurcation of that trunk. The inferior mesenteric artery deviates to the left side in the direction of the left iliac fossa, from which point it descends between the layers of the meso-rectum into the pelvis, and under the name of "superior haemorrhoidal" artery, runs down behind the rectum. It lies at first close to the aorta, on its left side, and then crosses over the left common iliac artery. Its branches are the following : The left colic artery (colica sinistra), is directed to the left side behind the peritoneum, and across the left kidney to reach the descending colon.
It divides into two branches, and forms a series of arches in the same way as the colic vessels of the opposite side. One of these two branches passes upwards along the colon, and inoculates with the descending branch of the middle colic ; whilst the other descends towards the sigmoid flexure, and anastomoses with the sigmoid artery. The sigmoid artery, runs obliquely downwards to the sigmoid flexure of the colon, where it divides into branches, some of which incline upwards and form arches with the preceding vessel, others turn downwards to the rectum and anastomose with the following branch. Instead of a single sigmoid artery, two or three branches are sometimes present.
The superior hemorrhoidal artery, (haemorrhoidalis, interna, — Haller, [s. sup.]), the continuation of the inferior mesenteric, passes into the pelvis behind the rectum, at first in the fold of the meso-rectum, and then divides into two branches which ramify one on each side of the intestine, extending to near its lower end, and anastomosing with the middle and inferior haemorrhoidal arteries. In this place it may be remarked that the arteries distributed to the alimentary canal communicate freely one with the other over the whole length of that canal. The arteries of the great intestine, derived from the two mesenteric arteries, form a range of vascular arches along the colon and rectum, at the lower end of which they anastomose with the middle and inferior haemorrhoidal arteries, given from the internal iliac and pudic arteries. The branches from the left side of the superior mesenteric form another series of arches along the small intestine, which is connected with the former by the ileo-colic artery. Further, a branch of the superior mesenteric joins upon the duodenum with the pancreatico-duodenal artery ; the latter, at its commencement, is in pyloric artery and so likewise, through the coronary artery of the stomach and its ascending branches, a similar connection is formed with the branch oesophageal arteries, even up to the pharynx.
From Quain's anatomy.