308 PHYSIOLOGY OF MUSCLES AND NERVES. 



3, we have really to do with a closing immediately suc- 

 cgeded by an opening, for the inductive current aiises and 

 a^^ain disappears as soon as it has reached a certain strength. 

 This may be imitated with suitable apparatus, by closing a 

 constant current for a very brief time. Such a ' current 

 shock ' may exhibit exactly the same phenomena as does an 

 inductive current. If its duration remains unaltered, but 

 the strength of the cuiTent is gradually increased, the height 

 of elevation at first increases, remains for a time at a fii-st 

 maximum, after which it again increases and reaches a second 

 maximum. The explanation is the same as was given in 

 Note 3 for inductive currents. At first only the beginning 

 of the current (the closing) acts excitingly; but when the 

 current is stronger, the cessation of the current (the opening) 

 can also act in the same way, and a combination of the two 

 irritants can be formed. 



If the duration of such a current-shock is very short, the 

 current must be stronger, if it is to exercise any exciting 

 effect at all, than would be necessary if the duration were 

 lonijer. It is evident that a current, if it lasts too short a 

 time, cannot effect a sufficient change in the molecular con- 

 dition of the nerve, and weaker currents require a longer 

 time to do this than stronger. 



From the curves in fig. 73 which represent the duration 

 of inductive currents, it appears that without exception the 

 commencement of the current results more abruptly than its 

 disappearance. The commencement of every inductive cur- 

 rent must therefore more easily excite than does its end, 

 especially as this is always the case even in the ordinary 

 closing and opening of every constant current, in which such 

 considerable differences in the duration do not occur. In 

 the case of weak inductive currents it is always only the 

 commencement which is active, in other words an inductive 

 current acts as does the closing of a continuous current. 

 Now let us suppose that an inductive current is passed 

 through a nerve in an ascending direction. So long as the 



