THE PHENOMENA OF MUSCLE AND NEKVE. 71 



found that the latent period remained about the same, that is to say, that in 

 all cases the latent period is chiefly taken up by changes in the muscular as 

 distinguished from the nervous elements. 



2. Such difference as does exist between the two curves in the figure 

 indicates the time taken up by the propagation, along the piece of nerve, of 



FIG. 22. 



Curves Illustrating the Measurement of the Velocity of a Nervous Impulse : The same muscle- 

 nerve preparation is stimulated (1) as" far as possible from the muscle, (2) as near as possible to 

 the muscle ; both contractions are registered in exactly the same way. 



In (1) the stimulus enters the nerve at the time indicated by the line a, the contraction begins 

 at b'; the whole latent period, therefore, is indicated by the distance from a to b'. 



In (2) the stimulus enters the nerve at exactly the same time a ; the contraction begins at b ; 

 the latent period, therefore, is indicated by the distance between a and b. 



The time taken up by the nervous impulse in passing along the length of nerve between 1 and 

 2 is, therefore, indicated by the distance between 6 and b', which may be measured by the tuning- 

 fork curve below ; each double vibration of the tuning-fork corresponds to 1-120 or 0.0083 second. 



the changes set up at the far end of the nerve by the induction-shock. 

 These changes we have already spoken of as constituting a nervous impulse ; 

 and the above experiment shows that it takes a small but yet distinctly 

 appreciable time for a nervous impulse to travel along a nerve. In the 

 figure the difference between the two latent periods, the distance between b 

 and 6', seems almost too small to measure accurately ; but if a long piece 

 of nerve be used for the experiment, and the recording surface be made to 

 travel very fast, the difference between the duration of the latent period 

 when the induction-shock is sent in at a point close to the muscle, and that 

 when it is sent in at a point as far away as possible from the muscle, may 

 be satisfactorily measured in fractions of a second. If the length of nerve 

 between the two points be accurately measured, the rate at which a nervous 

 impulse travels along the nerve to a muscle can thus be easily calculated. 

 This has been found to be in the frog about 28, and in man about 33 

 metres per second, but varies considerably, especially in warm-blooded 

 animals. 



Thus when a momentary stimulus, such as a single induction-shock, is sent 

 into a nerve connected with a muscle, the following events take place : a 

 nervous impulse is started in the nerve and this travelling down to the muscle 

 produces in the muscle, first the invisible changes which constitute the latent 

 period, secondly the changes which bring about the shortening or contraction 

 proper, and thirdly the changes which bring about the relaxation and return 

 to the original length. The changes taking place in each of these three phases 

 are changes of living matter ; they vary with the condition of the living sub- 

 stance of the muscle, and only take place so long as the muscle is alive. 

 Though the relaxation which brings back the muscle to its original length 

 is assisted by the muscle being loaded with a weight or otherwise stretched, 

 this is not essential to the actual relaxation, and with the same load the 

 return will vary according to the condition of the muscle ; the relaxation 

 must be considered as an essential part of the whole contraction no less than 

 the shortening itself. 



47. Not only, as we shall see later on, does the whole contraction vary 



