_Circulation_.--The next piece of mechanism for us to consider in this machine is the device for distributing this fuel to the various parts of the machine where it is to be used as a source of energy, corresponding in a sense to the fireman of a locomotive. This mechanism we call the circulatory system. It consists of a series of tubes, or blood vessels, running to every part of the body and supplying every bit of tissue. Within the tubes is the blood, which, from its liquid nature, is easily forced around the body through the tubes. At the centre of the system is a pump which keeps the blood in motion. The tubes form a closed system, such that the pump, or heart, may suck the blood in from one side to force it out into the tubes on the other side; and the blood, after passing over the body in this closed set of tubes, is finally brought back again to be forced once more over the same path. As this blood is carried around the body it conveys from one part of the machine to another all material that needs distribution. While in the intestine, as already noticed (Fig. 3), it receives the food, and now this food is carried by the circulation to the muscles or the other organs that need it. While in the lungs the blood receives oxygen, and this oxygen is then carried to those parts of the body that need it. The circulatory system is thus simply a medium by which each part of the machine may receive its proper share of the supplies needed for its action. Now in this circulation we have again to do with chemical and physical forces. All of its general phenomena are based upon purely mechanical principles. The action of the heart--leaving out of consideration for a moment its muscular power--is that of a simple pump. It is provided with valves whose action is as simple and as easy to understand as those of any water pump. By the action of these valves the blood is kept circulating in one direction. The blood vessels are elastic, and the study of the effect of a liquid pumped rhythmically into elastic tubes explains with simplicity the various phenomena associated with the circulation. For example, the rhythmically contracting heart forces a small quantity of blood into the arteries at short intervals. These tubes are large near the heart, but smaller at their ends, where they flow into the veins, so that the blood does not flow out into the veins so readily as it flows in from the heart. The jet of blood that is sent in with every beat of the heart slightly stretches the artery, and the tension thus produced causes the blood to continue to flow between the beats. But the heart continues beating, and there is an accumulation of the blood in the arteries until it exists under some pressure--a pressure sufficient to force it rapidly through the small ends of the arteries into the veins. After passing into the veins the pressure is at once removed, since the veins are larger than the arteries, and there is no resistance to the flow of the blood.