276 SOME CHAPTERS ON THE 



present in the latter but not in the axis cylinder. Macdonald 

 has given a different explanation of these results, but it is not 

 improbable that the original view is the more correct. 



We are now in a position to consider the bearing of these 

 observations on the theory of the nervous impulse. All the 

 modern workers agree in supposing that the dissolved electrolytes 

 are the cause of the electric currents, but as to the details of the 

 process each experimenter has a different view, and the more the 

 details are considered the less probable any given theory appears, 

 so that a very brief account here will be sufficient. 



In general, it is possible that the currents can be caused by 

 any one of three different processes. 



I. Chemical cells, of which the familiar type is the Leclanche. 



II. Concentration cells, where the current is caused by the 

 diffusion of ions with different velocities from a place of high 

 concentration to one of lower. 



III. Fluid cells, where the current is caused by the interchange 

 of the ions of two fluids separated by a membrane. 



I. The chemical-cell theories present more difficulties in the 

 case of nerve than the other two, and in spite of the distinguished 

 men who have been advocates on this side the general opinion is 

 now rather adverse to this view. It is quite unlikely that there 

 are no chemical changes in nerve. But the small amount of nerve 

 metabolism, the difficulty of showing any change in this meta- 

 bolism as a result of activity, and the apparent absence of any 

 heat produced, are all observations which tell against the proba- 

 bility that chemical activity is directly concerned with the nervous 

 impulse. 



One series of observations, however, can be taken as supporting 

 the chemical view, namely, those on the temperature coefficient. 

 It has been observed that the rate of change of a physical process 

 is less altered by variations in temperature than that of a chemical 

 one in other words, that the temperature coefficient given by the 



velocity at (T + 10) . , , . ,, , 



equation velocity at T c ' 18 U8ua ^y ^ e88 ^ an *"2 in the former 



case, more than this in the latter. 



Maxwell ( 55 ) has for this reason carefully measured the velocity 

 of the nervous impulse at different temperatures. He took the 

 pedal nerve of the giant slug (Ariolimax Columbianus) as the 

 object, and although the observations are not easy even in this 



