CHANGES IN A MUSCLE DURING CONTRACTION. 79 



number of vibrations of the tuning-fork which intervene between the mark 

 on the record which indicates the beginning of the rise of the near lever 

 (that is, the arrival of the contraction wave at this lever) and the mark 

 which indicates the beginning of the rise of the far lever, this will give us 

 the time which it has taken the contraction wave to travel from the near to 

 the far lever. Let us suppose this to be 0.005 second. Let us suppose the 

 distance between the two levers to be 15 mm. The contraction wave, then, 

 has taken 0.005 second to travel 15 mm., that is to say it has travelled at the 

 rate of 3 metres per second. And, indeed, we find by this, or by other 

 methods, that in the frog's muscles the contraction wave does travel at a 

 rate which may be put down as from 3 to 4 metres a second, though it 

 varies under different conditions. In the warm-blooded mammal the rate 

 is somewhat greater, and may probably be put down at 5 metres a second 

 in the excised muscle, rising possibly to 10 metres in a muscle within the 

 living body. 



If, again, in the graphic record of the two levers we count, in the case of 

 either lever, the number of vibrations of the tuning-fork which intervene 

 between the mark where the lever begins to rise and the mark where it has 

 finished its fall and returned to the base-line, we can measure the time inter- 

 vening between the contraction wave reaching the lever and leaving the lever 

 on its way on ward, that is to say we can measure the time which it has taken 

 the contraction wave to pass over the part of the muscle on which the lever 

 is resting. Let us suppose this time to be, say, 0.1 second. But a wave which 

 is travelling at the rate of 3 metres a second, and takes 0.1 second to pass 

 over any point must be 300 mm. long. And, indeed, we find that in the 

 frog the length of the contraction wave may be put down as varying from 

 200 to 400 mm., and in the mammal it is not very different. 



Now, as we have said, the very longest muscular fibre is stated to be at 

 most only about 40 mm. in length ; hence, in an ordinary contraction, dur- 

 ing the greater part of the duration of the contraction 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 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 further 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 construc- 

 tion 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 toward the middle of the fibre, and the contrac- 

 tion 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 the contraction wave includes not only the con- 

 traction proper and the thickening and shortening, but 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 onward corresponds to the relaxation. But we have already 



