THE APPARATUS OF THE CIRCULATION.
The blood being the source of nourishment of all the tissues of the body, its need of con stant renewal is apparent, for, otherwise, all nourishing material wouldspeedily be abstracted from it. Moreover, tissues not only remove from the blood what they require, but they pour into it the waste products of their activity. If the blood were stagnant in the tissues it would thus not only be deprived of all nourish ment, but would be loaded with waste and poi sonous material. Therefore, as fresh blood must come constantly streaming to an organ or tissue, so must it as constantly flow away again, carrying impurities with it. The main idea of the circulation is thus easy to under stand. There must be a central pump, so to speak, from which large pipes, to continue the illustration, pass off, leading th smaller and smaller pipes distributed throughout the whole body. The blood must be pumped into the large pipes and forced along till it reaches the smallest branches, so that it may have access to the remotest parts. There must also be a second system of pipes, by means of which the blood, after nourishing the tissues and being laden with their waste products, is brought back again to the central pump to be again distri buted. Somewhere in this circuit there must be means for purifying the blood from the waste products it has received. Now this central
pump is the heart, and the pipes leading from it and passing into smaller and smaller branches are the arteries, the fine vessels into which they ultimately pass being called the capillaries, while the pipes along which the blood is brought back to the heart are the veins. Of course there can be no break in the continuity between ar teries and veins. The arteries, beginning large at the heart, become smaller and more numer ous till they end in the tine, hair-like capillaries; and then the reverse process must go on, the blood passing, on its return journey, from smaller to larger vessels, till the large veins are reached which open into the heart. So that the arteries end in the tissues in fine, hair-like vessels—capillaries,—and the veins begin in the tissues in fine, hair-like vessels —capillaries,— and they must he mutually continuous, so that the capillaries of the arteries pass insensibly into capillaries of veins. Heart, arteries, capillaries, and veins form the apparatus of the circulation, and we must understand the apparatus before we proceed to examine the process of the circu lation itself.