74 VELOCITY OF NERVOUS IMPULSE. [BOOK i. 



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 occupy the latent period, secondly the 

 changes which bring about the visible shortening or contraction 

 proper, and thirdly the changes which bring about the relaxation 

 and return to the original length. The changes taking place in 

 these several phases are changes of living matter : they vary with 

 the condition of the living substance 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 con- 

 traction vary in extent and character according to the condition of 

 the muscle, the strength of the induction-shock, the load which the 

 muscle is bearing, and various attendant circumstances, but the 

 three phases may vary independently. The latent period may be 

 longer or shorter, the shortening may take a longer or shorter 

 time to reach the same height, and especially the relaxation may 

 be slow or rapid, complete or imperfect. Even when the same 

 strength of induction-shock is used, the contraction may be short 

 and sharp, or very long drawn out, so that the curves described on 

 a recording surface, travelling at the same rate in the two cases, 

 appear very different ; and, under certain circumstances, as when a 

 muscle is fatigued, the relaxation, more particularly the last part 

 of it, may be so slow, that it may be several seconds before the 

 muscle really regains its original length. We may add that the 

 latent period, which in an ordinary experiment on a frog's gastro- 

 cnemius is so conspicuous, may, under certain circumstances, be so 

 shortened as almost, if not wholly, to disappear. Indeed, it is 

 maintained by some that the occurrence of the latent period is 

 not an essential feature of the whole act. 



Hence, if we say that the duration of a simple muscular con- 

 traction of the gastrocnemius of a frog under ordinary circumstances 

 is about j 1 ^ sec., of which -^7 is taken up by the latent period, -^fa 

 by the contraction, and ^-^ by the relaxation, these must be taken 

 as 'round numbers,' stated so as to be easily remembered. The 

 duration of each phnse as well as of the whole contraction varies in 



