The further course of all these branches towards the small and large intestine affords a remarkable % instance of an arterial ana stomosis ; such as is almost unparalleled in the whole of the body for the freedom and frequency of its communications, and the size of thelvessels by which they are effected. Each of the primary branches just alluded to bifurcates : and its two resulting branches unite with those above and below them, so as to form a set (g,fig.277.) of arterial arches ; from the convexity of which spring new trunks, to divide and inosculate in a similar manner. This arrangement, which prevails throughout all the mesenteric branches that supply the intestine, is carried to such an ex tent in the jejunum and ileum, as to offer, in many parts, four or five successive sets of arches; which become smaller and more nu merous as they approach the bowel, and finally give off the minute arterial ramifications that enter and traverse the intestinal coats.
On reaching the intestine itself, the greatly diminished arteries break up into still smaller capillary branches. These inosculate freely with each other, by comparatively large branches of communication ; and thus unite and anastomose to form a dense stratum or flattened network of vessels, which occupies the layer of loose areolar tissue that separates the muscular from the mucous coat. This vas cular plexus gives off, on the one hand, the vessels of the mucous membrane ; and on the other, the not very numerous branches which run between and amongst the unstriped bun dles of the muscular coat.
The inferior mesenteric artery (b, fig. 278.), which supplies the descending and sigmoid portions of the colon, and the whole of the rectum, is also a branch from the aorta. It arises from the front and left side of this vessel (a, fig. 278.), about an inch below the place where it gives off the left renal artery, and nearly the same distance above its bi furcation into the two iliac vessels. From this origin it is directed downwards and slightly outwards, lying successively on the aorta, the left psoas muscle, and the left common iliac artery. During this part of its course, it lies at some distance from the in testine. But below where it crosses the com mon iliac vessels, it occupies the double fold of peritoneum (222eso-rectum) that attaches the rectum to the pelvis. This terminal portion of the vessel, which is called the superior bre morrhoidal artery (e,.fig. 278.), is continued to a point about opposite to the middle of the sacrum ; where it ends by bifurcating into two branches, which ramify on the opposite sides of the bowel, and are distributed to its various coats. These branches inosculate freely with the ramifications of the nziddle hxmorrhoidal artery, which is itself given off to the rectum by the internal iliac artery, or some of its branches.
The only named branches of the inferior mesenteric are the left colic or arteria colica sinistra (c, .fig. 278.), and the artery to the sigmoid flexure (d,,fig. 278.). The former of these two vessels passes upwards and out wards, across the psoas muscle and left kidney, to reach the descending colon at about the middle of its height. The latter, which is
sometimes double, also crosses the psoas, to enter the short nieso-colon which attaches the sigmoid flexure of the bowel. The fur ther distribution of both these arteries pre cisely recalls that of the similar colic branches from the superior mesenteric : each bifurcating into two branches ; which, by uniting with the similar trunks above and below, form the origin of a set of arches that ramify in a second and third series. And as the union of the upper twig of the colica sinistra with the lower or left branch of the colica media unites the superior and inferior mesenteric arte ries by a large anastomosing vessel (g,fig. 278.), all the arches of both these trunks have the most complete anastomosis with each other. So that it would be easy to trace out a con tinuous arterial channel of large size ; which begins as the superior mesenteric, and passes through the ileo-colic, left colic, median colic, and sigmoid branches, to end in the superior hmmorrhoidal artery.
Veins of the intestines.— The veins of the intestinal canal are chiefly characterized by the fact, that the trunks formed by their convergence and union do not open into the right auricle, like the veins of the body gene rally ; but undergo a second ramification and distribution, in their course from the capil laries of the intestine to the right side of the heart. This arrangement of course influences their distribution at two successive stages of their course. In the first place, their 'larger trunks fail to exhibit that close correspond ence with the arterial channels which is seen in the case of most other parts of the body. And, secondly, instead of seeking the large vessels on the spine, these trunks converge into a single channel, the portal vein (a, fig. 279.), which passes upwards to the liver at some distance from the aorta and primary in testinal branches.* The veins of the intestines commence by a dense network, that receives the minute venous radicles into which the capillaries of the mucous and muscular coats return their blood. This plexus has the same submucous situation, and flattened shape, as the cor responding arterial network alreauy men tioned; but,like the venous system in general, is composed of more numerous and larger branches. It gives off a number of veins ; which leave the intestine, and gradually unite into the vessels that converge to form the various trunks. These branches have a tole rable correspondence with the primary rami fications of the arteries from the cceliac axis and the two mesenteric vessels. Many of third portion of the duodenum, it swerves towards the right side, from what was hitherto an almost vertical course upwards ; and after crossing in front of the duodenum at nearly a right angle, ends by joining the splenic vein behind the pancreas. This junction gives rise to the portal trunk (a,fig. 279.).