88 THE WAVE OF CONTRACTION. [BOOK i. 



contraction, during the greater part of the duration of the contrac- 

 tion the whole length of the fibre will be occupied by the contrac- 

 tion wave. Just at the beginning of the contraction there will be a 

 time when the front of the contraction wave has reached for 

 instance only half way down the fibre (supposing the stimulus to be 

 applied, as in the case we have been discussing, at one end only), 

 and just at the end of the contraction there will be a time for 

 instance when the contraction has left the half of the fibre next 

 to the stimulus, but has not yet cleared away from the other half. 

 But nearly all the rest of the time every part of the fibre will be 

 in some phase or other of contraction, though the parts nearer the 

 stimulus will be in more advanced phases than the parts farther 

 from the stimulus. 



This is true when a muscle of parallel fibres is stimulated 

 artificially at one end of the muscles, and when therefore each 

 fibre is stimulated at one end. It is of course all the more true 

 when a muscle of ordinary construction is stimulated by means of 

 its nerve. The stimulus of the nervous impulse impinges, in this 

 case, on the muscle fibre at the end plate which, as we have said, is 

 placed towards the middle of the fibre, and the contraction wave 

 travels from the end plate in opposite directions toward each end, 

 and has accordingly only about half the length of the fibre to run 

 in. All the more therefore must the whole fibre be in a state of 

 contraction at the same time. 



It will be observed that in what has just been said the 

 contraction wave has been taken to include not only the con- 

 traction proper, the thickening and shortening, but also the 

 relaxation and return to the natural form ; the first part of the 

 wave up to the summit of the crest corresponds to the shortening 

 and thickening, the decline from the summit onwards corresponds 

 to the relaxation. But we have already insisted that the relax- 

 ation is an essential part of the whole act, indeed in a certain sense 

 as essential as the shortening itself. 



54. Minute structure of muscular fibre. So far we have 

 been dealing with the muscle as a whole and as observed with 

 the naked eye, though we have incidentally spoken of fibres. 

 We have now, confining our attention exclusively to skeletal 

 muscles, to consider what microscopic changes take place during 

 a contraction, what are the relations of the histological features 

 of the muscle fibre to the act of contraction. 



The long cylindrical sheath of sarcolemma is occupied by 

 muscle substance. After death the muscle substance may separate 

 from the sarcolemma, leaving the latter as a distinct sheath, but 

 during life the muscle substance is adherent to the sarcolemma, 

 so that no line of separation between the two can be made out ; 

 the movements of the one follow exactly all the movements of 

 the other. 



Scattered in the muscle substance but, in the mammal, lying 



