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] 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 flowing into the capillaries. In this way _they cause the intermittent flow from, the heart to become a steady stream in the capillaries_. The swelling of the arteries at each contraction of the ventricle is easily felt at certain places in the body, such as the wrist. This expansion, known as the "pulse," is the chief means employed by the physician in determining the force and rapidity of the heart’s action. *Purpose of the Valves in the Veins.*—The valves in the veins are not used for directing the _general_ flow of the blood, the valves of the heart being sufficient for this purpose. Their presence is necessary because of the pressure to which the veins are subjected in different parts of the body. The contraction of a muscle will, for example, close the small veins in its vicinity and diminish the capacity of the larger ones. The natural tendency of such pressure is to empty the veins in two directions—one in the same direction as the regular movement of the blood, but the other in the opposite direction. The valves by closing cause the contracting muscle to push the blood in one direction only—toward the heart. The valves in the veins are, therefore, an economical device for _enabling variable pressure_ in different parts of the body _to assist in the circulation_. Veins like the inferior vena cava and the veins of the brain, which are not compressed by movements of the body, do not have valves.