MECHANICAL RESPONSE OF NERVE 509 



cause. With mechanical response, however, provided this 

 could be rendered practicable, no such difficulty need arise. 

 For in that case it would be the direct effect of the exci- 

 tatory change, uncomplicated by any other disturbance, 

 which would be recorded. 



Finally, as regards the detection of the excitatory change 

 itself, the galvanometer is unable to indicate any change 

 below a certain high intensity of excitation. Thus it gives 

 no indication when excitation is due to one or to a few 

 shocks : it can only detect an excitatory effect which is much 

 stronger than this, having been brought about by the super- 

 posed effects of tetanic shocks of a certain duration. In 

 order to obtain even such effects, a galvanometer of very 

 high sensitiveness is necessary. That of a fairly delicate 

 instrument, detecting a current of about 'ooi ampere, will 

 have to be exalted some ten millions of times before it can 

 give efficient indications of excitatory effects in nerves ; and 

 in such a degree of galvanometric sensitiveness we approach 

 a limit which cannot be very much exceeded. 



Returning now to our original question, we have first to 

 determine whether excitation causes any motile effect in 

 nerve. Under observation, it is easily seen that when the 

 nerve is excited by tetanic electrical shocks it increases in 

 thickness and at the same time shortens in length. We 

 have here a phenomenon in every way analogous to the 

 thickening and shortening of muscle under excitation. The 

 contraction which occurs in nerve, moreover, is of an order 

 by no means microscopic. I give here a record (fig. 313) 

 of the contractile response of nerve under continuous 

 stimulation by fairly strong tetanising electric shocks. This 

 record was obtained by means of the ordinary lever- recorder, 

 the magnification employed being only three times. The 

 induced contraction in this particular case was about 14 per 

 cent. It will also be seen that this contraction reached a limit, 

 at which state of maximum contraction the nerve remained 

 for a considerable time. After this we observe a tendency 



