Page:Popular Science Monthly Volume 29.djvu/691

Rh so treated, discharge themselves in turn onward, and at a rate which corresponds to that with which the force-pump action of the heart charges them from behind. And so, tracing the hydraulics of the circulation through its phases we see, firstly, the heart over-distending the elastic arteries. We witness the arteries emptying themselves into their minute continuations, the capillaries, and through these latter into the veins or return-vessels. The economy is witnessed here in the easy means adapted for converting without complications a spasmodic flow of blood into a continuous stream; insuring also that the amount of of blood which flows from the arteries to the veins during the heart's stroke and pause exactly equals that which enters the circulation at each contraction of the ventricle. In other words, the tremendously high pressure of the arteries of our bodies saves at once the multiplication of bodily pumping-engines and conserves the force of the heart itself.

There are other points connected with the circulation, more or less intimately, to which a passing allusion may be made. The low-pressure flow of blood in the veins upward to the heart from the lower parts of the body is thus favored by the high pressure of the arterial system, and natural economy of energy is thus again exemplified. The arteries seem to be intent on the work of getting rid of their contents through the capillaries into the veins. There is no resistance, in fact, to the venous flow which is carried on at low pressure. Again, the ordinary muscular movements of the body are utilized in the economy of life, to favor the return of the venous blood. For the veins are compressed in the muscular movements, and, as they are provided with valves which prevent back-flow, the compression can act in one way only—namely, to aid the upward or backward return of blood to the heart's right side.

The overplus of the blood is known as lymph, and is gathered from the tissues by vessels known as absorbents or lymphatics. These return the lymph to the blood-current for future use. Nature "gathers up the fragments" here as elsewhere, and sees that the lymph or excess of the blood-supply is once more garnered into the vital stream of the circulation. If we ask how this lymph-flow is maintained from all parts of the body toward the great vein in the neck where the lymph joins the blood, we again light upon the question of high pressure in one side of matters and low pressure in the other side. All the ordinary movements of our bodies are economically pressed by Nature into the service of the lymph-flow. As in the veins, the valves of the lymphatics prevent backward movement, and as in the veins the muscles compress the vessels, and common movement thus assists a special end. Even the motions of breathing favor the return of the lymph. For, when we inspire, the pressure in the great veins becomes negative in character, and lymph is thus capable of being sucked into the circulation from the main tube or duct of the lymph-system. When we