CHAP, ii.] THE CONTRACTILE TISSUES. 137 



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 

 intensity, of the energy set free, is the chief difference between 

 various nervous impulses. Nervous impulses may differ in the 

 velocity 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, or by those changes in the central 

 nervous system which give rise to what may be called natural 

 motor nervous impulses as distinguished from those produced by 

 artificial stimulation of 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. 4, sc.) a long way off the 

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

 discharge of the induction-shock. 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 gives rise is unable 

 to stir the sluggish muscular substance to a visible contraction. 

 As we slide the secondary coil towards the primary, sending in an 

 induction-shock at each new position, we find that at a certain 

 distance between the secondary and primary coils, the muscle 

 responds to each induction-shock 2 with a contraction which makes 



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. 



2 In these experiments either the breaking or making shock must be used, not 

 sometimes one and sometimes the other, for, as we have stated, the two kinds of 

 shock differ in efficiency, the breaking being the most potent. 



