ARTERIES.] ANATOMY 903 behind the peritoneum, and the veins corresponding to them are rootlets of the inferior vena cava. The cceliac axis is a thick, short artery, which almost immediately divides into the coronary, hepatic, and splenic branches. The coronary artery subdivides into an cesoph- ageal branch for the lower end of the oesophagus, and a gastric branch for the coats of the stomach. The hepatic artery ends in the substance of the liver ; but gives off a cystic branch to the gall bladder, a pyloric branch to the stomach, a gastro-duodcnal branch, which divides into a superior pancreatico-duodenal for the pancreas and duode num, and a right gastro-epiploic for the stomach and omentum. The splenic artery ends in the substance of the spleen ; but gives off pancreatic branches to the pancreas, vasa ~brevia to the great end of the stomach, and a left gastro-epiploic to the stomach and omentum. The superior mesenteric artery gives off an inferior pancreatico-duodenal branch to the pancreas and duodenum ; about twelve intestinal branches to the small intestines, which form in the substance of the mesentery a series of arches before they end in the wall of the intestines ; an ileocolic branch to the end of the ileum, the cascum, and beginning of the colon ; a right colic branch to the ascend ing colon ; and a middle colic branch to the transverse colon. The inferior mesenteric artery gives off a left colic branch to the descending colon, a sigmoid branch to the sigmoid flexure of the colon, and ends in the superior hcemorrhoidal artery which supplies the rectum. The arteries which supply the coats of the alimentary tube from the cesophagus to the rectum anastomose freely with each other in the wall of the tube, or in its mesenteric attachment, and the anastomoses are usually by the forma tion of arches or loops between adjacent branches. The capsular arteries, small in size, run outwards from the aorta to end in the supra-renal capsules. The renal arteries pass one to each kidney, in which they for the most part end, but in the substance of the organ they give off small perforating branches, which pierce the capsule of the kidney, and are distributed in the sur rounding fat. The spermatic arteries are two long slender arteries, which descend, one in each spermatic cord, into the scrotum to supply the testicle. The corresponding arteries in the female, called the ovarian, do not leave the abdomen ; they supply the ovaries. The branches of the aorta which supply the walls of the thorax, abdomen, and pelvis, are the intercostal, the lumbar, the phrenic, and the middle sacral. The intercostal arteries arise from the back of the thoracic aorta, and are usually ten pairs. They run down the sides of the vertebral bodies as far as the commence ment of the intercostal spaces, when each divides into a dorsal and a proper intercostal branch ; the dorsal branch passes to the back of the thorax to supply the deep muscles of the spine; the proper inter costal branch runs outwards in the intercostal space to supply its muscles, and the lower pairs of intercostals also give branches to the diaphragm and wall of the abdomen. The lumbar arteries arise from the back of the abdominal aorta, and are usually four pairs. They run down the sides of the lumbar vertebrre, and divide into ndorsal branch, which supplies the deep mus cles of the back of the loins, and an abdominal branch FIG. 90. Diagram of a pair of inter costal arteries. Ao, the aorta trans versely divided, giving off at each Bide an intercostal artery; PB, tha posterior or dorsal branch ; AB, the anterior or proper intercostal branch; IM, a transverse section through the internal mammary artery. which runs outwards to supply the wall of the abdomen. The distribution of the lumbar and intercostal arteries exhibits a transversely-segmented arrangement of the vas cular system, similar to the transversely-segmented arrange ment of the bones, muscles, and nerves met with in these localities, but more especially in the thoracic region. The phrenic arteries, two in number, pass to supply the under surface of the diaphragm. The middle sacral artery, _ as already stated, is rather the continuation of the aorta than a branch. As it runs down the front of the sacrum it gives branches to the back of the pelvic wall. The statement has frequently been made that the visceral and parietal branches of the aorta do not anastomose with each other. Injections made by Turner have, however, shown that, both in the thoracic and abdominal cavities, slender anastomosing communications exist between the two sets of branches. In the abdominal cavity a wide meshed plexus of small arteries, named by him sub- or extra-peri toneal plexus, lies in the fat outside the peritoneum. It com municates, on the one hand, with the perforating branches of the renal arteries and with slender branches of the capsu lar, spermatic, colic, and pancreatic arteries, and in the re gion of the diaphragm with the hepatic artery. On the other hand, it communicates with the phrenic arteries, the lower intercostals, the lumbar branches of the aorta, and with the ilio-lumbar, circumflex ilii, and epigastric branches of the iliac arteries, which also go to the wall of the abdomen. In the pelvis also the visceral superior hsemorrhoidal artery anastomoses with the middle and lateral sacral arteries. The extra-peritoneal plexus supplies the fat and lymphatic glands lying outside the peritoneum, and it also gives origin to vasa vasorum for the coats of the aorta and vena cava. This plexus may, when the visceral branches of the aorta are obstructed, aid in an important manner in carrying on. the circulation. In a subject examined by J. Chiene, in the dissecting room of the University of Edinburgh, where the coeliac axis and the superior and inferior mesenteric arteries were obliterated at their origins, the blood flowed into these arteries and the viscera they supplied through a great enlargement of the arteries of this plexus. In the thoracic cavity a similar plexus, named the extra-pleural plexus, liea between the pleura and pericardium, which communicates on the one hand with the internal mammary arteries, and on the other passes in front of the root of the lung to join the bronchial system of vessels. Another portion of this plexus joins on the one hand the intercostal arteries near the dorsal vertebrae, and on the other passes to the lung at the back of its root. The branches for the head, neck, and upper limbs arise as three large arteries from the transverse part of the aorta ; they are named arteria innominata, left common carotid, and left subclavian. The arteria innominata is the largest ; it passes, upwards and to the right, to the root of the neck, and then divides into the right common carotid and the right subclavian. The carotid arteries supply the two sides of the head and neck; the subclavian arteries the two upper extremities. The subclavian artery is the commencement of the great Subc arterial trunk for the upper limb. It passes across the root s y st( of the neck and under the clavicle, when it enters the arm pit, and becomes the axillary artery ; by that name it extends as far as the posterior fold of the axilla, when it enters the upper arm, takes the name of brachial or humeral artery, and courses as far as the bend of the elbow, where it bifurcates into the radial and ulnar arteries. From the subclavian part of the trunk the following branches arise : a, Vertebral, which enters the foramen at the root of the transverse process of the 6th cervical
vertebra, ascends through the corresponding foramina in
