MECHANICAL RESPONSE OF NERVE 525 



condition of the tissue has been raised, by the application 

 of fresh energising stimulus, that the conducting power can 

 be gradually restored. 



This leads me to what is theoretically a very interesting 

 mode of determining the velocity of transmission in nerve, 

 by the mechanical response of the nerve itself, which will be 

 understood from the diagram already given (fig. 315). In 

 that figure, A B C is the nerve, so clamped at B as to prevent 

 any mechanical slip, but not tightly enough to obstruct the 

 transmission of excitation. The nerve, when brought to a 

 normal excitatory condition, is first excited at A a by a pair 

 of electrodes in connection with an induction coil. The 

 transmitted excitation, reaching B C, induces a contractile 

 mechanical response there, observed by the highly magnify- 

 ing optic lever. Records of the transmitted effect of stimulus 

 obtained in this manner will be given later in the chapter. 

 The interval of time, /, between the application of stimulus 

 and the initiation of response is accurately determined by 

 the usual methods. Stimulus is next applied at B b, and the 

 interval of time t' between stimulus and response again 

 determined. The difference (//') is the time required for 

 the stimulus to travel the intervening distance a b. By this 

 means, I found the velocity of transmission in a certain 

 specimen of nerve of fern to be 50 mm. per second. 



It is thus easy, by means of two successive experiments, 

 to eliminate from the observation the element of the latent 

 period. It is to be understood that the molecular change, 

 ultimately to be expressed as contraction, begins to be 

 initiated as soon as excitation reaches the responding area. 

 As the contractile effect exhibited by the nerve is relatively 

 small, we can see that a certain time will elapse before it 

 becomes sufficient to be perceptible, unless the magnification 

 employed is very high. With a magnification of the order 

 of 100,000 times, however which, as I shall show, is quite 

 practicable this loss of time is much lessened. 



It was while working out this investigation that I realised 

 how indefinite must be any determination of the velocity of 



