CHAP. IL] THE CONTRACTILE TISSUES. 87 



the nearer lever some little time before it has passed by the 

 farther lever ; and the farther apart the two levers are the greater 

 will be the difference in time between their movements. In other 

 words the contraction travels along the muscle in the form of a 

 wave, each part of the muscle in succession from the end 

 stimulated swelling out and shortening as the contraction reaches 

 it, and then returning to its original state. And what is true of 

 the collection of parallel fibres which we call the muscle is also 

 true of each fibre, for the swelling at any part of the muscle is 

 only the sum of the swelling of the individual fibres ; and if we 

 were able to take a single long fibre and stimulate it at one end, 

 we should be able under the microscope to see a swelling or 

 bulging accompanied by a corresponding shortening, i.e. to see a 

 contraction, sweep along the fibre from end to end. 



If in the graphic record of the two levers just mentioned 

 we count the 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 '005 sec. Let us suppose 

 the distance between the two levers to be 15 mm. The con- 

 traction wave then has taken '005 sec. to travel 15 mm., that is 

 to say it has travelled at the rate of 3 meters per sec. 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 meters a second, though it varies under different con- 

 ditions. In the warm-blooded mammal the rate is somewhat 

 greater, and may probably be put down at 5 meters a second 

 in the excised muscle, rising possibly to 10 meters 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 intervening between the 

 contraction wave reaching the lever, and leaving the lever on its 

 way onward, 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 

 *1 sec. But a wave which is travelling at the rate of 3 m. a 

 second and takes *1 sec. 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 



