76 ACTION OF THE CONSTANT CURRENT. [BOOK i. 



majority of cases what happens is as follows. At the moment that 

 the circuit is made, the moment that the current is thrown 

 into the nerve, a single spasm, a simple contraction, the so-called 

 making contraction, is witnessed ; but after this has passed away 

 the muscle remains absolutely quiescent in spite of the current 

 continuing to pass through the nerve, and this quiescence is 

 maintained until the circuit is broken, until the current is shut 

 off from the nerve, when another simple contraction, the so- 

 called breaking contraction, is observed. The mere passage of a 

 constant current of uniform intensity through a nerve does not 

 under ordinary circumstances act as a stimulus generating a 

 nervous impulse ; such an impulse is only set up when the 

 current either falls into or is shut off from the nerve. It is 

 the entrance or the exit of the current, and not the continuance of 

 the current, which is the stimulus. 



The quiescence of the nerve and muscle during the passage of 

 the current is however dependent on the current remaining 

 uniform in intensity or at least not being suddenly increased 

 or diminished. Any sufficiently sudden and large increase or 

 diminution of the intensity of the current, will act like the 

 entrance or exit of a current, and by generating nervous impulses 

 give rise to contractions. If the intensity of the current however 

 be very slowly and gradually increased or diminished, a very wide 

 range of intensity may be passed through without any contraction 

 being seen. It is the sudden change from one condition to another, 

 and not the condition itself, which causes the nervous impulse. 



In many cases, both a ' making ' and a ' breaking ' contraction, 

 each a simple spasm, are observed, and this is perhaps the 

 commonest event ; but when the current is very weak, and again 

 when the current is very strong either the breaking or the making 

 contraction may be absent, i.e. there may be a contraction only 

 when the current is thrown into the nerve or only when it is 

 shut off from the nerve. 



Under ordinary circumstances the contractions witnessed with 

 the constant current either at the make or at the break, are of the 

 nature of a ' simple ' contraction, but, as has already been said, the 

 application of the current may give rise to a very pronounced 

 tetanus. Such a tetanus is seen sometimes when the current 

 is made, lasting during the application of the current, sometimes 

 when the current is broken, lasting some time after the current has 

 been wholly removed from the nerve. The former is spoken of as 

 a ' making/ the latter as a ' breaking ' tetanus. But these excep- 

 tional results of the constant current need not detain us now. 



The great interest attached to the action of the constant 

 current lies in the fact, that during the passage of the current, 

 in spite of the absence of all nervous impulses and therefore 

 of all muscular contractions, the nerve is for the time both between 

 and on each side of the electrodes profoundly modified in a most 



