CHAP, ii.] THE CONTRACTILE TISSUES. 77 



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 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. 



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

 traction of the gastrocnemius of a frog, under ordinary circumstances 

 is about -Jg- sec., of which y^ is taken up by the latent period, y^ '' 

 by the contraction, and T |^ by the relaxation, these must be taken 

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

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

 different animals, in different muscles of the same animal, and in 

 the same muscle under different conditions. 



The muscle curve which we have been discussing is a curve of 

 changes in the length only of the muscle ; but if the muscle, instead 

 of being suspended, were laid flat on a glass plate and a lever laid 

 over its belly, we should find, upon sending an induction-shock 

 into the nerve, that the lever was raised, shewing that the muscle 

 during the contraction became thicker. And if we took a graphic 

 record of the movements of the lever we should obtain a curve 

 very similar to the one just discussed ; after a latent period the lever 

 would rise, shewing that the muscle was getting thicker, and after- 

 wards would fall, shewing that the muscle was becoming thin again. 



