Blood, containing the material for every tissue and cell of the body, furnishes nutriment for every organ, enabling it to perform its individual function; thus it supplies every possible physiological want in the animal economy.

It does this by the transudation of a portion of its plasma into the surrounding tissues through the capillary walls, by which the losses sustained by the cells on account of tissue metamorphosis are made good. According to modern biological views, this pabulum is a material sui generis, called irritable matter or protoplasm, and is the only living matter, and is universally diffused throughout the organism, of which it constitutes about one-fifth, the remaining four-fifths being organized and relatively, therefore, dead matter. In its physical character it is nitrogenous, pulpy, structureless, semi-fluid, translucent, homogeneous, similar to that of the ganglionic nerves and to the gray, nervous matter. In this transuded fluid appear fine granules, which unite to form germs, from which, again, cells develop. By the union of these cells are formed the tissue of every kind needed for the upbuilding of the whole organism. Two kinds of substances are needed in this process of tissue-building, and both are found in the blood - namely, the organic and the inorganic constitutents.

Among the former organic constituents are the sugar, fat and albuminous substances of the blood, serving as the physical basis of the tissues, while the water and salts - namely, potash, lime, silica, iron, magnesium and sodium - are the inorganic substances, which are believed to determine the particular kind of cell to be built up. Other salts may from time to time be found, but the foregoing, however, embrace all which are constantly present. Wherever, then, in the animal organism, new cells are to be generated and formed, there must be present, in sufficient quantity and proper relation, both these organic and inorganic substances. By their presence in the blood, all the organs, viscera and tissues in the body are formed, fixed and made permanent in their functions, and a disturbance here causes disturbed function.