Food is also demanded for this, and the non-nitrogenous food is the most readily combustible, especially the hydrocarbons, or fats; the carbo-hydrates—starch, sugar, etc.—also, but in lower degree. These? then, were described as fuel-food, or heat-producers.
This view is strongly confirmed by a multitude of familiar facts. Men, horses, and other animals can not do continuous hard work without a supply of nitrogenous food; the harder the work the more they require, and the greater becomes their craving for it. On the other hand, when such food is eaten in large quantities by idle people, they become victims of inflammatory disease, or their health otherwise suffers, according, probably, to whether they assimilate or reject it.
Man is a cosmopolitan as well as an omnivorous animal, and the variation of his natural demand for food in different climates affords very direct support to Liebig's theory. Enormous quantities of hydrocarbon, in the form of fat, are consumed by the Esquimaux and by Europeans when they winter in the Arctic regions. They can not live there without it. In hot climates some fuel-food is required, and the milder form of carbo-hydrates is chosen, and found to be most suitable; rice, which is mainly composed of starch, is an example. Sugar, also. Offer an Esquimau a tallow-candle and a rice-pudding, he will reject the latter, and eat the former with great relish.
A multitude of other facts might be stated, all supporting Liebig's theory.
There is one that just occurs to me as I write, which I will state, as it appears to have been hitherto unnoticed. Some organs which act in such wise that we can see their mode of action are visibly disintegrated and consumed by their own activity, and may be seen to demand the perpetual renewal described by Liebig. There are certain glands of cellular structure which cast off their terminal cells containing the fluid they secrete; do their work by giving up their own structural substance at their peripheral working surface.
Where, then, is the quicksand? It is here. If muscular and mental work were done at the expense of the nitrogenous muscular and cerebral tissues, the quantity of nitrogen excreted should vary with the amount of work done. This was formerly stated to be the case without hesitation, as the following passage from Carpenter's "Manual of Physiology" (third edition, 1856, page 256) shows: "Every action of the nervous and muscular systems involves the death and decay of a certain amount of the living tissue—as is indicated by the appearance of the products of that decay in the excretions."
More recent experiments by Fick and Wislicenus, Parkes, Houghton, Ranke, Voit, Flint, and others, contradict this by showing that the waste nitrogen varies with the quantity of nitrogenous food that is eaten, but not with the muscular work done. For the details of these experiments I must refer the reader to standard modern physiological treatises, as a description of them would carry me too far away
