24 EEPORT— 1887. 



Let ns examine by the light of modern science the truth of Liebig's 

 view — even now not uncommonly held — as to the functions of the two 

 kinds of food, and as to the cause of muscular exercise being the oxida- 

 tion of the muscular tissue. Soon after the promulgation of these views, 

 J. R. Mayer, whose name as the first expositor of the idea of the con- 

 servation of energy is so well known, warmly attacked them, throwing 

 out the hypothesis that all muscular action is due to the combustion of 

 food, and not to the destruction of muscle, proving his case by showing 

 that if the muscles of the heart be destroyed in doing mechanical work 

 the heart would be burnt up in eight days ! What does modern research 

 say to this question ? Can it be brought to the crucial test of experi- 

 ment ? It can ; but how ? Well, in the first place we can ascertain the 

 work done by a man or any other animal ; we can measure this work in 

 terms of onr mechanical standard, in kilogramme-metres or foot-pounds. 

 We can next determine what is the destruction of nitrogenous tissue at 

 rest and under exercise by the amount of nitrogenous material thrown ofi" 

 by the body. And here we must remember that these tissues are never 

 completely burnt, so that free nitrogen is never eliminated. If now we 

 know the heat-value of the burnt muscle, it is easy to convert this into its 

 mechanical equivalent, and thus measure the energy generated. What is 

 the result ? Is the weight of muscle destroyed by ascending the Faulhorn 

 or by working on the treadmill sufiicient to produce on combustion heat 

 enough when transformed into mechanical exercise to lift the body up to 

 the summit of the Faulhorn or to do the work on the treadmill ? Careful 

 experiment has shown that this is so far from being the case that the 

 actual energy developed is twice as great as that which could possibly be 

 produced by the oxidation of the nitrogenous constituents eliminated 

 from the body during twenty-four hours. That is to say, taking the 

 amount of nitrogenous substance cast off" from the body, not only whilst 

 the work was being done but during twenty-four hours, the mechanical 

 effect capable of being produced by the muscular tissue from which this 

 cast-off" material is derived would only raise the body halfway up the 

 Faulhorn, or enable the prisoner to work half his time on the treadmill. 



Hence it is clear that Liebig's proposition is not true. The nitro- 

 genous constitaents of the food do doubtless go to repair the waste of 

 muscle, which, like every other portion of the body, needs renewal, whilst 

 the function of the non-nitrogenous food is not only to supply the animal 

 heat, but also to furnish, by its oxidation, the muscular energy of the body. 



We thus come to the conclusion that it is the potential energy of the 

 food which furnishes the actual energy of the body, expressed in terms 

 either of heat or of mechanical work. 



But there is one other factor which comes into play in this question 

 of mechanical energy, and must be taken into account ; and this factor we 

 are as yet unable to estimate in our usual terms. It concerns the action 

 of the mind upon the body, and, although incapable of exact expression, 

 exerts none the less an important influence on the physics and chemistry 



