Excepting as regards the lungs, all the arteries distributed over the system have their origin in the aorta (fig. 193). This vessel, the largest in the body, arises from the base of the heart, where it is continuous with the left ventricle, from which it is separated by the semilunar valves. It receives the blood discharged from that cavity at each contraction.

The Aorta is a very capacious but short vessel, measuring only from 2 to 3 inches in length. After leaving the ventricle it soon divides into two unequal parts - the anterior and the posterior aortae; but before doing so it gives off two vessels, whose branches enter the substance of the heart for the nourishment of its tissue. These are the right and left coronary arteries.