GENERAL PHYSIOLOGY OF NERVE -TISSUE 



107 



character, the wave being 18 millimeters in length and occupying from 

 o 0006 to '0.0008 of a second in passing any given point. 



Absence of Diphasic Action Currents. When any two points on 

 the longitudinal surface which do not exhibit a current are connected 

 with the galvanometer and a single wave of excitation passes beneath the 

 electrodes, it might be expected that, as in the case of the muscle, a diphasic 

 action current would be observed, from the fact that the portions of the 

 nerve beneath the electrodes become alternately negative with reference 

 to all the rest of the nerve. This, however, is not the case, the absence of 

 the two opposing phases of the action current being explained on the suppo- 

 sition that the negativity of the two led-off points is of equal amount, and that, 

 owing to the great rapidity with which the excitation wave travels, the two 

 phases fall together too closely in time to alternately influence the galvan- 

 ometer needle During stimulation of the nerve, when two eventless or 

 isoelectric points are connected, there is also an absence of the action 

 current, as was observed first by du-Bois Reymond, and which is to be ex 

 plained on similar grounds. It is true that an apparent ac ion current 

 is sometimes seen when the stimulating current is very powerful or the seat 

 of stimulation too near the diverting electrodes. This, however, must be 

 attributed to an electrotonic state of the nerve. ron c t ant 



The Effects of a Galvanic Current on a Nerve. When a constant 

 galvanic current of medium strength is made to pass through a portion of a 

 nerve, several distinct effects are produced: 



i. The development of a nerve impulse at the moment the jcurror t ent ers 

 and at the moment the current leaves the nerve, *.,., **** Je 

 circuit is made and at the moment it is broken. The development 

 nerve impulse is made evident by the contraction of the ^scle if the nerve 

 muscle preparation be used. If the current be either very weak, or very 

 strong, the muscle contraction may not always take place. 



^AGALVANOMETER 



\ 



ANELECTROTONIC 

 CURRENTS 



KATELECTROTONIC 

 CURRENTS 





,. T* .i *, on e.cb _,ide of th, : 



S*i 



^ " the 



nerve according to the direction of the polarizing curr 



