THE PHENOMENA OF MUSCLE AND NERVE. 69 



cylinder of large radius with adequate gear, a high speed for instance, in a second, 

 can be obtained. In the spring myograph a smoked glass plate is thrust rapidly 



FIG. 21. 



! 

 p 



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 (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 Hg, and so by a wire (shown 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 Desprez 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 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 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 Hg, flows neither 

 through d nor through the Desprez signal. The core of the Desprez thus ceasing to be magnetized, 

 the marker flies back, being usually assisted by a spring (not shown in the figure). But since 

 the current ceases 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 down the marker of the signal, 

 and again by magnetizing the core of d 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. 



s 



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

 pendulum myograpli, Fig. 20, 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 shown in Fig. 17, 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, measured from the entrance 

 of the shock into the 'nerve to the return of the muscle to its natural length 



dotted line c'. The rod c is in electric continuity with the wire x of the primary coil of an 

 induction-machine. The screw d is similarly in electric continuity with the wire y of the same 

 primary coil. The screw d and the rod c are armed with platinum at the points at which they 

 are in contact, and both are insulated by means of the ebonite block e. As long as c and d are 

 in contact the circuit of the primary coil to which x and y belong is closed. When in its swing 

 the tooth a' knocks c away from d, at that instant the circuit is broken, and a " breaking " shock 

 is sent through the electrodes connected with the secondary coil of the machine, and so through 

 the nerve. The lever I, the end only of which is shown in the figure, is brought to bear on the 

 glass plate, and when at rest describes a straight line, or more exactly an arc of a circle of large 

 radius. The tuning-fork /, the ends only of the two limbs of which are shown in the figure 

 placed immediately below the lever, serves to mark the time. 



