STOMACH AND INTESTINE. 



361 



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 already 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 coeliac axis 

 and the two mesenteric vessels. Many of 



Fig. -279. 



Branches of the portal vein. 



a, trunk of the portal vein; b, superior mesenteric 

 vein; c, inferior mesenteric vein; d, splenic vein, 

 joined by the, e, gastro-epiploic and pyloric veins ; f, 

 pancreatico-duodenal veins ; </, branch of the portal 

 trunk to the left lobe of the liver ; //, similar branch 

 to the right lobe. 



(The remaining letters indicate as in the pre- 

 ceding figures.) 



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 porfee, which is formed 

 by the junction of it and these mesenteric 

 veins. 



The superior mesenteric vein (b, Jig. 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 bordtr of the 



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, Jig. 279.). 



The inferior mesenteric vein (c, Jig. 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 ((/, 

 fg. 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,Jig. 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 number 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 svs- 

 tem at the two extremities of the alimentary 

 tube. Thus many of the smaller veins at the 

 lower part of the oesophagus communicate with 

 both the azvgos and portal veins. While the 

 lowest branches of the inferior mesenteric vein 

 establish a similar and much more extensive 

 anastomosis of the two systems, bv their junc- 

 tion with a dense venous network the hcc- 

 morrhoidal plexus which 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 



* Compare the remarks on the vessels of the 

 stomach at p. 327. 



