From one-fourth to one-third of the whole body is fluid. If the skin be regarded as a water-tight bag, three-fourths or rather less of its contents are solid, one-fourth liquid; and even its apparently solid contents, the tissues, contain much water. Water is an essential constituent of protoplasm. It is also present in cell-juice. The estimate given above does not include the fluid within the cells, but only the fluid with which the cells are bathed. In a general sense this extracellular fluid, excluding blood, is termed =lymph=. It occupies the spaces of a gauzy “connective tissue,” which connects, or separates—the terms are equally appropriate—muscles, nerves, glands, and other tissues of specialized function. Nowhere, except, in a fashion, in the spleen, does blood come in contact with a cell. The lymph which more or less surrounds them is the bath from which cells receive their food and oxygen, into which they excrete carbonic acid and tissue-waste. The network of lymph-spaces is traversed by capillary bloodvessels with walls composed of flattened connective-tissue cells. Such cells are usually spoken of as elements of an “endothelium.” As the epithelium covers the surface of the body, so endothelium lines its cavities. Endothelial cells are thin scales or tiles with sinuous borders dovetailed one into another. That the tiles which form the walls of capillary vessels are not cemented together in any proper sense is shown by the facility with which white blood-corpuscles, leucocytes, by their amœboid movements, push them asunder when making their way from the blood-stream into the tissue-spaces, or _vice versa_. They offer no more resistance to a leucocyte than a pair of curtains hanging in front of a door offers to a child. Yet so long as the endothelial cells are alive they keep their edges in such close apposition as to constitute a continuous membrane which shuts off blood from lymph. They are always close enough together to prevent red blood-corpuscles from escaping from the capillary vessels; but their resistance to the passage of the different constituents of plasma varies greatly. The membrane which they compose is more complete and less pervious, or less complete and more pervious, in accordance with the nature of the tissues which surround it, and their varying needs. The blood-passages of the liver may be described as filters. The escape of red blood-corpuscles into lymphatic vessels is prevented, but they offer practically no resistance to the plasma. 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.