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 cup is full, it cannot become more full; not so with an artery. The elastic connective tissue allows it to expand as a rubber hose does under pressure. The first part of the aorta having expanded to receive the incoming blood, the stretched walls contract because of the elasticity of the outer connective tissue coat and force blood into the portion of the aorta just ahead, forcing it to expand in turn. Thus a wave of expansion travels along the arteries. This wave is called the pulse.

The Pulse may be most easily felt in the wrists and neck. As the artery stretches and springs back, one beat of the pulse is felt. In men there are about seventy heart beats or pulse beats a minute. In women the rate is about eighty a minute. It is slowest when one is lying down, faster while sitting, still faster when standing, and fastest of all during running or violent exercise. (Exp. 5.) It should not be thought that the muscular or middle layer of the artery actively contracts and helps to send along the pulse wave; for this wave is simply the passive stretching and contracting of the outer connective coat, and travels like a wave crossing a pond when a stone is dropped into the water. The force of the pulse is furnished, not by the muscle fibers in the artery, but by the beat of the heart; the outer, or connective tissue, coat enables the pulse to travel. Why must there be a middle, or muscular, coat for variation in size?

, A,, V, showing inner coat, e (endothelial); middle coat, m (muscular); and third coat, a (connective tissue).

Use of the Middle Coat; Quantity of Blood and its Distribution.—The body of an adult contains about five quarts of blood. The blood furnishes the nourishment needed for the activity of each organ. The more vigorous the work of any organ, the greater is the amount of blood needed. The whole amount of blood in the body cannot be suddenly increased, but the muscular coat of the arteries going to the working organ relaxes, and allows the arteries to become enlarged by the pressure from the heart. Consequently, more blood goes to the active organ, and the other organs get along with less blood for the time. When we are studying, our brains get more blood; when running, the