THE MUSCULAR SYSTEM. 427 



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 expression 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-nitro- 

 genous 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 carbonic acid, at least, is. 

 the result of chemical action in the system, and especially of the com- 

 bustion of non-nitrogenous food, although, doubtless, of nitrogenous 

 food also. We are, therefore, driven to the conclusion that the sub- 

 stance 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. 



ELECTRICAL CURRENTS is NERVES. 



The electrical condition of Nerves is so closely connected with the 

 phenomena of muscular contraction, that it will be convenient to con- 

 sider it in the present chapter. 



If a piece of nerve be removed from the body and su bjected to exam- 

 ination in a way similar to that adopted in the case of muscle which has 

 been described (p. 404), electrical currents are found to exist which cor- 

 respond exactly to the natural muscle currents, and which are called 

 natural nerve currents or currents of rest, according as one or other 

 theory of their existence be adopted, as in the case with muscle. One 

 point (equator) on the surface being positive to all other points nearer 

 to the cut ends, and the greatest deflection of the needle of the galvano- 

 meter taking place when one electrode is applied to the equator and the 

 other to the centre of either cut end. As in the case of muscle, these 

 nerve-currents undergo a negative variation when the nerve is stimulated, 

 the variation being momentary and in the opposite direction to the 

 natural currents; and are similarly known as the currents of action. 

 The currents of action are propagated in both directions from the point 

 of the application of the stimulus, and are of momentary duration. 



Rheoscopic Frog. This negative variation may be demonstrated 



