This is a large vessel of considerable length situated immediately beneath the spine, along which it runs from the seventh or eighth dorsal vertebra as far back as the sacrum. It is the largest division of the common aorta, and in the first part of its course describes an arch backward, termed the aortic arch. The anterior portion of the vessel is situated within the chest, and is hence distinguished as the thoracic aorta, while the posterior segment occupies the abdomen, and is known as the abdominal aorta. It is, however, one continuous vessel, and these different terms are only used to denote its anatomical relations. As it proceeds backwards it passes from the chest into the abdomen through the hiatus aorticus, an opening between the two pillars of the diaphragm.

The posterior aorta in its course beneath the spine gives off a number of branches, some of which are distributed to the walls of the chest and abdomen, while others go to the various organs they contain. The former set include: (l) 13 posterior intercostal vessels, which run downward between the ribs and give off branches upward to the muscles of the back and to the spinal cord. The first intercostal artery is derived from the superior cervical artery; the second, third, and fourth from the subcostal branch of the dorsal. (2) The phrenic, a branch going to the diaphragm or midriff. (3) Branches to the muscles of the loins.

The second group comprise: (1) The bronchial arteries to the air-tubes and oesophagus or gullet. (2) The cseliac axis, a short thick vessel, which, after leaving the under surface of the aorta, divides into three unequal branches - (a) The Splenic (b) The Gastric. (c) The Hepatic.

The first goes to the spleen, the second to the stomach, and the third to the liver. A little farther back it gives off (3) the great mesenteric artery, a short vessel of considerable size, whose branches are distributed to the large and small intestines. The next to appear are (4) the renal arteries, two short thick vessels, which spring from the sides of the aorta and enter the substance of the kidney. Still farther back come (5) the spermatic vessels. These arteries, two in number, are of considerable length, and take a peculiar winding course downward to reach the testicles. In the female the uterine and ovarian arteries spring from this point, and, as their names imply, supply the uterus and the ovaries. Then comes the small mesenteric, whose branches are distributed to the posterior part of the large bowel, which is not supplied by the large mesenteric. Finally, the posterior aorta divides into two pairs of vessels, one pair going to the right and the other to the left. These are distinguished as -

1. The Internal Iliac Arteries. 2. The External Iliac Arteries.

The Internal Iliac Artery breaks up into several divisions, which convey blood to the organs within the pelvis - the bladder, rectum, prostate gland, as well as parts of the uterus and vagina in the female and the penis in the male. They are: (1) the two last lumbar arteries; (2) the internal pudic artery; (3) the lateral sacral artery; (4) the ilio-lumbar artery; (5) the gluteal artery; (6) the ilio - femoral artery; (7) the obturator artery.

The External Iliac Artery, on leaving the aorta, runs down the inner side of the pelvis in an oblique direction backwards and outwards. On reaching the anterior border of the pubis it enters the thigh and takes the name of the femoral artery. In its course it gives off the circumflex iliac artery, which is distributed to the flank and to the thigh. A small branch goes to the spermatic cord, which in the mare goes to the uterus, and the prepubic artery, a vessel which divides into the posterior abdominal and the external pudic arteries.