158 ELECTRO-PHYSIOLOGY CHAP. 



portion is in no way abolished, so long as excitability is not 

 materially depressed at the points adjacent to the indifferent 

 point. For these must obviously play the same part in relation 

 to the effectuation or failure of the excitation proceeding from 

 the upper electrode as the fibre-ends in the parallel-fibred muscle 

 traversed by current, and injured at one end (Biedermann, 30). 



Certain chemical substances may also be employed for the 

 partial killing of medullated nerve, and Harless (31) has com- 

 municated observations on the effect of ammonia upon the 

 nerve-trunk, which agree essentially with the above results from 

 local death of the nerve. Ammonia, when applied in a con- 

 centrated solution, destroys the vital properties of the nerve at the 

 point of application without exciting it, or (at least at an early 

 stage) essentially altering its structural relations. Thus by 

 applying ammonia with a little brush (Harless) to points of the 

 intrapolar tract, the kathodic or anodic section of the nerve 

 traversed by current (i.e. the sum of all points which at given 

 strength of current and position of electrodes must be regarded 

 as exit- or entry-points of the current) may be functionally cut off, 

 and the intrapolar tract divided at the indifferent point, without 

 alteration of structure ; so that in the case before us only those 

 excitatory effects can come into play which correspond with the 

 peripheral electrode, i.e. with descending direction of current the 

 closing, with ascending direction the opening excitation. Since 

 the effect of ammonia (and to a lesser degree of other chemical 

 substances in solution) spreads in time with even carefully 

 localised application beyond the original point of contact, becoming 

 weaker in proportion with the distance from the seat of direct 

 action, it is clear that after applying ammonia in the region of the 

 central electrode, the modifications of excitability in successive 

 sections of the nerve must be very gradual. It is consequently 

 unnecessary to apply the ammonia to the intrapolar tract itself; 

 it is sufficient (especially if the electrodes are not too far apart) to 

 apply it to the points of the nerve which lie beneath the 

 central electrode, or even, if the excitation is in the continuity 

 of the nerve, on the far side of this, within the " centropolar " 

 region. When the advancing ammonia-effect has penetrated to 

 the region of the lower electrode, it is of course desirable to 

 shift the electrodes (at their original distance) farther along the 

 nerve. If, however, the electrodes are placed so that the distribu- 



