PRACTICAL EXERCISES 7 11 



time -tracing between these two lines can be readily determined, 

 and is the latent period. 



13. 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 12, 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. 655). 



14. Superposition of Contractions. -Smoke a drum arranged for 

 automatic stimulation as in Fig. 261. 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. 246, p. 656.) 



15. Composition of Tetanus. (a) Adjust a muscle-nerve prepara- 

 tion on a myograph plate, the nerve being laid on electrodes con- 

 nected 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. 81, p. 184). 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 rota- 

 tion, so as to give less magnification of the contraction. Set the 

 Xeef'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. 263 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 



