MULTIPLE RESPONSE OF NERVE 535 



We have seen that in order to obtain a galvanometric record 

 of the electrical response of nerve, one or even a few shocks 

 will not be sufficient to induce the necessary electromotive 

 change. For this, tetanic shocks of a certain duration are 

 necessary, and the responsive electromotive change is not 

 immediately perceptible. In consequence of this intensity of 

 stimulation, moreover, complete electrical recovery can only 

 take place after a perceptible interval. With the low magnifi- 

 cation Kunchangraph, too, tetanic shocks of something like 

 a second in duration are necessary, and complete recovery 

 here also requires a period of about one minute. But with 

 the greater sensitiveness available in the highly magni- 

 fying apparatus, response with a highly excitable specimen 

 of nerve is obtained with even so short-lived a stimu- 

 lation as that of two or three vibrations of the vibrating 

 interrupter of the secondary coil, lasting less than one-tenth 

 of a second. The responsive contraction of this short-lived 

 stimulus, and its recovery, are also quick. It is in conse- 

 quence of the rapidity of this response and recovery that the 

 responsive contractions due to the rapidly intermittent 

 tetanising shocks do not become fused, but show themselves 

 in the response-curve as consisting of successive twitches, 

 corresponding to the component shocks. Owing to the high 

 amplitude of these responses, and the trend of the base-line 

 either up or down, it is difficult in practice to obtain photo- 

 graphic records of these effects. But it is easy enough to obtain 

 definite visual demonstration of various characteristic effects 

 in the response by the employment of the following device. 

 The spot of light from the optic lever is made incident on 

 a revolving mirror, and reflected from it to a large white 

 screen at some distance. During a period of repose the 

 quiescent spot traces a more or less horizontal line of light. 

 This may trend either in a downward or an upward direction 

 continuously, according as there is induced a continuous 

 sub-tonic relaxation or a growing contraction, due to the 

 after-effect of stimulus. Somewhere between these two 

 extremes may be obtained a condition of more or less 



