ELECTRICAL PHENOMENA. 



109 



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 when properly connected with a galvanometer. Whether 

 this electrical change 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 

 nerve is altered at 

 each pole. At the 

 anode there is a 

 condition of de- 

 creased irritability 

 and conductivity 

 known as anelectro- 

 tonus; at the cath- 

 ode, in the begin- 

 ning, at least, a 

 condition of in- 

 creased irritability 

 known as catelec- 



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

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

 at each pole due to a spread of potential for some distance beyond 

 each pole. If any two points on the anodal side are connected a 

 current will be indicated which has the direction shown in Fig. 50. 

 This current is designated as the anelectrotonic current, and the 

 similar current which may be obtained on the cathodal side is the 

 catelectrotonic current. The spread of potential which gives rise 

 to these currents is probably the underlying cause of the physio- 

 logical changes in irritabihty and conductivity which are referred 

 to above. 



Fig. 50. — Schema to show the direction of the elec- 

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

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

 ode, and emerging at — , the cathode; fir', galvanometer 

 arranged with leading off electrodes to detect the anelec- 

 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 

 f?atelectrotonic currents continue as long as the polarizing 

 current is maintained. 



