Page:Popular Science Monthly Volume 29.djvu/689

Rh of the hand. While the "flexors" of the fingers placed on the front palm or surface of the limb close the hand, it is the "extensors" of the opposite aspect of the forearm (whose sinews we see in the back of the hand), which open or extend our digits. There may be multiplication of organs here, it is true; but, given the original power to produce them, there is a clear economy of vital wear and tear exercised in the avoidance of too onerous tasks being laid upon any one muscle.

It is something of this principle which we find reflected also in the circulation of the blood. Here we see the heart's left ventricle (or larger cavity of the left side) driving blood, as does a force-pump, out into the great system of arteries, which everywhere throughout the body carry the nutrient stream. No sooner, however, has the bloodstream, impelled by the contraction of the muscular walls of the heart's ventricle, passed into the great main artery (the aorta) which arises from the heart, than an economical principle of an important kind comes into play. This principle is represented by the elasticity of the arteries which bear the blood to the body. They possess a circular coating of muscle which diminishes in thickness as the vessels grow smaller and smaller, and are therefore removed from the influence of the pumping-engine of the circulation. The arterial coating is itself elastic, and the whole system of these vessels is thus endowed with a high amount of resiliency. Their internal coats are smooth and shining, as also is the lining of the heart's cavities, friction being thus reduced to its minimum. The united sectional area of the branches of the dividing artery is larger than the same area of its stem, so that the collective capacity of the vessels increases markedly as we pass from the heart outward to the minuter channels of the circulation.

The blood is thus driven through an elastic set of tubes presenting the least possible resistance to the flow of fluid through them, and economy of power is thus again witnessed in the details of the human estate. Nor is this all. That there exists resistance to the flow of blood is, of course, a necessary condition in any system wherein large tubes or arteries branch out into small tubes (the capillaries), and these, again, unite to form larger or return vessels—the veins. The problem of living Nature would here appear to resolve itself into the inquiry, how the apparently intermittent, or spasmodic, work of the heart may be converted into a constant and continuous action.

If we suppose that a pump drives water through a rigid pipe, we see, in such a case, just as much fluid to issue from the pipe's end as entered it at the stroke of the pump. Practically, also, the escape of the water from the pipe takes place almost simultaneously with its entrance therein. If we place some obstacle or resistance to the free flow through the pipe, while the pump acts as before, the quantity of water expelled will be less, because less fluid enters the pipe. Just as