CHAPTER VII 



Effect of Successive Stimuli upon a Muscle- Nerve 

 Preparation 



Superposition. Arrange the muscle-nerve preparation on the 

 myograph and connect the drum in the primary circuit in the manner 

 employed to record a simple muscle curve (p. 27 and Fig. 34). Place 

 the secondary coil at such a distance from the primary that the ex- 

 citation produced by a single pin projecting from its circumference 

 and striking the needle in its revolution produces a maximal effect ; 

 describe a normal muscle curve in the usual way. Then insert a 

 second pin at varying intervals so that the excitation which it produces 

 will affect the nerve at different intervals after the first excitation ; 

 viz., (a) during the rise of the first curve, (b) near the top of the first 

 curve, (c) during the decline of the first curve. Take these double 

 tracings at different levels of the paper, each one on its own abscissa. 



Effect of several successive stimuli ; tetanus. For studying the 

 effect on a nerve-muscle preparation of a rapid succession of stimuli a 

 vibrating steel reed ig used to make or break the primary circuit of 

 the induction coil by allowing a wire attached to its end to dip into 

 and out of a cup of mercury, the surface of which should be covered 

 with dilute alcohol. The rate of vibration of the reed depends upon 

 its length, which can be varied by clamping it at different places ; 

 it is marked at points for producing vibrations of ten, fifteen, twenty, 

 and thirty per second (Fig. 38). The secondary coil should be placed 

 at such a distance from the primary that only the break shock is 

 effective. The drum should revolve at moderate speed (one revolution 

 a minute). 



Attach the muscle to the lever of the myograph in the usual way ; 

 place the nerve upon the electrodes ; set the reed vibrating ; set the 

 drum revolving and bring the lever point against it, using the stop ; 

 open the key in the secondary circuit for about a second ; take the 

 lever point away from the drum. A tracing is to be taken in this way 

 at each of the above rates, each tracing on its own abscissa ; add a 

 time marking. 



Record of voluntary contraction. A voluntary muscular contrac- 



38 



