FATIGUE. 485 



It seems probable that the resistance of the synapses is hable to be 

 temporarily increased not only locally by the transmission of the nervous 

 excitation across them, but also generally by the influence of the waste 

 products of metabolism brought to them in the blood ; that they are, in 

 short, very subject to chemical influences of many kinds. ' 



If so, then such action of waste products upon them must play a large 

 part in that general increase of the ratio of resistance to energy which 

 constitutes general fatigue, and which manifests itself subjectively in 

 general tiredness and in sleepiness ; and also in the onset of sleep or 

 general quiescence of the brain, the most important of the modes in which 

 the organism protects itself against exhaustion. 



If we provisionally accept this view, that the synapses are the prin- 

 cipal seats of resistance, and that their resistances are liable to be thus 

 increased both locally and generally, then the simple picture of fatigue 

 which I have associated with the name of Professor Verworn becomes 

 considerably complicated. In addition to local ' Erschopfung ' of the 

 neurones, and to local and general ' Ermiidung ' of them by waste products, 

 the former of which means local, and the latter both local and general, 

 diminution of energy supply, we recognise local and general increase of 

 synaptic resistances. We have then, I think, a scheme much less in- 

 adequate to the explanation of those familiar and important peculiarities 

 of fatigue to which I drew your attention in my opening remarks. 



But I said there were two complications to be added to the simple 

 Verworn picture. We have added the resistances varying from local and 

 from general causes. The second complication concerns the sources of 

 the energies involved in bodily and mental activity. 



According to the Verworn scheme every cell-body i^^', I take it, to be 

 conceived as capable of generating or transforming or libei'ating a certain 

 quantity of energy, which, up to a certain upper limit, varies with the 

 intensity of the stimulus applied to the cell ; and this energy is to be 

 regarded as finding its field of operation wholly within the neurone or the 

 particular small functional group of neurones within which it is liberated ; 

 and all neurones are of appreciably equal value in this respect. 



But there is another conception of the mode of operation of nervous 

 energy which goes back at least as far as Descartes ; the conception — 

 namely, that the energy liberated by chemical change, by katabolic 

 process, in one part of the nervous system may be conducted through the 

 uer^ ous channels and may operate in other parts of the nervous system. 

 This may be called ilie hypothesis of the vicarious usage of nervous energy. 

 Now it seems impossible to get the physiologists of the laboratory, the 

 physiologists who are chiefly concerned with the organs rather than with 

 the organism, to consider this conception seriously and on its merits. If 

 they occasionally refer to it, it is only to put it aside contemptuously 

 as a naive survival from the dark ages. Yet those who are in the habit 

 of dealing with the problems of the organism as a whole, the physician 

 and the psychologist, constantly make use of this conception, for they find 

 it impossible to make progress in the understanding of their problems 

 without it.^ That fact gives the conception a claim to a more serious 

 consideration than it has commonly received from the physiologists. 



' There is something to be said for the view that they are the seats of the primary 

 and principal influence of various drugs, possibly of alcohol, chloroform, strychnine, 

 and others. 



- The necessity of conceiving nervous energy in this way was urged as long ago 



