Oxygen is required by the body pure, and, as it is uncombined with anything in the air, it needs no digestion to free it. A special organ, however, is necessary to absorb it. This is the lung. The lungs originate, just like a gland, by a pouching of the alimentary canal near its origin, but differ from a gland in their cells being very much flattened, to offer a large surface to the air on one side and to the bloodvessels on the other. Incessantly during life air is being drawn into the lungs; that the cells to which it is there exposed may transfer its oxygen to the blood; and then, after the cells have also transferred the carbonic acid gas from the blood to the air, driven out again to be replaced by fresh. (The mechanical means by which the lungs are filled and emptied come under another heading.) V. Food having been absorbed by cells set apart for the purpose, the next problem is, How is it distributed to those specialized for other work? The medium for this distribution is a liquid called lymph. All the spaces in the body are filled with lymph, all the organs bathed with it, every cell moistened with it; yet it is comparatively stagnant, and the food has to be conveyed from the walls of the alimentary canal to the lymph in the neighbourhood of the cell requiring nourishment by a more expeditious agent. This is done by the blood. [Illustration: DIAGRAM 11.—PRINCIPLE OF THE SYSTEM OF BLOODVESSELS.] [Illustration: DIAGRAM 12.—PRINCIPLE OF DOUBLE CIRCULATORY SYSTEM.] The blood is a fluid akin to the lymph, but confined in a system of tubes. Through these tubes it is driven at a considerable velocity, and in the course it takes passes within a reasonable distance of every cell in the body. As it passes the cells of the alimentary canal, they discharge the nutriment they have absorbed into it; as it passes through the other organs of the body, it discharges the requisite materials into the lymph bathing the actual cells: these are then able to help themselves. The lymphatic system is very simple. Lymph is practically fluid which has exuded through the walls of the bloodvessels, and is like the plasma of the blood, a thin solution of proteids in water containing just enough salt to hold them in solution. From different parts of the body a series of tubes run towards the heart, going up with increase in size and decrease in number as they near it. Into these tubes the lymph is forced with every movement of the body. At a slow rate, but varying with the activity of the animal, it is forced to flow along these tubes, regurgitation being prevented by valves at intervals, until it reaches the place where the lymphatic vessels join a large vein, and it is poured back into the blood-stream, thus completing its cycle.