84 THE MUSCLE-NERVE MACHINE. [BOOK i. 



Thus a weak stimulus gives rise to a small disturbance, that is a 

 weak nervous impulse, and a strong stimulus gives rise to a large 

 disturbance, that is a powerful nervous impulse. 



We are not in a position at present to speak definitely as to 

 the occurrence of other differences in the characters of nervous 

 impulses. As far as we know at present, nervous impulses what- 

 ever their origin are alike in nature l ; the impulses generated, in 

 a natural way, by the brain or spinal cord, or produced artificially 

 by mechanical stimuli, as by cutting or pinching, or by thermal 

 stimuli, as by touching the nerve with a red-hot wire, or by 

 chemical stimuli, or by electrical stimuli, may differ in intensity, and 

 in the rapidity with which they succeed each other, but, as far as 

 we know at present, not otherwise. Thus a drop of acid placed on 

 a nerve gives rise to tetanus in the muscle which differs from the 

 tetanus produced by repeated induction shocks applied to the nerve, 

 only so far as the tetanus is generally irregular, the individual 

 nervous impulses generated by the acid forming an irregular series, 

 not following each other at equal intervals and not being all of the 

 same intensity, whereas the impulses generated by the 'interrupted 

 current ' are generally of the same intensity and follow each other 

 at equal intervals. So also we are led at present to believe that 

 when muscles are thrown into action in a natural way in the living 

 body by the agency of the spinal cord, what goes 011 in the nerve 

 differs from what goes on in the same nerve when the interrupted 

 current is brought to bear on it, only in so far as in the former case 

 the impulses follow each other at a fixed rate (nineteen a second), 

 whereas in the latter, the rate of repetition varies according to 

 the rapidity with which in the induction-machine the shocks 

 follow each other; the individual impulses as far as we know at 

 present have the same characters in the two cases save only that 

 they may differ in intensity. 



Supposing that the irritability of a nerve-muscle preparation 

 remains for the period of the experiment fairly constant, care 

 being taken to avoid the effects of exhaustion, and that the 

 stimulus be applied to the same part of the nerve, we find that the 

 intensity of the nervous impulse generated (as measured by the 

 muscular contraction) varies up to a certain limit according to 

 what we may call the strength of the stimulus. Thus taking a 

 single induction shock as the most manageable stimulus, we find 

 that if, before we begin, we slide the secondary coil (Fig. 1, sc.) 

 a certain distance from primary coil pr. c., no visible effect at all 

 follows upon the discharge of the induction shocks. The passage 

 of the momentary weak current is either unable to produce any 

 nervous impulse at all, or the weak nervous impulse to which it 



1 It will be observed that we are speaking now exclusively of the nerve of a 

 muscle-nerve preparation, i.e. of what we shall hereafter term a motor nerve. 

 Whether sensory impulses differ essentially from motor impulses will be considered 

 later on. 



