IX 



ELECTRICAL EXCITATION OF NERVE 



123 



currents of exceedingly rapid onset, and induced currents have a 

 powerful excitatory action, even at low intensity, from the same 

 reason. This tendency of the ordinary nerve-muscle preparation 

 to react to the weakest currents, provided they are adjusted with 

 sufficient rapidity, renders it a valuable indication of the presence 

 of weak currents of brief duration (action currents in the muscle). 



An interesting fact in this connection, and one that also 

 depends mainly upon the influence which the onset of any 

 current exerts upon its excitatory action, is the unequal effect of 

 the make and break shock from an induction apparatus. The 

 excitatory action of the make shock is without exception much 

 lower than that of the break. This is plainly seen when the 

 secondary coil is a long way off' from the primary. There is 

 always a point at which the break shock is effective, when the 

 make shock fails to excite ; on approximating the coils, the latter 

 also takes effect. 



Since, as is easily shown on the galvanometer, the quantities 

 of electricity in the make and in 

 the break shock are equal, the 

 dissimilarity of physiological effect 

 must be fundamentally due to the 

 differences in time-distribution of 

 the two induction currents, caused 

 by the appearance of the extra cur- 

 rent on closure of the primary 

 circuit. Since the primary current 

 in the last case does not at once 

 reach its full strength, but increases 

 gradually, while it suddenly dis- 

 appears on opening the circuit, 

 the induced current must rise more 

 abruptly at break than at make of 

 the primary circuit (Fig. 174). Accordingly, the break shock 

 gives a sharp " crack " in the telephone, the make shock, on the 

 contrary, yields a dull, weak sound (Griitzner). 



This inequality of physiological action in the make and break 

 shocks, as due to inequality of time-distribution, is very inadequately 

 compensated by the contrivance of " Helmholtz's side wire," which 

 is attached to most induction coils, and there have been later 

 attempts at producing induction currents by other means, better 



A 



PI, Abscissa of primary current ; S, 

 abscissa of secondary current; A, 

 initial; E, terminal currents. 1, 

 Curve of rise of primary current (de- 

 layed by extra current) ; 3, opening of 

 the same ; 2 and 4, corresponding 

 secondary currents. (Hermann, Handb. 

 ii. 1.) 



