GENERAL PHYSIOLOGY OF NERVE -TISSUE 107 



ments to the nerve impulse, it may be said that it is a molecular disturb- 

 ance, traveling at the rate of about 28 meters a" second, is wave-like in 

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

 0.0006 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 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: 



GALVANOMETER 



ANELECTROTONIC KATELECTROTON 1C 



CURRENTS ' CURRENTS < 



FIG. 51. THE SITUATION AND DIRECTION OF ELECTROTONIC CURRENTS, DEVELOPED BY THE 

 PASSAGE OF AN ELECTRIC CURRENT THROUGH A PORTION OF A NEKVE.. 



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. 51), which can be led 

 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, 



