PRACTICAL EXERCISES 789 



12. 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 n, 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 (submaximal) stimuli can be summed in 

 the nerve. The same could be demonstrated for muscle (p. 730). 



13. Superposition of Contractions. Smoke a drum arranged for auto- 

 matic stimulation as in Fig. 278. Adjust the brass points with a 

 distance of, say, i 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 with the electrical tuning- 

 fork. (For specimen curve, see Fig. 259, p. 730.) 



14. Composition of Tetanus. (a) Adjust a muscle-nerve preparation 

 on a myograph plate, the nerve being laid on electrodes connected 

 through a short-circuiting key with the secondary of an induction 

 machine, the primary circuit of which contains a Daniell cell and is 

 arranged for an interrupted current (Fig. 93, p. 198). The lever should 

 be shorter than that used for the previous experiments, or the thread 

 should be tied in a hole farther from the axis of rotation, so as to give 

 less magnification of the contraction. Set the Neef's hammer going, 

 let the drum revolve (slow speed), and open the key in the secondary. 

 The writing-point at once rises, and traces a horizontal or perhaps 

 slightly-ascending line. Close the short-circuiting key, and the lever 

 sinks down again to the abscissa line. If it does not quite return, it 

 should be loaded with a small weight. This is an example of complete 

 tetanus. 



(b) Connect the spring shown in Fig. 280 with one of the upper 

 terminals of the primary coil, and the mercury cup with the other. 

 Fasten the end of the spring in one of the notches in the upright piece 

 of wood by means of a wedge, so that its whole length can be made to 

 vibrate. Let the drum off, set the spring vibrating by depressing it 

 with the finger, then open the key in the secondary. The muscle is 

 thrown into incomplete tetanus, and the writing-point traces a wavy 

 curve at a higher level than the abscissa line. Close the short-circuiting 

 key, and the lever falls to the horizontal. Repeat the experiment with 



