EFFECT OF THE REPETITION OF THE STIMULUS. 473 



ure may be wholly inadequate to evoke an indirect muscular response 

 when slowly applied to a motor nerve, whilst the same pressure, rapidly 

 produced, is fully adequate. A good illustration of the increased exciting 

 value of mechanical change when this is rapid is that afforded by the 

 use of the nerve shaker ; the nerve may be thus adequately stimulated 

 by very small, but rapidly effected, changes. 



In regard to chemical stimuli, such comparisons are hardly possible, 

 but in so far as these act by the withdrawal of water (glycerin, NaCl, 

 etc.), the rapidity of such withdrawal is obviously increased by the 

 strength of the chemical substance used. Provided that the nerve is 

 not injured by the agent, the exciting value of such substances as those 

 mentioned is in proportion to their degree of concentration. Finally, in 

 regard to thermal agencies, it is only sudden and extensive change which 

 causes these to have any exciting value at all. 



The effect of the repetition of the stimulus.— The indefatig- 

 ability of nerve.— To what extent can a stimulus be repeated and still 

 remain efficient ? The investigation of this question opens up a strictly 

 physiological line of inquiry : for the efficiency can only fail on repeti- 

 tion, in consequence of the inability of the living tissue to respond to 

 the constantly recurring physical change. 



The subject has been investigated in several ways, which may be 

 broadly divided into two groups. In the first, the nerve is excited with 

 an efficient stimulus recurring at comparatively long intervals, and of 

 uniform character and intensity. In the second, the nerve is excited by 

 a series of stimuli which recur at great rapidity. Does nerve show 

 signs of fatigue when subjected to excitations of the first kind ? Does it 

 show inability to respond when subjected to those of the second ? 



In the case of a motor nerve, the index of excitation afforded by the 

 indirectly evoked muscle response is itself susceptible of fatigue ; the 

 gradual failure in this contraction, when the nerve is excited by a series 

 of stimuli recurring every few seconds, is thus no indication of a failure 

 in the nerve. It is possible to avoid this source of error by eliminating 

 the muscle. This can be done in the mammal by the use of curare. A 

 sufficient dose is given to paralyse the motor nerve-endings, and artificial 

 respiration is employed until, after some hours, the effects of the drug 

 disappear, in consequence of its elimination. 1 The exposed nerve is 

 stimulated at intervals during the whole period, and, as the curare 

 paralysis wears off, it is seen by the muscular contractions which now 

 appear, that although the nerve has been subjected to this prolonged 

 excitation, it still responds to the action of the stimulus. 



Bernstein prevented the muscular contraction in the frog by 

 blocking the transmission from the seat of nerve excitation to the 

 muscle, this being effected by leading a galvanic current through a 

 portion of the nerve. The nerve was then excited for a prolonged 

 period above the block ; on opening the galvanic current, the muscular 

 contraction showed that the nerve was still being adequately excited. 2 



This method has been improved by using synchronous short periods 

 for closing both the excitation and the blocking currents ; by alter- 

 nate reversal of the latter, any injurious effect due to their prolonged 

 closure is largely got rid of. 3 It is found that after such excitation 



1 Bowditch, Arch./. Altai., Physiol, it. icissensch. Med., 1870. 



2 Bernstein, Arch. f. d. ges. Physiol., Bonn, 1877, Bd. xv. 



3 Wedenskii, Cenlralbl. /. d. vied. Wissensch., Berlin, 1884. 



