932 THE VASCULAK SYSTEM. 



number, are separable into two groups, jejunal and Heal ; they spring from the convexity 

 of the superior mesenteric artery, and pass obliquely forwards and downwards, between 

 the layers -of the mesentery, each dividing into two branches which anastomose with 

 adjacent branches to form a series of arcades, from which secondary branches are given 

 off. This process of division and union is repeated three or four times in the case of the 

 ileal arteries ; thus four or five tiers of arches are formed, from the most distal of which 

 terminal branches are given off to the walls of the jejunum and ileum. Branches from 

 the successive arcades are also given off to the mesenteric lymph glands. The terminal 

 branches anastomose together in the walls of the gut, forming a vascular network, which 

 communicates above with the inferior pancreaticb-duodenal artery and below with the 

 terminal branch of the superior mesenteric trunk. The arterial loops and branches are 

 accompanied by corresponding veins, lymphatics, and nerves. 



(b) The inferior pancreatico-duodenal artery. This branch arises either from the 

 trunk of the superior mesenteric, at the upper border of the third part of the duodenum, 

 or from the first jejunal branch. It runs to the right, between the head of the 

 pancreas and the third part of the duodenum, and terminates by dividing into two 

 branches, anterior and posterior, which ascend, the former in front, and the latter behind 

 the head of the pancreas ; they supply the head of the pancreas, the descending and 

 inferior parts of the duodenum, and they anastomose. with the similar branches of the 

 superior pancreatico-duodenal artery. 



(c) The middle colic artery is a large branch which springs from the front of the 

 superior mesenteric as it enters the root of the mesentery. It runs downwards and < 

 forwards, in the transverse mesocolon, and terminates by dividing into two branches, 

 right and left, which anastomose respectively with the right and left colic arteries, 

 forming arcades. Secondary and tertiary loops are sometimes formed and the terminal 

 branches are distributed to the walls of the transverse colon. 



(d) The right colic artery springs from the right or concave side of the superioi 

 mesenteric, either alone or in the form of a common trunk which divides into right colic 

 and ileo-colic branches. It runs to the right, posterior to the peritoneum on the posterioi 

 wall of the abdomen, and in front of the right psoas major, the ureter, and the interna 

 spermatic vessels, towards the ascending colon, near which it divides into ai 

 ascending and a descending branch. The former passes upwards, and anastomoses, in th( > 

 transverse mesocolon, with the middle colic artery. The latter descends to anastomose 

 with the upper branch of the ileo-colic, and from the loops thus .formed branches are dis 

 tributed to the walls of the ascending colon and the beginning of the transverse colon. 



(e) The ileo-colic artery arises by a common trunk with the right colic, or separately 

 from the right side of the superior mesenteric, and passes downwards and to the right 

 behind the peritoneum, towards the lower part of the ascending colon, where it terminate: 

 by dividing into an ascending branch which anastomoses with the lower branch of th< 

 right colic, and a descending branch which communicates with the colic termiua 

 branches of the superior mesenteric trunk. 



(/) Terminal. The lower end of the superior mesenteric artery divides into fiv 

 branches (i.) ileal, (ii.) appendicular, (iii.) anterior ileo-csecal, (iv.) posterior ileo-caecal 

 and (v.) colic. 



The ileal branch, turns upwards and to the left in the lowest part of the mesentery, an* 

 anastomoses with the intestinal arteries. The appendicular branch passes behind the termina 

 portion of the ileum, and through the mesentery of the vermiform process to the vermiform process 

 upon which it ends. The anterior ileo-caecal crosses the front of the ileo-caecal junction in 

 fold of peritoneum ; the posterior ileo-caecal crosses the ileo-caecal junction posteriorly, and th 

 colic runs upwards to the ascending colon. The ileo-caecal branches supply the walls of th 

 caecum, and, like the colic branch, anastomose with branches of the ileo-colic artery. In som 

 cases the majority or all of the above terminal branches spring from the ileo-colic. 



3. Arteria Mesenterica Inferior. The inferior mesenteric artery (Fig. 77 

 arises from the front of the aorta towards the left side, 37 mm. above the bifurca 

 tion ; it passes downwards and slightly to the left, lying posterior to the peritoneui 

 and anterior to the left psoas major muscle, to the upper and left border of th 

 left common iliac artery, where it becomes the superior hseniorrhoidal. 



Branches. (a) The left colic artery arises from the left side of the inferic 

 mesenteric near its origin, and almost immediately divides into an upper and a low< 

 branch. The upper branch runs upwards and to the left towards the left colic flexure, and 1 

 the lower pole of the left kidney, where it divides into (i.)a branch which enters the transver 

 mesocolon, and, turning medially, terminates by joining the left branch of the middle col 



