The aorta, which is the main trunk of the arterial system which carries the red blood, is the largest artery in the system. It commences at the upper portion of the left ventricle, not far from the ventriculo-aortic orifice; it has three valves, called the sigmoid or semi-lunar valves, which serve to prevent the reflux of the blood, and which completely close the vessel when expanded.

The aorta runs upward and to the right, and is here the ascending aorta; then it turns to the left, passing in front of the spinal column, and taking a new turn downward, it forms the arch of the aorta; it runs along and to the left of the spine through the posterior mediastinum, and is called the descending aorta, and passes through the opening in the diaphragm. On reaching the abdomen it becomes the abdominal aorta, up to about the fourth lumbar vertebræ, where it bifurcates and forms the two primitive iliac arteries.

From its upper portion the aorta throws off important branches, of which the principal are the following:—The brachio-cephalic or innominate artery, which springs from the arch, and is its representative on the right side of the chest This trunk gives off the right common carotid and subclavian; the left common carotid and subclavian spring directly from the arch of the aorta.

The common carotid arteries run upward along the outside of the neck on a level with the upper border of the thyroid cartilage, and each divides into the external and internal carotid.

The external carotid gives off the superior thyroid, the facial, lingual, and occipital arteries. At the level of the condyle of the jaw it divides into the temporal and internal maxillary.

The internal carotid runs upward along the cervical vertebræ, enters the skull, gives off the ophthalmic artery, and is distributed through the brain.

The subclavian runs outside, behind and below the clavicle, as its name indicates, and gives off, among other branches, the vertebral artery, and the internal mammary; and on reaching the arm-pit (axilla) it takes the name of the axillary artery, and gives off important vessels to the shoulder and chest; and then descending along the humerus under the name of the brachial artery, it divides below the elbow, and forms the radial and ulnar arteries, which furnish the vessels of the fore-arm and those of the hand.

Among the arteries arising from the descending aorta we will mention only the caeliac trunk, which divides into three branches, destined to the liver, the stomach, and the spleen; the superior and inferior mesenteric, which go to the mesentery and intestines; and the renal or emulgent arteries.

Iliac Arteries

The common iliac arteries, formed by the bifurcation of the aorta, run obliquely downward to the right and left After attaining a length of about two and a half inches, each one divides into the internal iliac, which ramifies on the inside and on the outside of the pelvic cavity, and the external iliac, which at the point where it leaves the pelvis gives off the epigastric artery. This artery runs upward behind the anterior wall of the abdomen, and unites by anastomosis with the lower extremity of the internal mammary artery. On leaving the pelvis the external iliac takes the name of the femoral artery, gives off large branches to the muscles of the thigh, and on reaching the lower third of this region, becomes the popliteal artery, or artery of the ham. This last gives off the anterior tibial, and then divides into the posterior tibial and the peroneal artery. The anterior tibial at the point of the articulation of the foot with the leg takes the name of dorsal artery of the foot, and ramifies over the upper surface of the foot; while the peroneal and posterior tibial, after having, like the anterior tibial, distributed branches to the leg, terminate in the plantar region, or sole of the foot.