ELECTRICAL PHENOMENA. 



107 



D 



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 change. 

 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 con- 

 dition of decreased 

 irritability and con- 

 ductivity known as 

 anelectrotonus ; a t 

 the cathode, in the 

 beginning, at least, 

 a condition of in- 

 creased irritability 

 known as catelec- 

 trotonus. 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 

 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 condition in the 

 nerve that makes it possible is designated as anelefctrotonus. A 

 similar current can be led off from the nerve on the cathode side 

 for a considerable distance beyond the cathode; this is known as 

 the catelectrotonic current, and the electrical condition leading 

 to its production as catelectrotonus. Within the nerve these 

 electrotonic currents have the same direction as the battery or 

 polarizing current, as is shown in the diagram (Fig. 49), The 

 terms anelectrotonus and catelectrotonus are used, therefore, 

 in physiology to designate both the physiological and the elec-* 

 trical changes around the poles when a battery current is led 

 into a nerve. Whether the physiological and the electrical changes 

 have a causal connection or are two independent phenomena is 

 at present undecided. 



Fig. 49. 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- 

 ode, and emerging at , the cathode; a*, 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 

 catelectrotonic currents continue as long as the polarizing 

 current is maintained. 



