CHAP. IL ] THE CONTRACTILE TISSUES. 139 



fairly suppose that in two experiments we may in the 

 experiment bring the induction-shock or other stimulus to bear 

 on a few nerve fibres only, and in the other experiment on many 

 or even all the fibres of the nerve. In the former case only those 

 muscular fibres in which the few nerve fibres stimulated end will 

 be thrown into contraction, the others remaining quiet, and the 

 shortening of the muscle as a whole, since only a few fibres take part 

 in it, will necessarily be less than when all the fibres of the nerve 

 are stimulated and all the fibres of the muscle contract. That is 

 to say, the amount of contraction will depend on the number of 

 fibres stimulated. For simplicity's sake however we will in what 

 follows, except when otherwise indicated, suppose that when a 

 nerve is stimulated, all the fibres are stimulated and all the 

 muscular fibres contract. 



In such a case the stronger or larger nervous impulse leading 

 to the greater contraction will mean the greater disturbance in 

 each of the nerve fibres. What we exactly mean by the greater 

 disturbance we must not discuss here ; we must be content with 

 regarding the greater or more powerful or more intense nervous 

 impulse as that in which, by some mode or other, more energy is 

 set free. 



So far as we know at present this difference in amount or in- 

 tensity, of the energy set free, is the chief difference between various 

 nervous impulses. Nervous impulses may differ in the velocity 

 with which they travel, in the length and possibly in the form 

 of the impulse wave, but the chief difference is in strength, in, so 

 to speak, the height, of the wave. And our present knowledge will 

 not permit us to point out any other differences, any differences 

 in fundamental nature for instance, between nervous impulses 

 generated by different stimuli, between for example the nervous 

 impulses generated by electric currents and those generated by 

 chemical or mechanical stimuli ; nor can we, in the present state 

 of our knowledge at least, recognise any essential difference 

 between what may be called natural motor nervous impulses, that 

 is to say, those set going by changes in the central nervous 

 system and those produced by the artificial stimulation of the 

 motor nerves 1 . 



This being premised, we may say that, other things being equal, 

 the magnitude of a nervous impulse, and so the magnitude of the 

 ensuing contraction, is directly dependent on 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 place the secondary coil (Fig. 5, sc.) a long way off the 

 primary coil pr. c., no visible effect at all follows upon the 



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. 



