or even more, in number, and, by their mutual inoculations, they form a very remarkable series of arches before they finally reach the bowel. At first they run parallel to one another, but soon they divide into two branches, each of which joins its neighbour, and in this way a succession of arterial arcades is formed. From these smaller twigs proceed, which divide and unite in a similar manner to form a second series of arches, and so on, until three, four, or perhaps even five tiers of arterial arcades are produced. From the lowest arches a multitude of minute twigs pass directly to the wall of the intestine.

The colic branches (arteriæ colica) spring from the concavity or right side of the superior mesenteric artery.

The ileo-colic artery (arteria ileo-colica), the lowest of the three branches which go to the great intestine, proceeds downwards, between the two layers of the mesentery, towards the right iliac fossa, and divides into a descending and an ascending branch. The descending branch joins the terminal part of the superior mesenteric, and from the arch thus formed twigs are given to the lower end of the ileum, the vermiform appendix, and the cæcum. The ascending branch inosculates with a branch of the right colic, and sends offsets to the ascending colon.

The right colic artery (arteria colica dextra) frequently arises in common with the ileo-colic. Escaping from the root of the mesentery it takes a horizontal course to the right, behind the parietal peritoneum on the back wall of the abdomen, and divides into two branches, a superior and an inferior. The superior branch ascends between the two layers of the transverse meso-colon to inosculate with the middle colic; whilst the inferior branch joins the ascending part of the ileo-colic. From the convexity of these

arches twigs proceed to the colon.

The middle colic artery (arteria colica media) is the highest of the three branches which spring from the concavity of the superior mesenteric. It passes between the two layers of the transverse meso-colon, and divides into a right and a left branch. The right branch joins the superior

part of the right colic, whilst the left branch inosculates with the ascending part of the left colic artery, which is derived from the inferior mesenteric.

The Superior Mesenteric Vein (vena mesenterica superior) lies to the right of the artery and receives. tributaries, which come from those parts of the intestinal canal which are supplied by branches from the superior mesenteric artery, and also the right gastro-epiploic vein from the great curvature of the stomach. Leaving the mesentery, it passes upwards in front of the duodenum, and then disappears under cover of the pancreas. Here it unites with the splenic vein to form the vena portæ.

The Superior Mesenteric Nervous Plexus (plexus mesentericus superior) is a dense plexus of sympathetic twigs, which surrounds the superior mesenteric artery like a sheath. From it filaments are prolonged to the gut along the various branches of the artery. As the nerves approach the bowel, some of the twigs leave the vessels and effect a series of communications with each other in the intervals between the arteries.

The superior mesenteric plexus is an offshoot from the solar plexus, and it distributes twigs to the jejunum, ileum, and to the right half of the great intestine.

Mesenteric Lymphatic Glands (lymphoglandulae mesentericae). These are very numerous, indeed considerably over a hundred in number. In health they rarely attain a size greater than that of a bean or a pea, and they are scattered between the two layers of the mesentery. The larger glands lie along the superior mesenteric artery, whilst the others are placed in the intervals between its branches. It should be noted that they are most numerous opposite the jejunum, and that the mesentery in the immediate vicinity of the gut is free from them.

A few lymphatic glands will also be noticed in connection with the great intestine.

The lacteal vessels enter the mesentery from the walls of the intestine in enormous numbers. As they proceed upwards they pass through the succession of glands which

they meet, and greatly reduced in numbers, although considerably enlarged in calibre, they usually terminate near the origin of the superior mesenteric artery in one or perhaps more large trunks which pour their contents into the receptaculum chyli of the thoracic duct.

Dissection. The coils of the small intestine must now be pulled over to the right side of the body, and the peritoneum carefully removed by the fingers from the lower part of the aorta and the left side of the spine and psoas muscle. The inferior mesenteric artery is thus exposed, and its branches can be followed to their distribution. The ureter and the inferior mesenteric vein will be seen lying upon the psoas muscle.

The Inferior Mesenteric Artery (arteria mesenterica inferior), considerably smaller than the superior mesenteric, springs from the left side of the abdominal aorta, about an inch and a half above its terminal bifurcation, and descends with a slight inclination to the left, towards the left iliac fossa. At first it is applied to the left side of the aorta, to which it is bound by peritoneum ; it then crosses the left common iliac artery and enters the pelvis, where it receives the name of superior hæmorrhoidal. Before leaving the abdomen proper it gives off the left colic and the sigmoid branches.

The left colic artery (arteria colica sinistra) proceeds to the left, over the left kidney, and divides into two branches, of which one ascends in the transverse meso-colon to inosculate with the middle colic, whilst the other descends behind the peritoneum lining the posterior wall of the abdomen to unite with the sigmoid. From the arches thus formed twigs are supplied to the colon.

The sigmoid arteries (arteriæ sigmoidea) proceed obliquely downwards, and, entering the sigmoid meso-colon, break up into several branches. These form arches with each other, from which twigs are ultimately given to the sigmoid flexure of the colon. The upper arch is in connection with the lower branch of the left colic, whilst the lower arch inosculates with the superior hæmorrhoidal artery. The arcading of the sigmoid vessels resembles that of the vasa intestini tenuis of the small intestine, and the extent

to which it is carried varies with the length of the sigmoid meso-colon.

FIG. 113.-Diagram of the inferior mesenteric artery and
its branches. (From GRAY'S Anatomy.)

Most frequently in place of one there are two or three sigmoid arteries, each with an independent origin from the inferior mesenteric artery.

The superior hæmorrhoidal artery will be followed out in the dissection of the pelvis.

The Inferior Mesenteric Vein (vena mesenterica inferior) receives tributaries corresponding with the branches of the inferior mesenteric artery. It passes upwards upon the psoas muscle under cover of the peritoneum, to the left of, and at some distance from, the artery, and, disappearing behind the pancreas, it ends in the splenic vein.

The Inferior Mesenteric Plexus of Nerves (plexus mesentericus inferior) is an offshoot from the left side of the aortic plexus. It closely surrounds the artery, and sends twigs along the branches of the vessel to supply the left half of the great intestine.

Dissection.—If the peritoneum has been carefully stripped off the lower part of the aorta, there will be little difficulty in recognising and following out the delicate nerves which form the aortic plexus. Raise the third part of the duodenum from the surface of the aorta, and trace these nervous twigs upwards.

The Aortic Plexus of Nerves (plexus aorticus abdominalis) is placed upon the aorta between the origins of the two mesenteric arteries. It is more strongly marked upon the sides of the artery than in front of it. Superiorly it will be found to be continuous with the solar and renal plexuses, whilst inferiorly it sends numerous large branches downwards in front of the common iliac arteries to join the hypogastric plexus-a plexus which is situated in front of the fifth lumbar vertebra, and which will be afterwards dissected. Upon each side the aortic plexus will be observed to be reinforced by several small twigs from the gangliated cord of the sympathetic. The inferior mesenteric plexus accompanying the artery of that name, and the spermatic (or ovarian) plexus of nerves which accompanies the spermatic (or ovarian) artery, are offsets from it.

Removal of the Intestines.-The jejunum, ileum, cæcum, and colon may now be removed from the abdominal cavity. Apply two