NOT ADMITTED. 203 



physiological laws. All life, with Fletcher, implies the con- 

 sumption of protoplasm under the operation of stimuli, but 

 this, especially in the activity of the nervous system, does not 

 always go on pari passu with the regeneration of the life-sub- 

 stance ; on the contrary, it exceeds it at times ; hence, exhaustion 

 and modification of living matter, so that the same stimulus 

 loses its effect and can no longer rouse to activity as before. 

 The living matter now passes into the resting stage and sleep 

 comes on, during which the regeneration overbalances the con- 

 sumption, and in due time the susceptibility is restored to its- 

 previous acuteness, so that the same stimuli, which are still 

 present, are now sufficient to excite the nerve centres into the- 

 wakiug state of activity. Of course it is not pretended here to 

 explain the nature of sleep, or of the resting stage of living 

 matter as contrasted with its special activity. But even without 

 the above connection of the phenomenon in question with con- 

 sumption and regeneration of protoplasm, there is no doubt that 

 the repetition of every stimulus produces enfeebled or exhausted 

 susceptibility towards itself, which susceptibility is revived again 

 after rest. In this sense it may be said, for a time, in respect 

 to some stimuli, " a constant stimulus is no stimulus at all." 

 In respect also to the storing up of the living matter, and 

 even that chemical substance supposed to be consumed im- 

 mediately in nerve and muscle action (see p. 159), the facts 

 agree with Bain's view of the exuberant movements of the 

 young and the vigour and freshness after rest, only the dis- 

 charge of energy is not spontaneous. In fact it is as if the 

 boiler of an engine were well stored with steam you would have 

 to turn the cock all the same, although the machine would go 

 faster and farther than when half cold and exhausted. 



But the strongest proof of the non-existence of spontaneity is 

 given by Beale's absolute denial of the existence of uni-polar 

 nerve cells. There are always two poles at least, and often many 

 more. If the nerve cell could start into activity spontaneously, 

 and if any purpose were to be served thereby, one cord would 

 be enough to convey the evolved energy as far as we know, for 

 we do not know that the second is necessary for any electrical 

 reason, i.e., to complete the circuit. The second is no doubt an 



