from fifteen to twenty in number. They arise from the convexity of the artery, and after passing a short distance nearly parallel to each other, between the layers of the mesentery, each one bifurcates. By the inosculation of these bifurcations with each other, a series of arches, or one continuous arch, is formed, from the convexity of which another set of branches arise. These, much more numerous than those which arise directly from the artery, inosculate with each other, and form a second series of arches, from the convexity of which another set of branches arise and inosculate with each other. By the repetition of this process the third, fourth, and sometimes the fifth series of arches are formed by the time the middle portion of the small intestine is reached. Having reached the intestine, the last branches divide into two sets, which ramify on the opposite sides of the bowel; some of them between the serous and muscular coats; others perforate the muscular layer, and terminate in the mucous membrane.

To dissect the arteries to the small intestine, the mesentery should be spread out and made tense. It is not necessary that the student should dissect all of them, in order to understand their general arrangement.

The branches to the large intestine arise from the concavity of the main trunk. There are three in number, the iliocolic, the right colic, and the middle colic. They pass between the layers of the meso-colon, and bifurcate to form a single series of arches, from which branches proceed to the large intestine, upon which they ramify in the same manner as the arteries do on the small intestine.

The ileo-colic, or inferior colic, Fig. 156 (15), descends to the cæcum to divide into branches, to be distributed to the lower part of the ileum, the cæcum, the appendix vermiformis cæci, and the lower part of the ascending colon. It sends a branch upwards to anastomose with the right colic. The right colic, Fig. 156 (14), or, according to some, the middle colic, goes horizontally to the ascending colon, and divides into an ascending and a descending branch, to anastomose, the one with the middle colic, and the other with the ileo-colic.

The middle colic, or the superior colic, Fig. 156 (11), proceeds upwards to the right half of the colon, and like the preceding, divides into two branches. One of these anasto

moses with the ascending branch of the right colic, and the other with the left colic branch of the inferior mesenteric artery.

spleen. 4. The pancreas. 5. A section of the duodenum. 6. The ascending colon. 7. The small intestines. 8. The descending colon. a. The portal vein. b. The splenic vein. c. The right gastro-epiploic. d. The inferior mesenteric. superior mesenteric. f. Section of the superior mesenteric artery.

e. The

The SUPERIOR MESENTERIC VEIN, Fig. 157 (e), corresponds to the artery just described, and originates in that portion of the intestine which is supplied by that artery. Its main trunk passes upwards over the inferior transverse portion of the duodenum, and beneath the pancreas where, in front of the aorta and to the left of the ascending vena cava, it unites with the splenic vein to form the vena porta. Its branches are the same as those of the artery which it accompanies.

The SUPERIOR MESENTERIC PLEXUS, Fig. 142 (16), proceeds from the lower part of the solar plexus. The nervous cords in it are large and numerous, forming a sheath for the trunk of the artery. It divides into secondary plexuses, corresponding to the divisions of the mesenteric artery. In the mesentery, the filaments are long, slender, and straight. Some of them unite to form arches just before they penetrate between the coats of the intestine. This is the largest plexus in the body.

The INFERIOR MESENTERIC ARTERY, Fig. 155 (11), and 158 (9), arises from the aorta, from one to two inches above its bifurcation into the common iliacs. It passes downwards along the aorta, and across the left common iliac, to enter the pelvis. It is much smaller than the superior mesenteric. It supplies the left portion of the transverse colon, the descending colon, the sigmoid flexure, and the upper part of the rectum. In its course it sends off the following branches:

The left colic artery, Fig. 158 (10), passes over the left kidney, between the layers of the meso-colon, to the descending colon. It divides into an ascending and descending branch. The former anastomoses with the middle colic branch of the superior mesenteric, thus establishing a free anastomotic connection between the two mesenteric arteries; the latter joins the sigmoid artery below.

The sigmoid arteries, Fig. 158 (12), proceed transversely to the sigmoid flexure, and divide into branches to anastomose above with the left colic, and below with the superior hemorrhoidal.

The superior hemorrhoidal artery, Fig. 158 (13), passes to the upper part of the rectum, between the laminae of the meso-rectum. Its branches anastomose with the sigmoid above, and the middle hemorrhoidal below.

The INFERIOR MESENTERIC VEIN, Fig. 153, d, is formed by branches which correspond to those of the inferior mesenteric artery. A free anastomosis exists between the superior and middle hemorrhoidal veins. It passes upwards to empty into the splenic vein behind the pancreas. Sometimes it opens into the superior mesenteric vein.

The INFERIOR MESENTERIC PLEXUS, Fig. 142 (16), is de

rived from the aortic plexus. It supplies that portion of the large intestine to which the inferior mesenteric artery is distributed.

THE DISTRIBUTION AND BRANCHES OF THE INFERIOR MESENTERIC ARTERY.-1.1. The superior mesenteric artery, with its branches and the small intestines turned over to the right side. 2. The cæcum and appendix cæci. 3. The ascending colon. 4. The transverse colon raised upwards. 5. The descending colon. 6. Its sigmoid flexure. 7. The rectum. 8. The aorta. 9. The inferior mesenteric artery. 10. The colica sinistra, inosculating with 11, the colica media, a branch of the superior mesenteric artery. 12, 12. Sigmoid branches. 13. The superior hæmorrhoidal artery. 14. The pancreas. 15. The descending portion of the duodenum.

The SPERMATIC ARTERIES, Fig. 159 (10), usually arise a little below the renal, from the forepart of the aorta. Each

descends on the side of the spine, and over the psoas magnus and iliacus internus muscles to the internal inguinal ring, where it joins the spermatic cord. They cross the ureters, and are accompanied by the spermatic veins. In the female, they go to the ovaries, and are called the ovarian arteries. There are sometimes two on the same side. The right one occasionally passes beneath the vena cava. They supply the testicles.

The SPERMATIC VEINS in the abdomen accompany the The spermatic arteries. The left one usually terminates in the renal, and the right one in the ascending vena cava. latter passes beneath the right and lower portion of the mesentery, and the former beneath the sigmoid meso-colon.

The SPERMATIC PLEXUS accompanies the spermatic artery to the testicle. In the female, the corresponding plexus follows the ovarian artery to the ovary and the uterus. It is derived from the renal plexus.

The RENAL ARTERIES, Fig. 159 (7), sometimes called the emulgent arteries, arise from the aorta opposite to the kidneys. They are very large in proportion to the size of the organs which they supply. The right one is longer than the left, and passes beneath the vena cava. The corresponding veins are usually situated in front of them. When the arteries reach the fissures of the kidneys, they divide into several branches. Sometimes, instead of one there will be two or three on the same side. They are also subject to considerable variation in their origin.

The RENAL VEINS convey the venous blood from the The left is the longest, kidneys to the ascending vena cava. and passes over the aorta; it also receives the left spermatic vein. The veins are in front of the corresponding arteries. They open into the vena cava at right angles. The suprarenal vein on the left side usually opens into the renal of the same side.

The RENAL PLEXUS, Fig. 142 (14), is formed on each side by filaments from the solar plexus and from the lesser It accompanies the renal artery to the splanchnic nerve.

kidney.

The SUPRA-RENAL ARTERIES, Fig. 159 (6), arise from the