CHAPTER VII 



EFFECT OF SUCCESSIVE STIMULI UPON A MUSCLE-NERVE 



PREPARATION 



Summation. Make a muscle-nerve preparation and place the 

 nerve upon electrodes connected through a short-circuit key with the 

 secondary coil. Include the Neef hammer in the primary circuit, as 

 in fig. 23. Shift the secondary coil to such a distance from the primary 

 coil that a single-break induction shock, made by moving the Neef 

 hammer by hand, just fails to produce a contraction of the muscle. 

 The stimulation is therefore subliminal. Now allow the Neef hammer 

 to vibrate so that a succession of stimuli of the same strength act 

 upon the nerve. The muscle will contract, owing to the summation 

 of the effects of the repeated stimulations of its nerve, although indi- 

 vidually these stimulations were ineffective. 



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. 33). 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, (6) 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 is 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 rate of vibration of the reed 



36 



