814 THE POPULAR SCIENCE MONTHLY. 



amount of water running past. So in the body the decomposition of 

 proteine, which is the source of power to the muscles, goes on con- 

 stantly, 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 phys- 

 iologists 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 un- 

 interesting. 



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 non- 

 nitrogenous 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 nitro- 

 gen. 



The animal body may for our present purpose be regarded as con- 

 sisting, 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 mat- 

 ter 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 part- 

 ly 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. 



