PRACTICAL EXERCISES 623 



With the short-circuiting key closed, press the release and allow 

 an abscissa line to be traced Again shove back the frame till it is 

 caught. Push home the rod by means of which the prongs of the 

 tuning-fork are separated, and rotate it through 90. Close the 

 knock-over key, open the short-circuiting key, shoot the plate again, 

 and a muscle-curve and time-tracing will be recorded. Again close 

 the short-circuiting key, withdraw the writing-point of the tuning- 

 fork, push back the plate, close the trigger-key, then open the short- 

 circuiting key, and holding the travelling frame with the hand, allow 

 it just to open the knock-over and stimulate the nerve. The writing- 

 point now records a vertical line (or, rather, an arc of a circle), which 

 marks on the tracing the moment of stimulation. The latent period 

 is obtained by drawing a parallel line (or arc) through the point of 

 the muscle-curve where it just begins to diverge from the abscissa 

 line. The value of the portion of the time-tracing between these two 

 lines can be readily determined, and is the latent period. 



17. Summation of Stimuli. Arrange two knock-over keys on the 

 spring myograph at such a distance from each other that the plate 

 travels from one to the other in a time less than the latent period. 

 Connect each key with the primary circuit of a separate induction 

 coil having a couple of Daniells in it. Join two of the binding-screws 

 of the secondaries together : connect the other two through a short- 

 circuiting key with electrodes, on which the nerve of a muscle-nerve 

 preparation is arranged. Push up the secondaries till the break 

 shocks obtained on opening the two knock-over keys are maximal. 

 Then shoot the plate as described in 16, first with one trigger key 

 closed, and then with both. The curves obtained should be of the 

 same height in the two cases, as a second maximal stimulus falling 

 within the latent period is ignored by the nerve or muscle. Repeat 

 the experiment with submaximal stimuli, i.e., with such a distance of 

 the coils that opening of either trigger key does not cause as strong 

 a contraction as is caused when the coils are closer. The curve will 

 now be higher when the two shocks are thrown in successively than 

 when the nerve is only once stimulated. This shows that (sub- 

 maximal) stimuli can be summed in the nerve. The same could be 

 demonstrated for muscle (p. 571). 



18. Superposition of Contractions. Smoke a drum arranged for 

 automatic stimulation as in Fig. 192. Adjust the brass points with 

 a distance of, say, one centimetre between them, so that a second 

 stimulus may be thrown into the nerve at an interval greater than the 

 latent period of muscle. Put two Daniells in the primary circuit. 

 Lay the nerve of a muscle-nerve preparation on electrodes connected 

 through a short-circuiting key with the secondary. Allow the drum 

 to revolve (fast speed) ; open the short-circuiting key till both brass 

 points have passed the projecting wire, then close it. Now bend 

 back the second brass point, and take a tracing in which the first 

 curve is allowed to complete itself. This will not rise as high as the 

 second curve obtained when the two stimuli were thrown in. Repeat 

 the experiment with varying intervals between the brass points 

 that is, between the two successive stimuli. Put on a time-tracing 



