70 GRAPHIC RECORD OF A CONTRACTION. [Book i. 



induction-shock, will make the signal lever fly up or come down. 

 Hence we shall have on the recording surface, under the muscle, a 

 mark indicating the exact moment at which the primary current was 

 hroken or made. Now, the time taken up by the generation of the 

 induced current and its passage into the nerve between the electrodes 

 is so infinitesimally small, that we may, without appreciable error, take 

 the moment of the breaking or making of the primary current as 

 the moment of the entrance of the induction-shock into the nerve. 

 Thus we can mark below the muscle-curve, or, by describing the arc of 

 the muscle lever, on the muscle-curve itself, the exact moment at which 

 the induction-shock falls into the nerve between the electrodes, as is 

 done at a in Figs. 7, 8, 9. 



In the pendulum myograph a separate signal is not needed. If, 

 having placed the muscle lever in the position in which we intend to 

 make it record, we allow the glass plate to descend until the tooth a' 

 just touches the rod c (so that the rod is just about to be knocked 

 down, and so break the primary circuit) and make on the base line, 

 which is meanwhile being described by the lever marker, a mark to 

 indicate where the point of the marker is under these circumstances, 

 and then bring back the plate to its proper position, the mark which 

 we have made will mark the moment of the breaking of the primary 

 circuit and so of the entrance of the induction-shock into the nerve. 

 For it is just when, as the glass plate swings down, the marker of the 

 lever comes to the mark which we have made that the rod c is knocked 

 back and the primary current is broken. 



Fig. 11. Diagram of an Arrangement of a Vibrating Tuning-fork 



with a Desprez Signal. 



The current flows along the wire /connected with the positive (+) pole or end 

 of the negative plate (.V) of the battery, through the tuning-fork, down the pin 

 connected with the end of the lower prong, to the mercury in the cup Hg, and so by 

 a wire (shewn in the figure as a black line bent at right angles) to the binding 

 screw e. From this binding screw part of the current flows through the coil a 

 between the prongs of the tuning-fork, and thence by the wire c to the binding 

 screw a, while another part flows through the wire g, through the coil of the 

 1 >esprez signal back by the wire b, to the binding screw a. From the binding 

 screw a the current passes back to the negative (— ) pole or end of the positive 

 element (P) of the battery. As the current flows through the coil of the Desprez 

 signal from g to b, the core of coil becoming magnetized draws to it the marker of 

 the signal. As the current flows through the coil d, the core of that coil, also 

 becoming magnetized, draws up the lower prong of the fork. But the pin is so 

 adjusted that the drawing up of the prong lifts the point of the pin out of the 

 mercury. In consequence, the current, being thus broken at 1I<], flows neither 

 through d nor through the Desprez signal. In consequence, the core of the Desprez 

 thus ceasing to be magnetized, the marker flies back, being usually assisted by a 



