Diagram 11.—Principle of the System of Bloodvessels.
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.
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.