132 THE MECHANISM OF LIFE 



What we know about the passage of a nervous impulse along 

 a nerve suggests that the former passes without any of the 

 substance of the nerve being used up. Arriving at the first 

 synapses, it passes into the nerve cells there and transforms into 

 the same quantity of energy, which then passes along the intra- 

 cerebral tract into the second synapses and cells, where another 

 equal quantity of energy is transformed and is propagated along 

 the efferent nerve (again without loss), and thrown into the 

 muscle. But there it releases a much greater quantity of energy, 

 which is represented by the force with which the muscle con- 

 tracts or relaxes. The latter contained energy in the potential 

 (chemical) form, and the minute quantity entering it as a nervous 

 impulse sets free or transforms this chemical energy in the same 

 way as a small electric current can release (or fire) the huge 

 quantity of energy contained in an explosive charge. 



Now, from the point of view that we have taken so far, every- 

 thing that happens in the sensori-motor system of an animal 

 conforms to the structural scheme of reflex arcs and to the 

 dynamical scheme of the billiard-ball model. The peripheral 

 and central nervous system is a means whereby the energy 

 received by the stimulation of the receptor organs is transmitted 

 as a nervous impulse to the nerve centres, and is there trans- 

 formed into another kind of impulse which is transmitted to an 

 effector organ, where, finally, potential chemical energy is 

 released, and muscles contract and relax or glands secrete. 

 The impulse which results from the stimulation of a sense organ 

 goes to the central nervous system, but there it may pass along 

 one or more of a great number of different paths. Upon the 

 path it takes depends its effect; thus the stimulation of the 

 retina may lead to a reflex act of winking, or the " mouth may 

 water," or the man may start violently, or run, or sit down, or 

 laugh. Nothing, then, is explained by our structural analysis 

 of the nervous system; all that we have studied is the means 

 whereby one of a number of effects that may be produced by a 

 stimulus is produced. 



Consider now the energetical side of the process of stimulus 

 and response. Just that quantity of energy which is received by 

 the receptor organ is transmitted along the afferent nerve into 

 the centre, and the same quantity again is sent through the 

 central nervous system from centre to centre, and still the same 

 quantity is transmitted along the efferent nerve into the effector 



