Page:Popular Science Monthly Volume 19.djvu/668

650 sectional area. The capillaries have a sectional area several times that of the aorta, the purpose of this being to delay the blood at the time it is brought into most intimate contact with the tissues.

The walls of the capillaries are of extreme tenuity, and easily permeable under the physical action called osmosis. Even the corpuscles can pass outward through the walls.

 o d, right auricle; n d, right ventricle; o g, left auricle; v g, left ventricle. The arrows indicate the course of the blood.

To what degree the heart is aided by other forces is yet a matter of investigation. Probably there are several forces assisting. The elasticity of the arteries increases their carrying capacity. They are firm, elastic tubes, which expand under the pressure from each heart-contraction, and then by their own elasticity contract and help the onward flow of the blood. In the smaller arteries the flow loses the intermittent character it possesses in the larger arteries, and becomes a steady stream. The elasticity of the arteries serves precisely the same purpose as the air-chamber of any force-pump, that of equalizing the flow, and so increasing the amount delivered. The whole force is derived from the heart; the arteries cause the force to act continuously.

The veins are lax tubes, somewhat larger than the arteries, and capable of holding all the blood of the body. They convey the same amount of blood as the latter, but more slowly. In the larger veins, however, near the auricles, the velocity may be two hundred millimetres per second. They are provided with valves which effectually prevent the blood from flowing backward toward the heart. Any compression, produced by muscular contraction, or otherwise, will therefore assist the forward flow of venous blood. This is one explanation why