72 A SIMPLE MUSCULAR CONTRACTION. [BOOK i. 



bearings at C. The contrivances by which the glass plate can be removed and 

 replaced at pleasure are not shewn. A second glass plate so arranged that the 

 first glass plate may be moved up and down without altering the swing of the 

 pendulum is also omitted. Before commencing an experiment the pendulum is 

 raised up (in the figure to the right), and is kept in that position by the tooth a 

 catching on the spring-catch b. On depressing the catch b the glass plate is set 

 free, swings into the new position indicated by the dotted lines, and is held in that 

 position by the tooth a' catching on the catch b'. In the course of its swing the 

 tooth a' coming into contact with the projecting steel rod c, knocks it on one side 

 into the position indicated by the 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 in 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 shewn 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 shewn in the figure 

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



occupying yj^ sec., the whole curve has taken ^ sec. to make. 

 In the same way we 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. 5) 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 vibration is known, may be used for the purpose ; 

 thus a reed, made to vibrate by a blast of air, is sometimes em- 

 ployed. 



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 light 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 magneti- 

 sation of the coil ceasing, the lever by help of the spring flies up. The 

 marker of such a lever is placed immediately under (i.e. 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 magnetise the coil of the signal, we use, as we may 

 do, the primary current which generates the induction-shock, the break- 

 ing or making of the primary current, whichever we use to produce the 



