very large measure by the simple expedient of injecting some substance like mucilage directly into the blood stream. We are to think of the capillary walls, then, as allowing materials to pass freely through them in either direction, from the blood into the tissue spaces or from the tissue spaces into the blood, with the exception of the red corpuscles, platelets, and the blood proteins, and as thus keeping the tissue fluids supplied with whatever materials the blood contains or taking from the tissue fluids the waste products of cell metabolism, which the cells are pouring out. With this arrangement clearly in mind, all that remains for the understanding of the conveyer system is to see how the blood is kept in motion and distributed among the various organs of the body, and then to consider where the blood in turn gets its supplies of materials which it can pass on to the tissue fluids, or how it gets rid of the substances which the tissue fluids have passed on to it from the cells. At the beginning of the chapter we said something about the arteries and veins and their branching into smaller and smaller subdivisions with the final connecting link between the smallest arteries and the smallest veins in the form of capillaries. We are now to consider in detail the movement of the blood through these tubes, and to do that it will be necessary to speak again of this arrangement. In describing the circulation we usually begin with the heart. The heart itself will be taken up presently. First let us trace the blood vessels from the heart through the body and back to it again. The large main artery leading from the heart is known as the _aorta_. This springs from the upper side of the heart, bends over in an arch, and passes down through the chest into the abdomen. [Illustration: THE HEART AND BLOOD VESSELS DIAGRAMMATICALLY REPRESENTED _L_, lung; _M_, intestine; _P_, liver; dotted lines lymphatics. (Martin’s “Human Body”)]