CHAP, ii.] THE CONTRACTILE TISSUES. 127 



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 a 

 nervous impulse give rise to a contraction. If the intensity of the 

 current however be very slowly and gradually increased or di- 

 minished, 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 twitch, 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 application 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 

 peculiar manner. This modification, important both for the light 

 it throws on the generation of nervous impulses and for its practical 

 applications, is known under the name of electrotonus. 



