THE PHENOMENA OF MUSCLE AND NERVE. 67 



can measure the duration of the rise of the curve or of the fall, or of any 

 part of it. 



Though the tuning-fork may, by simply striking it, be set going long 

 enough for the purposes of an observation, it is convenient to keep it going 

 by means of an electric current and a magnet, very much as the spring in 

 the magnetic interrupter (Fig. 15) is kept going. 



It is not necessary to use an actual tuning-fork ; any rod, armed with a 

 marker, which can be made to vibrate regularly, and whose time of vibra- 

 tion is known, may be used for the purpose : thus a reed, made to vibrate 

 by a blast of air, is sometimes employed. 



The exact moment at which the induction shock is thrown into the 

 nerve may be recorded on the muscle-curve by means of a " signal," which 

 may be applied in various ways. 



A large steel lever armed with a marker is arranged over a small coil by means 

 of a light spring in such a way that when the coil by the passage of a current 

 through it becomes a magnet it pulls the lever down to itself; on the current being 

 broken, and the magnetization of the coil ceasing, the lever by help of the spring 

 Hies up. The marker of such a lever is placed immediately under i. c. , at some 

 point on the arc described by the marker of the muscle (or other) lever. Hence 

 by making a current in the coil and putting the signal lever down, or by breaking 

 an already existing current, and letting the signal lever fly up, we can make at 

 pleasure a mark corresponding to any part we please of the muscle (or other) curve. 



If, in order to magnetize the coil of the signal, we use, as we may do, the pri- 

 mary current which generates the induction-shock, the breaking or making of the 

 primary current, whichever we use to produce the 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 cur- 

 rent 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 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. 17, 18, 19. 



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. 



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 purpose the signal is intro- 

 duced into a circuit the current of which is continually being made and broken by 

 a tuning-fork (Fig. 21 ). 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 other- 

 wise; such a cylinder driven by clockwork is shown in Fig. 13, B. By using a 



