STIMULUS BY ELECTRIC CURRENT. 101 



time during which it was applied ; that, so long as the current passed along 

 the nerve, nervous impulses would be generated ; and that these would throw 

 the muscle into something, at all events, like tetanus. And, under certain 

 conditions, this does take place ; occasionally it does happen that at the 

 moment the current is thrown into the nerve the muscle of the muscle-nerve 

 preparation falls into a tetanus, which is continued until the current is shut 

 off ; but such a result is exceptional. In the vast majority of cases what 

 happens is as follows : At the moment that the circuit is made, the mo- 

 ment that the current is thrown into the nerve, a single twitch, 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 nerv- 

 ous 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 quies- 

 cence 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 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 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 con- 

 stant 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 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 exceptional 

 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 practi- 

 cal applications, is known under the name of electrotonus. 



73. Electrotonus. The marked feature of the electrotonic condition is 

 that the nerve, though apparently quiescent, is changed in respect to its irri- 

 tability ; and that in a different way in the neighborhood of the two elec- 

 trodes respectively. 



