CHAP, ii.] THE CONTRACTILE TISSUES. 71 



spring (not shewn in the figure). But, in consequence of the current ceasing to flow 

 through d, the core of d ceases to lift up the prong, and the pin, in the descent of 

 the prong, makes contact once more with the mercury. The re-establishment of the 

 current, however, once more acting on the two coils, again pulls upon the marker of 

 the signal, and again, by magnetizing the core of c?, pulls up the prong and once 

 more breaks the current. Thus the current is continually made and broken, the 

 rapidity of the interruptions being determined by the vibration periods of the 

 tuning-fork, and the lever of the signal rising and falling synchronously with the 

 movements of the tuning-fork. 



A 'signal' like the above, in an improved form known as Desprez's, 

 may be used also to record time, and thus the awkwardness of bringing 

 a large tuning-fork up to the recording surface obviated. For this pur- 

 pose the signal is introduced into a circuit, the current of which is 

 continually being made and broken by a tuning-fork (Fig. 10). The 

 tuning-fork, once set vibrating, continues to make and break the current 

 at each of its vibrations, and, as stated above, is kept vibrating by the 

 current. But each make or break caused by the tuning-fork affects 

 also the small coil of the signal, causing the lever of the signal to fall 

 down or fly up. Thus the signal describes vibration curves synchronous 

 with those of the tuning-fork driving it. The signal may similarly be 

 worked by means of vibrating agents other than a tuning-fork. 



Various recording surfaces may be used. The form most generally 

 useful is a cylinder covered with smoked paper, and made to revolve by 

 clockwork or otherwise ; such a cylinder driven by clockwork is shewn 

 in Fig. 3, B. By using a cylinder of large radius with adequate gear, 

 a high speed, some inches for instance in a second, can be obtained. In 

 the spring myograpk a smoked glass plate is thrust rapidly forward 

 along a groove, by means of a spring suddenly thrown into action. In 

 the pendulum myograph, Fig. 9, a smoked glass plate attached to the 

 lower end of a long frame, swinging like a pendulum, is suddenly let go 

 at a certain height, and so swings rapidly through an arc of a circle. 

 The disadvantage of the last two methods is that the surface travels at 

 a continually changing rate, whereas, in the revolving cylinder, careful 

 construction and adjustment will secure a very uniform rate. 



46. Having thus obtained a time record, and an indication 

 of the exact moment at which the induction-shock falls into the 

 nerve, "we may for present purposes consider the muscle-curve 

 complete. The study of such a curve, as for instance that shewn 

 in Fig. 7, taken from the gastrocnemius of a frog, teaches us the 

 following facts : 



1. That although the passage of the induced current from 

 electrode to electrode is practically instantaneous, its effect, meas- 

 ured from the entrance of the shock into the nerve to the return 

 of the muscle to its natural length after the shortening, takes 

 an appreciable time. In the figure, the whole curve from a to d 

 takes up about the same time as eleven double vibrations of the 

 tuning-fork. Since each double vibration here represents 100th of 

 a second, the duration of the whole curve is rather more than 



