Colic arteries. The superior mesenteric sends off three, sometimes only two, branches from its concavity, called right colic arteries, dis tinguished as superior or colica media, middle or colica dextra, and inferior or ileo-colic ; when there are but two, the superior and middle form but a single trunk ; the inferior is generally distinct.
The right superior colic or colica media arises a few inches distant from the origin of the supe rior mesenteric; it passes forwards between the layers of the meso-colon towards the middle of the transverse colon, and divides into a right rs 0 and left branch ; the right follows the right part of the transverse colon, and anastomoses with the superior branch of the colica dextra ; the left branch follows the left portion of the transverse colon, and communicates with the left colic branch of the inferior mesenteric artery.
The colica dextra or middle right colic artery arises close to the colica media, sometimes from a trunk common to both, and sometimes from the ileo-colic. After its origin it passes for wards, upwards, and to the right in the meso colon towards the ascending colon, and divides into two branches ; one superior ascends to anastomose with the right branch of the colica media, the other descends along the concavity of the ascending colon, and communicates with the ascending branch of the ileo-colic.
The ileo-colic, cwcal, or inferior right colic passes downwards, and to the right towards the ccecum, and then divides into three branches ; the first ascends in the meso-colon, and anas tomoses with the descending branch of the colica dextra ; the second communicates in the mesentery with the termination of the superior mesenteric; and the third, arising in the angle between the two preceding, passes behind the junction of the ileum with the ccecum : at this place it gives off a branch which forms a small arch in the mesentery of the vermiform appen dix, and then divides into two branches, one of which passes upwards on the colon, and the other descends on the ccecum. The colic arte ries, by their anastomoses with each other, form arches, from the convexities of which, turned towards the intestine, numerous branches arise ; each of these again divides into two, which with the contiguous vessels form smaller arches, and straight branches finally arise from the ultimate arches, which, passing on either side of the intestine, include it between them, and anastomose on its convex edge.
In the foetus we have the omphalo-mesen teric artery arising from the superior mesen teric ; this vessel, which passes along the um bilical cord to the vesicula alba, becomes obliterated towards the end of the second month of gestation.
The inferior mesenteric artery arises from the front of the aorta to its left side, at about an inch or an inch'and a half above the origins of the primitive iliacs ; it sometimes arises from the left primitive iliac, especially when the aorta has divided higher than usual ; instances of the absence of this artery are very rare, but interest ing as presenting an example of the normal condition in birds and reptiles, in which the inferior mesenteric artery is much reduced in size or entirely absent.
The inferior mesenteric artery runs obliquely downwards and to the left, and gets between the layers of the left iliac meso-colon, where it divides into many branches, distributed to the left portion, and sigmoid flexure of the colon and the rectum ; the superior branches are dis tributed to the descending portion and sigmoid flexure of the colon, and are called left colic arteries, while the lower branches go to the rectum under the name of superior hamor rhoidal arteries. The left colic arteries are three in number, the superior, middle, and in ferior. The superior left colic is the largest of the three ; it arises from the inferior mesen teric immediately after its origin, passes trans versely to the left, and divides near the left lumbar colon into two branches, one of which ascends to the transverse meso-colon, and anastomoses with the colica media of the supe rior mesenteric ; the other branch descends towards the left iliac meso-colon, where it anastomoses with the ascending branch of the middle left colic.
The middle left colic is sometimes a branch of the preceding. It divides into two branches; one ascends along the left colon, and anas tomoses with the descending branch of the left superior colic ; the other, inferior, smaller com municates with the ascending branch of the left inferior colic.