614 THE ARTERIES. 



transverse and descending portions of the duodenum, anastomosing with the 

 superior pancreatico-duodenal artery. 



The vasa intestini tenuis arise from the convex side of the superior mesenteric 

 artery. They are usually from twelve to fifteen in number, and are distributed to 

 the jejunum and ileum. They run parallel with one another between the layers 

 of the mesentery, each vessel dividing into two branches, which unite with a sim- 

 ilar branch on each side, forming a series of arches the convexities of which are 

 directed toward the intestine. From this first set of arches branches arise, 

 which again unite with similar branches from either side, and thus a second series 

 of arches is formed ; and from these latter, a third, and a fourth, or even fifth, 

 series of arches is constituted, diminishing in size the nearer they approach the 

 intestine. From the terminal arches numerous small straight vessels arise which 

 encircle the intestine, upon which they are distributed, ramifying thickly between 

 its coats. 



The ileo-colic artery is the lowest branch given off from the concavity of the 

 superior mesenteric artery. It descends between the layers of the mesentery to 

 the right iliac fossa, where it divides into two branches. Of these, the inferior 

 one inosculates with the lowest branches of the vasa intestini tenuis, from the 

 convexity of which branches proceed to supply the termination of the ileum, the 

 caecum and appendix caeci, and the ileo-caecal valve. The superior division inos- 

 culates with the colica dextra and supplies the commencement of the colon. 



The colica dextra arises from about the middle of the concavity of the superior 

 mesenteric artery, and, passing behind the peritoneum to the middle of the 

 ascending colon, divides into two branches a descending branch, which inoscu- 

 lates with the ileo-colic ; and the ascending branch, which anastomoses with the 

 colica media. These branches form arches, from the convexity of which vessels 

 are distributed to the ascending colon. The branches of this vessel are covered 

 with peritoneum only on their anterior aspect. 



The colica media arises from the upper part of the concavity of the superior 

 mesenteric, and, passing forward between the layers of the transverse meso-colon, 

 divides into two branches, the one on the right side inosculating Avith the colica 

 dextra ; that on the left side, with the colica sinistra, a branch of the inferior 

 mesenteric. From the arches formed by their inosculation branches are distrib- 

 uted to the transverse colon. The branches of this vessel lie between two layers 

 of peritoneum. 



The Inferior Mesenteric Artery (Fig. 371). 



In order to expose this vessel draw the small intestines and mesentery over to the right 

 side of the abdomen, raise the transverse colon toward the thorax, and divide the peritoneum 

 covering the left side of the aorta. 



The Inferior Mesenteric Artery supplies the descending and sigmoid flexure of 

 the colon and the greater part of the rectum. It is smaller than the superior 

 mesenteric, and arises from the left side of the aorta, between one and two 

 inches above its division into the common iliacs. It passes downward to the left 

 iliac fossa, and then descends, between the layers of the meso-rectum, into the 

 pelvis, under the name of the superior hcemorrhoidal artery. It lies at first in close 

 relation with the left side of the aorta, and then passes as the superior haemor- 

 rhoidal in front of the left common iliac artery. Its branches are the 



Colica Sinistra. Sigmoid. 



Superior Haemorrhoidal. 



The colica sinistra passes behind the peritoneum, in front of the left kidney, 

 to reach the descending colon, and divides into two branches an ascending 

 branch, which inosculates with the colica media; and a descending branch, which 

 anastomoses with the sigmoid artery. From the arches formed by these inoscu- 

 lations branches are distributed to the descending colon. 



The sigmoid artery runs obliquely downward across the Psoas muscle to the 



