ELECTRICAL PHENOMENA. 101 



points of the muscle are led off to a galvanometer, has been shown 

 by Bernstein to precede the wave of contraction in muscle, that 

 is, in a stimulated muscle fiber the electrical change at any point 

 precedes the mechanical process of shortening. The electrical change 

 passes over the fiber from the point stimulated in the form of a 

 wave of definite velocity, but at any one point the electrical change 

 reaches its maximum before the process of contraction is visible. 

 We may suppose, therefore, that the electrical change is an indication 

 of the excitation or possibly constitutes the excitation that sets 

 up the chemical change of contraction, or else that the change IP 

 electrical potential is caused by the chemical change of contraction 

 and precedes the mechanical result of shortening, since the latter 

 process will have a certain latent period. It has been shown, 

 indeed, by Demoor that a completely fatigued muscle may still 

 conduct an excitation (muscle impulse), although unable to con- 

 tract, and the same fact has been demonstrated by Engelmann 

 for the heart muscle. In the nerve the action current, or the 

 negative change causing it, has been considered as simultaneous 

 with or possibly identical with the nerve impulse. The velocity 

 of the two is identical; the action current is given whenever the 

 nerve is stimulated, and, so far as experiments have gone, the 

 nerve cannot enter into activity without showing an action 

 current, that is, without showing a moving electrical charge. 

 Whether this electrical charge constitutes the nerve impulse or 

 is simply an accompanying phenomenon will be discussed briefly 

 in the paragraph upon the nature of the nerve impulse in the 

 following chapter. 



The Electrotonic Currents. In speaking of the effect of 

 passing a galvanic current through a nerve attention was called 

 to the fact that the 



condition of the / - - -- - - A 

 nerve is altered at 

 each pole. At the 

 anode there is a con- 

 dition of decreased 

 irritability and con- 

 ductivity known as 



a t Fi g- 45. Schema to show the direction of the elec- 



a I trotonic currents in an excised nerve: P, The battery for 



thp Pflthnrlp in thp the polarizing current sent into the nerve at +, the an- 



1G > 1] ode, and emerging at , the cathode; g', galvanometer 



beginning at least, arranged with leading off electrodes to detect the anelec- 

 trotonic current, the direction of which is indicated by 



a Condition Ol in- the arrows (in the nerve it is the same as that of the po- 



i -j. I_'TJ. larizing current) ; gr, galvanometer similarly arranged to de- 



creased irritability tect the catelectrotonic current. The anelectrotonic and 



i M i catelectrotonic currents continue as long as the polarizing 



known as CaieieC- current is maintained. 



trot onus. In addi- 



tion to these changes in the physiological properties of the nerve 



there is a change also in its electrical condition at each pole, of 



