THE SOURCE OF MUSCULAR ENERGY. 383 



Having thus glanced at some of the more important experimental 

 results bearing on this subject, let us return to the consideration of 

 the two questions previously enunciated. First, then, " What is the 

 fuel-material for muscular force ? is it albuminoid and nitrogenous, or 

 is it non-nitrogenous ? " That it is not essentially nitrogenous will ap- 

 pear from the experiments last described, for if such were the case we 

 should find nitrogen eliminated in much greater quantities during mus- 

 cular work than during rest, which is not the case. The material which 

 supplies the force by its decomposition must, then, be mainly non-nitro" 

 genous. Here, again, are various possibilities. Fats, sugars, glycogen, 

 are all non-nitrogenous, and we have next to inquire whether the fuel- 

 material be fats, sugars, or glycogen. The facts above stated of the 

 constant occurrence of glycogen in the muscular tissues, and its dis- 

 appearance in part during muscular exercise, suggest at once the pos- 

 sibility of this substance being a fuel-material. We shall obtain light 

 on this question from the facts regarding the influence of muscular 

 exertion on the ratio of the volume of carbonic acid expired to that of 

 the oxygen taken up. The three principal classes of foods consumed 

 in the animal body are the fats, carbohydrates (starch, sugars, glyco- 

 gen, etc.), and nitrogenous substances. For the present purpose it 

 may be considered that the fats and carbohydrates are ultimately con- 

 verted into carbonic acid and water, and that the nitrogenous sub- 

 stances are ultimately converted into carbonic acid, water, and urea. 

 The nitrogenous foods are usually subdivided into albuminoids proper, 

 and substances not albuminoids. All these nitrogenous substances are 

 composed mainly of carbon, hydrogen, oxygen, and nitrogen, and 

 usually also sulphur, in proportions which vary with different sub- 

 stances, but within very narrow limits. For the sake of simplicity, 

 therefore, it will be permissible to take a certain average composition 

 to represent the entire class, and the deductions will apply with suffi- 

 cient accuracy to the nitrogenous foods as a body. For the sake of 

 easy comparison we may also represent this average composition by a 

 formula which may be considered as representative of the class ; e. g., 

 ^i4sH5a8^3R^4t^- ^^ ^^ "o^ consider this to be oxidized to carbonic 

 acid, water, and urea (and the sulphur to be oxidized to SO,, as would 

 be the case in the formation of a sulphate), we might represent the pro- 

 cess by the following equation : 



Cm,H„.N,,0,.S - 2990 = 124CO, -h 75H,0 -f 19C0N,H, H- SO,. 



Albuminoids, etc Urea. 



This would give 248 volumes CO, produced for 299 volumes of 

 oxygen taken up, or a ratio of ^%% = 0-83. 



If we consider the fats, and take stearine as a fair example of this 

 class, we should have for such an equation — 



C„H„„0, + 1630 = 57CO, + 55H,0. 



Stearine. 



or the ratio of volumes of carbonic acid and oxygen would be -J-H 



