The blood is entirely confined in a closed system of tubes, along which it moves always in the same direction. The main principle of the system is that of a ring. One side of the ring is split into a vast number of fine tubes to give a large surface for absorption and discharge of food among the cells; the other side is a single tube, with an enlargement in which the blood from different parts is mixed (see Diagram 11). This enlargement, which is contractile and fitted with valves, rhythmically draws the blood in from one direction and pumps it out in another. (The mechanics of the process we shall study later.) As a matter of fact, this system is twofold, as in Diagram 12. In passing through one-half of its course the blood absorbs oxygen in the lungs; in the other it yields oxygen to the tissues, and absorbs, whilst passing over the alimentary canal, proteid, carbohydrate, water, and salts, which are duly distributed to the other organs. Fat is absorbed by the lymph direct, but poured into the blood for distribution. The blood which passes over the alimentary canal on its way back to the heart goes through the liver. In this gland it leaves the carbohydrate which it has taken up, and a large store is laid down there after a meal, to be doled out as it is wanted. Blood also passes through the liver from the spleen, where it has been, so to speak, overhauled for repairs. [Illustration: DIAGRAM 13.—SCHEME OF THE CIRCULATORY SYSTEM. Blood system on the right, lymph system on the left.] As the medium for chemical communication throughout the community of cells, the blood has another all-important and obvious function, viz., that of clearing away the waste products of life. Of these there is, of course, the same quantity as of new material introduced. Carbonic acid gas is discharged into the lungs, but all the nitrogen and most of the other elements in the new combinations which protoplasm has made them assume leave by the kidneys, plus a little water by the skin as sweat and a few items discharged into the last part of the alimentary canal amongst the unabsorbed portions of the food. In their constituents, blood and lymph resemble one another, being both weak solutions of salts and proteid material; but the blood is distinguished from the lymph by the presence of innumerable extremely minute bodies, which give it its red colour. These corpuscles, to give them their proper name, are the vehicle by which oxygen is transported from the lungs to the tissues. They consist of an envelope of protoplasm filled with a red fluid (hæmoglobin), which combines loosely and easily with oxygen. In shape they are discoid, with a thickened rim and biconcave sides, another device for increasing surface and reducing bulk. (See Diagram 14.) [Illustration: DIAGRAM 14.—A RED BLOOD CORPUSCLE.]