1. The inner coat consists of a delicate lining of flat cells resting upon a thin layer of connective tissue. The inner coat is continuous with the lining of the heart and provides a smooth surface over which the blood glides with little friction.
2. The middle coat consists mainly of non-striated, or involuntary, muscular fibers. This coat is quite thin in the veins, but in the arteries it is rather thick and strong.
3. The outer coat is made up of a variety of connective[pg 048] tissue and is also much thicker and stronger in the arteries than in the veins.
Fig. 19—Artery dissected to show the coats.
Marked differences exist between the arteries and the veins, and these vessels are readily distinguished from each other. The walls of the arteries are much thicker and heavier than those of the veins (Fig. 19). As a result these tubes stand open when empty, whereas the veins collapse. The arteries also are highly elastic, while the veins are but slightly elastic. On the other hand, many of the veins contain valves, formed by folds in the inner coat (Fig. 20), while the arteries have no valves. The blood flows more rapidly through the arteries than through the veins, the difference being due to the fact that the system of veins has a greater capacity than the system of arteries.
Fig. 20—Vein split open to show the valves.
Why the Arteries are Elastic.—The elasticity of the arteries serves a twofold purpose. It keeps the arteries from bursting when the blood is forced into them from the ventricles, and it is a means of supplying pressure to the blood while the ventricles are in a condition of relaxation. The latter purpose is accomplished as follows:
Contraction of the ventricles fills the arteries overfull, causing them to swell out and make room for the excess of blood. Then while the ventricles are resting and filling, the stretched arteries press upon the blood to keep it[pg 049] flowing into the capillaries. In this way they cause the intermittent flow from, the heart to become a steady stream in the capillaries.