FATIGUE. 483 



nerves. On the othei' hand, there are many considerations tending to 

 show that the principal seats of these resistances may be the synapses or 

 places of junction of the neurones. I have not time to attempt to display 

 the strength of tlie grounds of this assumption, and most of you arc 

 familiar with them.' 



If we assume that the synapses are the seats of the resistance, and 

 that the process of transmission of the impulse across the synapse is one 

 that is very liable to result in a raising of tlie resistance of the synapses, 

 then we have a conception which enables us to account satisfactorily for 

 many manifestations of fatigue. I will point out a few of the facts which 

 seem to point to the synapse rather than to the cell-bodies as the principal 

 seats of the increased resistance. It has long been notorious that nerve- 

 fibres are but little, if at all, liable to fatigue ; and there is good reason 

 to believe that the cell-bodies also are fatigued, not easily or rapidly, but 

 only under prolonged activity or when deprived of the normal means of 

 recuperation— deprived, that is, of the normal blood-supply. Yet we 

 have evidence that local fatigue may be rapidly induced, and may as 

 rapidly pass away, in the various levels of the nervous system. 



Such local fatigue occurs in the reflected arcs of the spinal cord if 

 a stimulus is continuously applied to the afferent path.^ It occurs in 

 the motor region of the cerebral cortex if one spot be continuously or 

 repeatedly stimulated.^ It occurs still more readily in the higher levels 

 of the brain concerned in perception— e.^., in all the cases of ambiguous 

 figures which may be perceptually interpreted in two or moi-e different 

 ways, and of which the different modes thrust themselves alternately 

 upon consciousness. I have studied these alternations and shown reason 

 to suppose that a rapidly induced local fatigue of the higher cortical 

 paths concerned is the principal condition of this alternating activity.'* 

 In the same way the extreme liability to fatigue of the paths of the 

 higher cortical levels seems to underlie the extreme instability of all our 

 thought processes ; the fact that our attention can never be held fixedly 

 and unchangingly upon any single feature of an object, but always, in 



' See especially Professor Sherrington's Integrative Action of the Nervovs Syitem, 

 Lecture I. In spite of all that has been written of the synapse in recent years, one 

 still finds widely accepted the following argument : The nervous system consists 

 of cell-bodies and nerve-fibres, therefore all those of its properties that are not 

 displayed by peripheral nerves are due to the cell-bodies. Verworn himself still 

 uses this argument, and so little is the conception of the synapse understood that 

 Verworn, in a recent paper, interprets Sherrington as meaning by the term ' synapse' 

 the place of junction of the axis cylinder and the cell-body of a single neurone. 

 That, I take it, is implied when he writes, ' Goldscheider und Sherrington denken 

 an die Uebergangstelle des Nerven in den Ganglionzellkorper, an die " Synapse," wie 

 Sherrington sich ausdriickt' (op. cit., p. 134). Sherrington inclines to the view that 

 the process of transmission of the excitation across the synapse is a purely physical 

 one {oj). cit., p. 17) ; but if, as he also inclines to believe, it is a process very 

 readily affected by the presence of toxic substances in the blood, and if it has the 

 effect of immediately and temporarily raising the synaptic resistance (fatigue), and 

 of ultimately leaving it (when the fatigue effect has passed away) permanently 

 diminished (habit), these effects would seem to demand for their explanation the 

 assumption of some metabolic activity at the synapse. In a paper in Brain 

 (vol. xxiv.) I have adduced other arguments in support of the assumption of 

 intercellular substances, the seats of synaptic metabolism of a highly specialised kind. 



^ Sherrington, op. cit., p. 214. 



' A. G. Levy, ' An Attempt to Estimate Fatigue of the Cerebral Cortex,' Joiirn. 

 of Physiol., vol. xxvi. 



* ' Physiological Factors of the Attention-process,' Mind, N.S., vol. xv. p. 340. 



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