The heart empties itself into the large arteries; the left heart into the aorta, the right heart into the pulmonary artery. Both these arteries, as well as their subdivisions, are highly elastic. The very best quality of rubber tubing is not superior to our arteries as samples of elastic tubes. The blood, as we have already seen, is quite sticky, and the capillaries through which it must pass in its course around the body are microscopically tiny. The heart pumps the blood out of itself at the rate of four or five quarts a minute or more, according to whether it is working moderately or at high speed. To force this amount of the sticky blood through the tiny capillaries evidently requires very considerable force. As a matter of fact, the force is sufficient so that if it were applied to working a fountain it would force a jet to the height of nearly eight feet. Evidently pumping blood into elastic arteries with this force and against the resistance offered by the tiny capillaries causes the arteries themselves to be not only filled but overfilled, so that their walls are greatly stretched. This fact, that our arteries are elastic and are kept on the stretch by the pressure of the blood within them, is of the very greatest importance to the proper flow of blood and this in turn is so important to our well-being that some of our most serious chronic diseases are traceable to the loss of elasticity on the part of the arteries. We must remember that once every second, or oftener, the heart is shooting a jet of blood into the large artery which is already stretched with blood and which can empty itself only through the tiny capillaries at the tips of its finest subdivisions. On account of the inertia of the blood stream, room is made for this additional jet of blood by stretching the arteries near the heart more than they were stretched before. The result is that there is an inequality in the amount to which the arteries are stretched, those near the heart being stretched more than those farther along. As quickly as possible this inequality of stretch will be equalized by a spreading of the additional tension out over all the arteries in the form of a wave. This wave makes up what we know as the pulse. It can be felt in any artery that is near enough to the surface so that the finger tips can press upon it. The radial artery at the wrist is the one commonly used by physicians for feeling it. There is a large artery in the neck in which the pulse can also be felt readily, and if one takes the pulse of another person in the neck with one hand and in the wrist with the other he can easily satisfy himself that the pulse in the neck always comes an instant earlier than that in the wrist. This is simply because the pulse spreads from the heart as a wave, and the distance to the neck is not so great as that to the wrist. By the time the finest subdivisions have been reached, the tension is