NOTES AND ADDITIONS. 299 



3. Excitability and Strength of Irritant. Combination 



OF Irritants (p. 119). 



"When the coils of a sliding inductive apj^aratus aro 

 brought nearer together, the strength of the inductive current 

 does not increase in exact proportion with the decreasing* 

 distance betweeen the two, but in a complex way, which 

 must ba provided for in ea^h apparatus separately, Fick, 

 Kronecker, and others have shown methods by which this 

 calibration of the apparatus may be accomplished. If the 

 real strength of the irritating current is compared with 

 the height of the pulsation which it elicits, it appears that 

 when the current is very weak no action is observable ; 

 action first appears, in the form of a slight, just visible pulsa- 

 tion, when the current has reached a certain strength, greater 

 or le^;s according to the condition of excitability of the 

 nerve. As the currents increase further in strength, the 

 heights of elevation increase in exact proportion to the 

 strength of the currents, till a certain maximum has been 

 reached. If the strength of the current becomes yet greater, 

 the pulsations remain constant for a time; but then they 

 again increase and reach a second maximum, above which 

 they do not pass. 



These so-called ' over maximum ' pulsations are due to 

 a combination of two irritants. An inductive shock is, as 

 we have seen, a very brief current, in w^hich the commence- 

 ment and the end succeed each other very rapidly. For 

 reasons which will be further explained in Note 7, the com- 

 mencement of an inductive current is a more powerful 

 irritant than its end. As long, therefore, as the current 

 does not pass a certain strength, only the commencement of 

 the current irritates ; but in the case of very powerful cur- 

 rents the end may be sufficiently efiective : this gives two 

 irritations following each other in rapid succession, and these 



