CAUSES AND PHENOMENA OF MOTION. 45 



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 carbonic 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-nitrog- 

 enous 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. 



The electrical condition of Nerves is so closely connected with the 

 phenomena of muscular contraction, that it will be convenient to consider 

 it in the present chapter. 



Electrical currents in Nerves. If a piece of nerve be removed from 

 the body and subjected to examination in a way similar to that adopted in 

 the case of muscle which has been described (p. 22, Vol. II.), electrical cur- 

 rents are found to exist which correspond 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 (corresponding to the equator) on the sur- 

 face being positive to all other points nearer to the cut ends, and the 

 greatest deflection of the needle of the galvanometer 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. The negative variation of the nerve current 

 may be demonstrated by means of the following experiment. The new 

 current produced by stimulating the nerve of one nerve -muscle prepara- 

 tion may be used to stimulate the nerve of a second nerve-muscle prepa- 

 ration. The fore-leg of a frog with the nerve going to the gastrocnemius 

 cut long is placed upon a glass plate, and arranged in such a way that its 

 nerve touches in two places the sciatic nerve, exposed but preserved in 



