in protoplasm, cell growth in a legitimate sense, and especially cell division, are dependent upon the presence of an active nucleus. While, therefore, histologists no longer formulate the cell theory in the restricted terms in which it was enunciated some years ago, they still regard the cell as the unit of structure and unit of function. The body is built of cells, and whatever is done by the body as a whole is done by its individual cells. CHAPTER IV THE FLUIDS OF THE BODY 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.