ARTERIES are the elastic tubes or ves sels that carry blood from the heart to the tissues of the body. They owe their name, which was connected by the older anatomists with the Greek aer, air, to the belief that they contained air, since they were found empty after death. The arteries spring from the heart in two great trunks, the pulmonary artery which carries blood to the lungs, and the aorta through which the blood is sent throughout the body. Each of these two main trunks divides and redivides until they become the minute vessels known as arterioles. Arteries show a ready adaptation in change of position attendant upon evolutionary development or mechanical obstruction in the individual organism. The main trunks occupy relatively constant positions but as the branches become smaller and smaller their position is less constantly determined. As they approach the periphery of any organ their minute branches connect through the hair like capillaries with the minute divisions of the veins, which carry the blood back to the heart, thus completing the round of • the circulation. The carotid is the branch of the aorta which mainly supplies the blood to the head, the superficial or external carotid supplying the outer structures and the deep or internal carotid nourishing the brain and deeper lying parts. There are numerous anastomoses between the branches of the caro tid arteries. The most important of these form a vascular circle, the circle of Willis, which affords freedom of arterial circulation by the anastomoses between arteries not only on the same side but also on opposite sides of the medial plane. For the other main branches of
the aorta see AORTA.
The' minute structure of the arteries is well adapted to the varying functions that these ves sels perform. Their walls consist of several coats. The outer coat, or the tunica adven titia is composed of white, fibrous connective tissue. Next to this is the yellow elastic and within this the muscular coat composed of in voluntary muscular tissue. The elastic coat is much thicker in the large arteries than the muscular but in the smaller there is a relatively stronger muscular coat. In the first part of the aorta, pulmonary artery, and arteries of the retina there is no muscular coat. The vaso motor nerves terminate in the muscular coat. Within this layer is a smooth elastic coat perforated by small apertures. The innermost coat is a layer of endothelial cells which forms the free surface over which the blood flows with a minimum of friction. The extreme toughness of the outer layer strengthens and protects while the elasticity with which the walls are provided permits the artery to return to its average diameter after it has been contracted or dilated by the muscular layer. The large arteries are thus more elastic and less con tractile while for the smaller ones the reverse is true. • The arteries are nourished by their own blood supply through minute vessels, vasa vasorum, distributed through the fibrous, elas tic and muscular coats.