Plasma—“lymph,” as it is termed as soon as it is outside bloodvessels—passes through the walls of the capillaries of the liver unchanged in constitution. Where they traverse glands (other than the liver), muscles, skin, and various other structures, the walls of capillary vessels, while offering practically no resistance to water and diffusible salts which can pass through membranes, prevent proteid substances from passing from blood to lymph, except in extremely small quantities. In this way an exquisite balance is automatically maintained. Water and salts pass out as they are needed. But they never pass out in excess, because the protein-containing blood-stream tends to keep them in, in virtue of the same attractive force which enables it to suck in the oxidized products thrown into the lymph by the tissues. Whatever a tissue needs it takes from the lymph. Suppose that bone is being formed. Large quantities of lime and phosphates are needed for the calcification of the cartilage in which it is modelled. The cartilage absorbs lime and phosphates from the lymph which bathes it. Lime salts and phosphates immediately begin to diffuse from blood into lymph. The hurrying blood-stream brings up further supplies from the walls of the intestine, products of digested milk and other foods. Lymph contains (although not in the same proportions) everything which blood contains. Many an analogy may be found in the world of economics, although no illustration would be sufficiently complete. From the lymph tissues take the fuel that they need, the oxygen with which to burn it, the foods for their own repair, the raw materials for their arts. Into it they throw their smoke, their drainage, the slag and refuse of their factories. The blood replaces the supplies as they disappear. It absorbs all waste. Lymph occupies streets, market-place, passages, corridors. The blood-stream is a closed system, rolling down the streets and through the market-place, on its never-ceasing circuit from port and mine to open air and open sea. From the alimentary canal it picks up food and fuel; the lungs give it oxygen, and disperse its carbonic acid; the kidneys purge it of non-gaseous waste. [Illustration: FIG. 3.—A DUCTULE AND TWO ACINI OF A MUCOUS GLAND OF THE MOUTH, WITH A MUSCLE-FIBRE CUT LONGITUDINALLY; CAPILLARY BLOODVESSELS AND CONNECTIVE TISSUE. Stellate connective-tissue cells form a labyrinth of intercommunicating lymph-spaces which separate the gland-cells and the muscle-fibre from the walls of the capillary bloodvessels. The capillaries contain circular red blood-corpuscles and nucleated leucocytes. Some of the leucocytes are squeezing their way either out of a capillary into a lymph-space or _vice versa_. A granular leucocyte is to be seen in a lymph-space at the bottom of the picture.]