CHAP, ii.] 



THE CONTRACTILE TISSUES. 



73 



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

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

 mark indicating the exact moment at which the primary current was 

 broken 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 infinitesinially 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 bi'oken. 



FIG. 11. DIAGRAM OF AN ARRANGEMENT OF A VIBRATING TUNING-FORK WITH A 



DESPRETZ SIGNAL. 



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

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

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

 a wire (shewn in figure) to the binding screw e. From this binding screw part of the 

 current flows through the coil d 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 Despretz 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 Despretz signal from g to b, the core of coil becoming magnetized draws down 

 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 piu 

 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 Hg, flows neither 

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

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

 spring (not shown 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 



