70 A SIMPLE MUSCULAR CONTRACTION. [BOOK i. 



found that both these extreme curves are funda- 

 mentally the same as the medium one, when 

 account is taken of the different rapidities of the 

 travelling surface in the several cases. 



In order to make the ' muscle-curve ' complete, 

 it is necessary to mark on the recording surface the 

 exact time at which the induction-shock is sent into 

 the nerve, and also to note the speed at which the 

 recording surface is travelling. 



In the pendulum myograph the rate of move- 

 ment can be calculated from the length of the 

 pendulum ; but even in this it is convenient, and 

 in the case of the spring myograph and revolving 

 cylinder is necessary, to measure the rate of move- 

 ment directly by means of a vibrating tuning-fork 

 or of some body vibrating regularly. Indeed it is 

 best to make such a direct measurement with each 

 curve that is taken. 



A tuning-fork, as is known, vibrates so many 

 times a second according to its pitch. If a tuning- 

 fork, armed with a light marker on one of its prongs 

 and vibrating say 100 a second, i.e. executing a 

 double vibration, moving forwards and backwards, 

 100 times a second, be brought while vibrating to 

 make a tracing on the recording surface immedi- 

 ately below the lever belonging to the muscle, we 

 can use the curve or rather curves described by the 

 tuning-fork to measure the duration of any part or 

 of the whole of the muscle-curve. It is essential 

 that at starting the point of the marker of the 

 tuning-fork should be exactly underneath the marker 

 of the lever, or rather, since the point of the lever 

 as it moves up and down describes not a straight 

 line but an arc of a circle of which its fulcrum is 

 the centre and itself (from the fulcrum to the tip 

 of the marker) the radius, that the point of the 

 marker of the tuning-fork should be exactly on 

 the arc described by the marker of the lever, either 

 above or below it, as may prove most convenient. 

 If then at starting the tuning-fork marker be thus 

 on the arc of the lever marker, and we note on the 

 curve of the tuning-fork the place where the arc 

 of the lever cuts it at the beginning and at the end 

 of the muscle-curve as at Fig. 7, we can count the 

 number of vibrations of the tuning-fork which have 

 taken place between the two marks, and so ascer- 

 tain the whole time of the muscle curve ; if for 

 instance there have been 10 double vibrations, each 



FIG. 9. 



