DETERMINATION OF DIFFERENTIAL EXCITABILITY 2?7 



ential excitation as between A and B. In this way records 

 may be taken qf a series of the after-effects of brief 

 excitations, at intervals of, say, a minute. The direction of 

 this responsive current, which in the tissue is from the more 

 to the less excitable, enables us to determine the relative 

 excitabilities of the two points, A and B. 



Much more delicate is the second method, that, namely, 

 which depends on the record of the Direct-Effect of equi- 

 alternating shocks ; but for its perfect working, certain 

 difficulties have to be overcome. One of the first conditions 

 to be fulfilled lies in the perfect equality of the alternating 

 shocks. The importance of this will be understood on 

 observing the effect of the alternating shocks given by a 

 Ruhmkorff's coil, when actuated by a vibrating hammer. 

 Here, the make- and break-shocks are of unequal intensity 

 and duration, and the following sources of disturbance come 

 into play: (i) a galvanometric drift in one direction or the 

 other ; (2) a resultant inequality of polarisation-effects ; and 

 (3) the inequality of the excitatory values of the two shocks. 



The galvanometer-drift, owing to the inequality of the 

 induction shocks, becomes very troublesome, when we have 

 to employ, as is necessary, an instrument of high sensibility. 

 If the differential excitability of the specimen be very great, 

 this drift may be masked by the predominant excitatory 

 effect. In other cases, however, the excitatory effect itself 

 may be overpowered by the drift. The difference of 

 intensity as between the make- and break-shocks in the 

 Ruhmkorff's coil described, thus becomes a strongly dis- 

 turbing element. The necessity to make the two shocks 

 absolutely equal will be understood when we find that 

 alternating telephonic currents, which are generally re- 

 garded as equal and opposite, induce a drift of the 

 galvanometer in one direction or the other, on account of a 

 slight difference of intensity between the two alternating 

 currents. 



The difficulty arising from inequality of polarisation- 

 effects is too obvious to require further elucidation. Still 



