The inferior mesenteric vein (c, fig.279.), —the origin of which also corresponds to the region supplied by the artery of the same name — generally commences as a single trunk at or near the border of the pelvis. From hence it ascends almost vertically, but with a slight inclination inwards, beneath the perito neum, and on the psoas muscle ; until, finally, it crosses under the transverse meso-colon, to end by a junction with the splenic vein (d, fig. 279.). In the latter part of this ascent, it is of course unaccompanied by the inferior mesenteric artery : and even below where this vessel is given off from the aorta, the artery and vein diverge so as to be compara tively distant from each other. Its junction with the splenic vein (d,fig. 279.), is usually about one or two inches from the point where this meets with the superior mesenteric vein. But it occasionally approaches much more closely to the latter vessel, or even joins with it prior to its union with the splenic to form the portal vein.
The branches of both these mesenteric veins resemble those of the corresponding arteries in their nutnber and size, and in the remarkable freedom of' their anastomosis. And this copious and frequent inosculation,—which coincides with an absence of all valves, — not only holds good of the several primary branches which converge into the portal vein, but also applies in some degree to those smaller ramifications, by which the portal sys tem inosculates with the general venous sys tem at the two extremities of the alimentary tube. Thus many of the smaller veins at the lower part of the cesophagus communicate with both the azygos and portal veins. While the lowest branches of the inferior mesenteric vein establish a similar and much more extensive anastomosis of the two systems, by their junc tion with a dense venous network—the bee morrhoidal encircles the lower part of the rectum, and gives origin to the middle and inferior hcemorrhoidal branches of the internal and external iliac veins.
The foregoing peculiarities in the vascular arrangements of the human alimentary canal are at present only susceptible of a very im perfect explanation.
As regards the arteries, their great number and size, and their large anastomosing chan nels, would probably be attended by several ad vantages. The variety of these channels would concede to the circulation, not only a large supply of blood, but one such as no ordinary • local accident could at all interfere with.* The
muscular fibre contained in their walls would allow these numerous tubes to exercise an unusual control over the amount of blood them unite to form two chief trunks, the superior and the inferior mesenteric veins. 'While others open directly into the splenic vein ; or into the vena portce, which is formed by the junction of it and these mesenteric veins.
The superior mesenteric vein (b, fig. 279.), which receives the venous blood from that portion of intestine supplied by the artery of the same name, travels for some distance in company with the latter vessel ; lying on its right side, and somewhat superficially to it, and surrounded by very numerous lacteals and nerves. But near the lower border of the they from time to time convey. While on very simple hydraulic principles, these re peated cross branches would so diminish the various resistances (of impact and adhesion) offered to the blood within the vessels, as to permit, either a greater rapidity of the cur rent, and hence a more rapid renewal of the mass of blood contained in the capillaries ; or a more forcible pressure upon the latter fluid ; or even both of these effects simultaneously.
It is perhaps a corroboration of the above conjectures, to trace their close relation to those which might be gathered from an inde pendent consideration of the circumstances of the portal system. The trunk vein of this set of vessels leads to a second set of capillaries in the liver ; through which there is nothing to propel the portal blood, save the force of the heart, aided by a small amount of suc tion, which the thorax exerts as it ex pands during inspiration. And hence, how ever large a quantity of the original cardiac pressure may be again amassed by the con vergence of the various intestinal veins, still there can be no doubt that at least so much of it will have been lost, as to require all the aid which the above disposition of the arteries can afford it. But in spite of all such assistance, it seems probable, that the current of the portal blood is both far slower, and much more feeble, than that which occupies any of the arteries. Still it is no doubt quite sufficient for the exigencies of the circulation in the liver ; and especially for that secretion of bile, to which the various details of the organization of this gland chiefly refer.