474 VOICE AND SPEECH. 



to believe that the waste of muscle-substance can be expressed, 

 with unimportant exceptions, in any other way than by an 

 increased excretion of urea, it is evident that we must look 

 elsewhere than in destruction of muscle, for the source of 

 muscular action. For, it need scarcely be said, all force 

 manifested in the living body must be the correlative expres- 

 sion of force previously latent in the food eaten or the tissue 

 formed ; and evidences of force expended in the body must be 

 found in the excreta. If, therefore, the nitrogenous excreta, 

 represented chiefly by urea, are not in sufficient quantity to 

 account for the work done, we must look to the non-nitrogenous 

 excreta as carbonic acid and water, which presumably, cannot 

 be the expression of wasted muscle-substance. 



The quantity of these non-nitrogenous excreta is undoubtedly 

 increased by active muscular efforts, and to a considerable 

 extent ; and whatever may be the source of the water, the car- 

 bonic acid, at least, is the result of chemical action in the 

 system, and especially of the combustion of non-nitrogenous 

 food, although, doubtless, of nitrogenous food also. We are, 

 therefore, driven to the conclusion, that the substance of 

 muscles is not wasted in proportion to the work they perform ; 

 and that the non-nitrogenous as well as the nitrogenous foods 

 may, in their combustion, afford the requisite conditions for 

 muscular action. The urgent necessity for nitrogenous food, 

 especially after exercise, is probably due more to the need of 

 nutrition by the exhausted muscles and other tissues for which, 

 of course, nitrogen is essential, than to such food being superior 

 to non-nitrogenous substances as a source of muscular power. 



CHAPTER XVIII. 



OF VOICE AND SPEECH. 



IN nearly all air-breathing vertebrate animals there are 

 arrangements for the production of sound, or voice, in some 

 part of the respiratory apparatus. In many animals the sound 

 admits of being variously modified and altered during and 

 after its . production ; and, in man, one of the results of such 

 modification is speech. 



Mode of Production of the Human Voice. 

 It has been proved by observations on living subjects, by 



