802 THE VASCULAR SYSTEM. ~ 
the right iliac fossa, and crossing in this part of its course obliquely in front of the 
aorta, the lower part of the inferior vena cava, the right ureter, and the right psoas 
muscle. At its origin it lies behind the body of the pancreas and the splenic vein ; 
where it passes in front of the duodenum it is crossed anteriorly by the transverse 
colon, and in the lower part of its extent it is concealed by coils of small intestine. 
Branches.—It gives off numerous branches which supply the duodenum and the 
pancreas in part, the whole of the small intestine below the duodenum, and the large 
intestine nearly as far as the splenic flexure. 
The branches are as follows :— 
(a) Rami intestini tenuis (aa. intestinales), or branches to the small intestine, 
varying from ten to sixteen in number; 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; 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 glands. The terminal branches anastomose together in the walls of the gut, 
forming a vascular network which communicates above with the inferior pancreatico- 
duodenal artery and below with the terminal branch of the superior mesenteric trunk. 
The vascular loops and branches are accompanied by corresponding veins, lymphatics, and 
nerves. 
(b) The inferior pancreatico-duodenal artery (a. pancreatico-duodenalis inferior). It 
arises either from the trunk of the superior mesenteric, at the upper border of the third part 
of the duodenum, or from the first of the rami intestini. 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 third part of 
the duodenum, and they anastomose with the similar branches of the superior pancreatico- 
duodenal artery. 
(c) The middle colic artery (a. colica media) is a large branch which springs from the 
front of the superior mesenteric as it enters the root of the mesentery. It runs down- 
wards 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 from which secondary and tertiary loops are derived, the terminal 
branches being distributed to the walls of the transverse colon. 
(dq) The right colic artery (a. colica dextra) springs from the right or concave side of 
the superior mesenteric, either alone or in the form of a common trunk which divides 
into right and ileo-colic branches. It runs to the right, behind the peritoneum on the 
posterior wall of the abdomen, and in front of the right psoas, the ureter, and the 
spermatic or ovarian vessels, towards the ascending colon, near which it divides into an 
ascending and a descending branch. The former passes upwards, and anastomoses in the 
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 (a. ileo-colica) arises by a common trunk with the right colic, 
or separately from the right side of the superior mesenteric, and passes downwards and 
outwards, behind the peritoneum, towards the lower part of the ascending colon, where 
it terminates by dividing into an ascending branch which anastomoses with the lower 
branch of the right colic, and a descending branch which communicates with the ileo-cecal 
terminal branches of the superior mesenteric trunk. 
(f) Terminal.—The lower end of the superior mesenteric artery divides into five 
branches—(i.) ileal, (ii) appendicular, (iii.) anterior ileo-cwcal, (iv.) posterior ileo-caecal, 
and (v.) colic. 
The ileal branch (a. ilea) turns upwards and to the left in the lowest part of the mesentery, 
and anastomoses with the rami intestini. The appendicular branch (a. appendicularis) passes 
behind the terminal portion of the ileum, and through the meso-appendix to the vermiform pro- 
cess, upon which it ends. The anterior ileo-cecal crosses the front of the ileo-caecal junction in 
a fold of peritoneum ; the posterior ileo-czcal crosses the ileo-ceecal junction posteriorly, and the 
colic runs upwards to the ascending colon. The ileo-cxeeal branches supply the walls of the 
cecum, and, like the colic branch, anastomose with branches of the ileo-colic artery. 
3. The inferior mesenteric artery (a. mesenterica inferior, Fig. 566) arises 
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