66 



THE CONTRACTILE TISSUES. 



FIG. 19. 



"tuning-fork being the same as that used in Fig. 17; indeed, with a very 

 slow movement, the two may be hardly separable from each other. On 

 the other hand, if the surface travel very rapidly the 

 curve may be immediately long drawn out, as in* Fig. 

 19, which is a curve from a gastrocnemius muscle of 

 a frog, taken with a very rapidly moving pendulum 

 myograph, the tuning-fork making about 500 vibra- 

 tions a second. On examination, however, it will be 

 found that both these extreme curves are fundamen- 

 tally the same as the medium one, when account is 

 taken of the different rapidities of the travelling sur- 

 face 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 movement 

 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 neces- 

 sary, to measure the rate of movement 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 forward and backward, 100 times 

 a second be brought while vibrating to make a trac- 

 ing on the recording surface immediately 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 ex- 

 actly 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. 17, 

 we can count the number of vibrations of the tuning- 

 fork which have taken place between the two marks, 

 and so ascertain the whole time of the muscle-curve ; 

 if, for instance, there have been 10 double vibrations, each occupying T ^Q- 

 second, the whole curve has taken y 1 ^ second to make. In the same way we 



