Structure.—An artery consists of an internal, a middle and an external coat.
The inner coat consists of endothelial cells and elastic fibrous tissue, sometimes arranged longitudinally, but usually they form a distinct fenestrated membrane (similar to a doorscreen).
The middle coat consists mostly of elastic tissue and white fibrous tissue.
The external coat is called the fibrous coat. It contains fibrous connective tissue and elastic tissues.
Vasa-Vasorum.—Running in the outer wall of the artery, we find small capillary vessels, and their function is that of nourishing the outer wall, for the blood which passes through the artery does not nourish the artery from within, but depends on these small capillaries, called vasa-vasorum, for their nutrition.
The individual sheath, or arterial sheath, the covering for the artery, is composed of connective tissue, and at places may adhere very tightly to the artery.
Fig. 9—Valves of the veins.
Fig. 10—Cross section through a small artery and vein. (Klein and Noble Smith)