8o2 HUMAN ANATOMY. 



transverse colon. The lower portions of the duodenum and ileum and the large intestine are 

 supplied by branches given off from the concave surface of the artery, while the rest of the small 

 intestine receives its supply from a somewhat variable number of branches which arise from 

 the convex surface. 



(a) The inferior pancreatico-duodenal (a. pancreaticoduodenalis inferior) is a small vessel 

 which usually arises from the superior mesenteric just as it emerges from beneath the pancreas, 

 although it occasionally is given off by the uppermost of the intestinal branches. It passes towards 

 the right along the upper border of the third portion of the duodenum, and supplies that portion 

 of the intestine, as well as the neighboring portions of the pancreas, and anastomoses with the 

 superior pancreatico-duodenal branch of the hepatic artery. 



(If) The intestinal branches (rami imestiuales), also called z'asa intesiitn tenuis, are from 

 ten to sixteen in number, and arise from the convex surface of the artery, those branches which 

 arise from the upper portion of the parent stem being, in general, larger than the lower ones. 

 The first two or three branches, as they pass towards the intestine between the two layers of 

 the mesentery, divide into an ascending and a descending branch, and these branches inosculate 

 to form a series of primary arches, which run, in a general way, parallel with the intestine. 

 Lower down, in addition to these primary arches, secondary ones are formed by the inosculation 

 of branches given off proximally to those which form the primary arches ; still later, tertiary arches 

 make their appearance, and finally the arrangement becomes so complicated as to resemble 

 a net-work rather than a definite series of arches. From the convex surfaces of the primary 

 arches a large number of parallel straight branches pass to the intestine and are distributed to 

 its walls. They rarely branch in their course through the mesentery, and are usually distributed 

 to one side of the intestine and then to the other alternately. The rich anastomosis which 

 occurs between the various intestinal branches, and which varies greatly in its complexity, serves 

 to equalize the supply of blood to the entire length of the intestine and to permit of abundant 

 and rapidly collateral circulation to any portion of the tract from which the direct supply may 

 be cut off by pressure exerted during peristalsis. 



(r) The ileo-colic artery (a. ileocolica) arises about half-way down the concave surface of 

 the superior mesenteric either independently or in common with the right colic branch. It 

 passes downward and outward, beneath the peritoneum, towards the ileo-cjecal junction, giving 

 off branches which inosculate with the right colic above, with the terminal portion of the supe- 

 rior mesenteric below, and, in the interval, with one another to form a series of arches from 

 which branches are supplied to the terminal portion of the ileum, to the ctecuni and the vermi- 

 form appendix (a. appendicularis) and to the lower third of the ascending colon. 



{d ) The right colic artery (a. colica dextra) arises from the concave surface of the superior 

 mesenteric either a short distance above or in common with the ileo-colic. It runs towards the 

 right, behind the peritoneum, passing over the right psoas muscle, the ureter, and the spermatic 

 (or ovarian) vessels, and as it approaches the ascending colon it divides into an ascending and 

 a descending branch. These inosculate respectively with the middle colic and the ileo-colic to 

 form arches, from which branches pass to the upper two-thirds of the ascending and to a portion 

 of the transverse colon. 



(^) The middle colic artery (a. colica media) arises from the concave surface of the superior 

 mesenteric a little below the origin of the inferior pancreatico-duodenal branch. It passes for- 

 ward and downward between the two layers of the trans\-erse mesocolon, and divides into a 

 right and left branch which inosculate respectively with the right colic and with the left colic 

 branch of the inferior mesenteric to form arches, from which branches pass to the transverse 

 colon. 



Variations. — Considerable variation occurs in the number and position of the branches of 

 the superior mesenteric artery and also in the complexity of the anastomoses which occur 

 between these. In addition to those usually present, branches may be sent to any of the neigh- 

 boring organs, such as the liver, stomach, and spleen, and the artery may give rise to the hepatic, 

 as already pointed out, or to the gastro- duodenal, or even the gastric or renal artery. It has 

 been observed to supply the place of the inferior mesenteric artery when that vessel was lacking, 

 giving off left colic, sigmoid, and superior hemorrhoidal branches. 



From the embryological stand-point the superior mesenteric represents the intestinal 

 branch of the omphalo-mesenteric artery, which, during the early months of foetal life, passes 

 outward through the umbilicus to be distributed upon the surface of the yolk-sac. Usually this 

 artery disappears, except in so far as it is concerned in the formation of the superior mesenteric 

 artery; but it has been observed to persist, appearing as a branch of the superior mesenteric 

 which is continued forward in a strand of connective tissue from the ileum to the umbilicus, 

 where it anastomoses with the epigastric artery and sends a branch upward along with the 

 round ligament of the liver. 



3. The Inferior Mesenteric Artery. — The inferior mesenteric artery (a. 

 mesenterica inferior) (Fi^-. 723) arises from the anterior surface of the abdominal 

 aorta from 3-4 cm. above the bifurcation of that vessel into the two common iliacs. 



