CHAP, ii.] THE CONTRACTILE TISSUES. 45 



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 6 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 hi the figure 

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



the glass plate, may be calculated from the length of the pendulum, but it 

 is simpler and easier to place a vibrating tuning-fork immediately under 

 the point of the lever. If the vibrations of the tuning-fork are known, 

 then the number of vibrations which are marked on the plate between any 

 two points on the line described by the lever gives the time taken by the 

 lever in passing from one point to the other. An easy arrangement 

 permits the exact time at which the shock is sent through the nerve to 

 be marked on the line of the lever. To avoid the confusion of too 

 many markings on the plate the pendulum after describing an arc is 

 caught by a spring catch on the opposite side. 



A complete muscle-curve, such as that shewn in Fig. 3, 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 after the shortening, takes an appreci- 

 able time. In the figure, the whole curve from a to d takes up 

 about the same time as eighteen double vibrations of the tuning- 

 fork. Since each double vibration here represents -^ of a second, 

 the duration of the whole curve is ^ sec. 



2. In the first portion of this period, from a to b, there is no 

 visible change, no shortening of the muscle, no raising of the lever. 



3. It is not until 6, that is to say after the lapse of 



IoU 



i.e. about -^sec., that the shortening begins. The shortening as 

 shewn by the curve is at first slow, but soon becomes more rapid, 

 and then slackens again until it reaches a maximum at c; the 

 whole shortening occupying about -^ sec. 



4. Arrived at the maximum of shortening, the muscle at once 

 begins to relax, the lever descending at first slowly, then very 



. 



