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The abundant provision for the multiplication of leucocytes shows that the destruction of these cells must occur on an equally large scale. Every day large numbers die. Where this occurs, and how their dead bodies are removed, is not certainly known. Doubtless they are eaten by their fellows, their substance oxidized, and the products—carbonic acid, water, and nitrogenous waste—thrown into the lymph. There is some reason for thinking that a part of the nitrogenous waste is excreted in the form of uric acid (cf. p. 216). The daily production, and consequent destruction, of leucocytes shows that their metabolism is a factor which cannot be overlooked when we are making up the body’s accounts.

The fixed tissues receive their nutriment in a digested condition. Leucocytes digest it for themselves. In many cases, although not in all, the cells of fixed tissues last throughout life, so far as their outer form is concerned, although their molecules are oxidized and replaced by new material. It is not improbable, therefore, that there is a difference between the metabolism of the fixed tissues and the metabolism of leucocytes. The whole of a wandering cell, its nucleus included, breaks down and has to be removed. We do not know that this occurs in the case of a fixed cell. On the strength of evidence which points, apparently, to a chemical relationship between nuclear substances and uric acid, it has been inferred that the two chief nitrogenous products which are excreted by the kidney are divisible into the one which in the main represents the oxidation of fixed cells, urea, and the other, uric acid, largely derived from the oxidation of wandering cells.