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 appear from the experiments last described, for if such were the case we should find nitrogen eliminated in much greater quantities during muscular work than during rest, which is not the case. The material which supplies the force by its decomposition must, then, be mainly non-nitrogenous. 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 disappearance in part during muscular exercise, suggest at once the possibility 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, glycogen, etc.), and nitrogenous substances. For the present purpose it may be considered that the fats and carbohydrates are ultimately converted into carbonic acid and water, and that the nitrogenous substances 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 substances, 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 sufficient 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., C143H326N38O46S. If we now consider this to be oxidized to carbonic acid, water, and urea (and the sulphur to be oxidized to SO3, as would be the case in the formation of a sulphate), we might represent the process by the following equation:
|C143H326N38O46S 124CO2 75H2O 19CON3H4 SO3299O|
This would give 248 volumes CO2 produced for 299 volumes of oxygen taken up, or a ratio of 248 0·83.
If we consider the fats, and take stearine as a fair example of this class, we should have for such an equation—
|C57H110O6 163O 57CO2 55H2O.|
or the ratio of volumes of carbonic acid and oxygen would be 114