380 



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 (6, 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 



Fig. 278* 



STOMACH AND INTESTINE. 



Distribution of the inferior mesenteric artery to the 

 large intestine. 



a, abdominal aorta ; b, inferior mesenteric artery ; 

 c, left colic artery ; d, artery to the sigmoid flexure ; 

 e, superior hsemorrhoidal artery; /, middle colic 

 artery; g, large communicating branch between the 

 left and middle colic artery, sf, sigmoid flexure 

 of the colon ; r, rectum, (t c, a c, p, du, as in 

 fig. 279.) 



* In several of the preceding microscopic figures, 

 the artist has been indebted to Koelliker's beautiful 

 woodcuts for some details, which require this spe- 

 cific acknowledgment. 



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 (mesa-rectum) that attaches the 

 rectum to the pelvis. This terminal portion 

 of the vessel, which is called the superior hce- 

 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 middle 

 hcemorrhoidal 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 meso-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 

 haemorrhoidal 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 (, fig. 

 279.), which passes upwards to the liver at 

 some distance from the aorta and primary in- 

 testinal branches,* 



* See Art. VENOUS SYSTEM. 



