CHAP, ii.] THE CONTRACTILE TISSUES. 85 



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. 



54. We may now turn to the question, What takes place in 

 a muscular fibre when a contraction wave sweeps over it ? 



Optical Changes. Although undoubtedly the optical features 

 of a muscular fibre change while it is contracting, it is very diffi- 

 cult to make an authoritative statement as to what those changes 

 are. In the first place a contraction wave, even when it is travel- 

 ling with relative slowness, travels so rapidly that the individual 

 features cannot be seized by the eye. We are confined to con- 

 clusions drawn from the study of short local contractions, local 

 thickenings and shortenings which may be obtained in the living 

 fibre and fixed by the action of osmic acid vapour or by other 

 means ; and it has to be assumed that these local bulgings give a 

 true picture of a normal contraction wave by which, as we have 

 just seen, the whole length of a fibre is occupied at the same time. 

 In the second place the minute structure of a muscular fibre has 

 been and still is the subject of fierce dispute. 



If we adopt the view that the fibre is made up of dim 

 bands or discs of dim substance alternating with bright bands 

 or discs of bright substance, with transverse markings in the 

 middle of each bright band forming a line " intermediate " be- 

 tween the two adjacent dim bands, we may, according to some 

 observers, say that during a contraction there seems to be an 

 interchange between the dim and bright bands so that, in ordinary 

 light, at the height of the contraction, in the broadest part of one 

 of the bulgings just spoken of, the previously obscure " interme- 

 diate line " becomes a conspicuous dark band, the interval between 

 two such changed intermediate lines becoming relatively and uni- 



