ELECTRICAL PHENOMENA. 



101 



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 in 

 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 impulses), 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 experi- 

 ments have gone,the 

 nerve can not enter 

 into activity with- 

 out showing an ac- 

 tion current, that 

 is, without showing 

 a moving electrical 



charge. Whether ode> - and emerging at , the cathode; a', galvanometer 

 this electrical Charge arranged with leading off electrodes to detect the anelec- 



constitutes the 

 nerve impulse or 

 is simply an accom- 

 panying phenome- 

 non 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 nerve is altered at each pole. 

 At the anode there is a condition of decreased irritability and con- 

 ductivity known as anelectrotonus ; at the cathode, in the begin- 

 ning, at least, a condition of increased irritability known as cate- 

 lectrotonus. In addition to these changes in the physiological prop- 

 erties of the nerve there is a change also in its electrical conditions 

 at each pole of such a character that if the nerve is led off from 

 two points on the anode side a current will be indicated. The 

 current can be obtained at a considerable distance from the anode, 

 and is known as the anelectrotonic current, while the electrical con- 

 dition in the nerve that makes it possible is designated as anelec- 



Fig. 45. Schema to show the direction of the elec- 

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

 the polarizing current sent into the nerve at +, the an- 



trotonic current, the direction of which is indicated by 

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

 larizing current); g, galvanometer similarly arranged to de- 

 tect the catelectrotonic current. The anelectrotonic and 

 catelectrotonic currents continue as long as the polarizing 

 current is maintained. 



