OTHER THEORIES 275 



considerable support from Sherrington's researches 1 . Reflex action 

 as compared with the conduction of an impulse along a nerve, is charac- 

 terised by " lost time " or appreciable delay. Sherrington has brought 

 forward abundant evidence to show that the delay is due to resistance 

 to the passage of the impulses and that the resistance probably has its 

 seat in the synapses. It is further probable that in the lower levels of 

 the nervous system, e.g., in the spinal cord, where the reflex paths are 

 paths of high degree of constancy of function, the synapses are very 

 thoroughly organised, i.e., their degree of resistance has been reduced 

 to a minimum by frequent repetition of the particular reflex action in 

 the individual and in the race, while in the higher parts of the nervous 

 system the resistance, and therefore the loss of time, occasioned by the 

 synapses is greater in the inverse order of their degree of organisation. 



Repeated stimulation causes fatigue, which also probably is due to 

 changes occurring in the synapses. This is the explanation of the 

 " complete fading " of retinal images under certain conditions (v. p. 261). 

 Complete fading may be brought about with greater ease by the simul- 

 taneous effect of a second stimulus. Thus, if a small white patch a is fixated 

 for 15 sees, or more, and then a second similar patch b is suddenly exposed, 

 so that the image of b falls on another part of the same retina or on a 

 non-corresponding part of the other retina, a will usually disappear from 

 consciousness at once, remain absent several seconds, and then return 

 suddenly, appearing equally bright with b. McDougall has described 

 numerous instances of these " mutual inhibitions " of visual images, 

 amongst which those of binocular and uniocular struggle are the most 

 striking. Thus, if a bright white surface is fixated w r ith a red glass 

 before one eye and a blue glass before the other, the surface usually 

 appears alternately red and blue. If the white surface is less bright 

 there is greater tendency for the colour impressions to fuse 2 . This 

 alternation is most easily explained by supposing that one excitation 

 ceases to pass through the neurones of the visual cortex, and that the 

 factor which determines the regular alternation of the two images is 

 the rapidly oncoming and rapidly disappearing fatigue of the synapses 

 of the cortex. 



In support of this view we have the following facts : (1) A very 

 similar alternation in consciousness, i.e., alternate predominance of an 

 image and inhibition of it by another image, may occur in the case of 

 two after-images formed on adjacent areas of one retina ; (2) if, when a red 



1 See The Integrative Action of the Nervous System, London, 190(5. 



2 Cf. Rivers, Camb. Phil. Soc. vin. 



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