90 THE WAVE OF CONTRACTION. [B OK l - 



to be at most only about 40 mm. in length ; hence, in an ordinary 

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

 traction the whole length of the fibre will be occupied by the 

 contraction 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 



