The nutrient vessels of the arteries and veins, the vasa vasorum, are present in all the vessels except those less than one millimeter in diameter. The vasa vasorum course in the external coat and send capillaries into the media, supplying the outer portion of the coat and the externa with nutritive material. The nutrition of the intima and inner portion of the media is obtained from the blood circulating through the vessel. Lymphatics and nerves are also present in the middle and outer layers of the vessels. Arteries The structure of the arteries varies notably, depending upon the size of the vessel. A cross section of the thoracic aorta reveals a dense network of elastic fibers, occupying practically all of the space between the single layer of endothelial cells and the loose elastic and connective tissue network of the outer layer. Smooth muscle fibers are seen in the middle coat, but, in comparison with the mass of elastic tissue, they appear to have only a limited function. In a cross section of the radial artery one sees a wavy outline of intima, caused by the endothelium following the corrugations of the elastica. The endothelium is seen as a delicate line, in which a few nuclei are visible. The media is comparatively thick, and is composed of muscle cells, arranged in flat bundles, and plates of elastic tissue. Between the media and the externa the elastic tissue is somewhat condensed to form the external elastic membrane. The adventitia varies much in thickness, being better developed in the medium-sized than in the large arteries. It is composed of fibrous tissue mixed with elastic fibers. "Followed toward the capillaries, the coats of the artery gradually diminish in thickness, the endothelium resting directly upon the internal elastic membrane so long as the latter persists, and afterward on the rapidly attenuating media. The elastica becomes progressively reduced until it entirely disappears from the middle coat, which then becomes a purely muscular tunic, and, before the capillary is reached, is reduced to a single layer of muscle cells. In the precapillary arterioles the muscle no longer forms a continuous layer, but is represented by groups of fiber cells that partially wrap around the vessel, and at last are replaced by isolated elements. After the disappearance of the muscle cells the blood vessel has become a true capillary. The adventitia shares in the general reduction, and gradually diminishes in thickness until, in the smallest arteries, it consists of only a few fibroelastic strands outside the muscle cells." (Piersol's Anatomy.)