Page:Popular Science Monthly Volume 15.djvu/834

814 amount of water running past. So in the body the decomposition of proteine, which is the source of power to the muscles, goes on constantly, independently of whether the energy which is set free takes the form of motion or appears in some other shape.

These are, in outline, the views of the two schools into which physiologists are divided upon this point. Professor Flint, in his book already referred to, advocates, and seeks to sustain by experimental evidence, a theory which may fairly be said to have been abandoned by both sides; and a review of his book, which appeared in this journal in April, 1878, having given some prominence to the subject, a brief review of the present state of our knowledge upon it may not be uninteresting.

It will facilitate an intelligent comprehension of the matter to preface our study of the main question with some explanation of the means by which our knowledge of the amount of nitrogenous and nonnitrogenous matter decomposed in the body is gained, and by some considerations regarding the effect of the kind and quantity of food upon the nutrition of the muscular system and the excretion of nitrogen.

The animal body may for our present purpose be regarded as consisting, besides water, of proteine and non-nitrogenous matter, chiefly fat: the latter contains the elements carbon, hydrogen, and oxygen; the former, in addition to these three, nitrogen. Both classes of matter are gradually oxidized in the body, and are finally converted into carbonic acid, water, and urea, the former of which is excreted through the lungs and skin, the latter through the kidneys, and the water partly by all three channels.

In the urea (together with small amounts of uric acid and other products) is contained the nitrogen resulting from the oxidization of the proteine.

It has been established, by an overwhelming mass of evidence, that all the nitrogen which leaves the system does so in the urine, and that the amount of this element in the latter is an accurate measure of the amount of proteine destroyed in the body. A determination of ureal nitrogen thus informs us of the amount of albuminoids oxidized; while a determination of the amount of carbon excreted in carbonic acid and urea, taken together, enables us, by a little calculation, to find the amount of fat oxidized.

By means of experiments conducted on this basis a tolerably full knowledge has been obtained of the effect of food upon the formation of flesh (muscular substance) and fat, and facts have been discovered which have an important bearing, both on our views of the origin of muscular power and on the precautions necessary in experimenting upon this subject. The earliest workers in this field were Bidder and Schmidt, followed by Karl Voit, in conjunction, first with Bischoff and later with Pettenkofer.