GENERAL PHYSIOLOGY OF NERVE-TISSUE. 105 



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 currentless 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 action 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. 



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: 



1. The development of a nerve impulse at the moment the current enters 

 and at the moment the current leaves the nerve, i.e.,. at the moment the 

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

 nerve impulse is made evident by the contraction of the muscle 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. 



2. The development of electric currents on each side of the positive pole 

 or anode, and the negative pole or kathode (see Fig. 52), which can be led 



* J / ^X 



GALVANOMETER 



\ 



ANELECTROTONIC KATELECTROTONIC 



CURRENTS CURRENTS 



FIG 52. ELECTROTONIC CURRENTS. 



off by means of wires into a galvanometer circuit from either the artificial 

 transverse and longitudinal surfaces, or from any two points on the longi- 

 tudinal surface as shown by the deflection of the galvanometer needle. 

 The direction of these electric currents in the nerve coincides with that of 

 the galvanic or "polarizing current." The " natural nerve currents," the 

 currents of injury or demarcation currents, as they are variously termed, 

 are at the same time increased and decreased at opposite extremities of the 

 nerve according to the direction of the polarizing current. 



To this changed condition of the electromotive forces in a nerve the 

 term electrotonus was given (du Bois-Reymond). The currents them- 

 selves are known as electrotonic currents; from their relation to the anode 

 and kathode, they are termed anelectrotonic and katelectrotonic currents. 

 The condition of the nerve around the poles both in the intra-polar and 

 extra-polar regions is known as anelectrotonus and katelectrotonus. 



The electrotonic currents vary considerably in strength and extent, 

 according to the intensity of the polarizing current, increasing steadily 

 with the intensity of the latter up to the point at which the polarizing current 



