574 



PHYSIOLOGY 



simultaneous contrast, the black half being seen against the surrounding yellow, while 

 the luminosity of the blue half is increased by the effect of the surrounding black. In 



the inner ring the blue half is dark - 

 ened by contrast with the surround- 

 ing yellow, while the black half is 

 not evident at all. If the disc be 

 rotated, we get two concentric rings 

 on an apparently homogeneous 

 field. It is found however that 

 the outer ring flickers long after 

 complete fusion has taken place in 

 the inner ring, showing that the 

 stimulation of the retina by the 

 outer ring is increased under the 

 influence of contrast. 



CAUSE OF AFTER IMAGE. 

 Sherrington has shown that 

 in the case of muscles there 

 is what is called reciprocal 

 FlG - 296 - innervation. Thus stimula- 



tion of the cortex which causes the contraction of one muscle also 

 brings about a corresponding relaxation of its antagonist, in order 

 that a rapid and economical motion may take place. The contraction and 

 corresponding relaxation are therefore analogous to the response of one 

 part of the retina, which is accompanied by an inhibition of the surrounding 

 parts of the retina to the same kind of stimulus (simultaneous contrast). In 

 a similar manner the inclination to extension which is found to accompany 

 the prolonged flexion of a limb, finds its analogy in the phenomena of after 

 images and adaptation of the eye, since the tendency is to suppress a similar 

 stimulus in the part of the retina stimulated and to encourage its comple- 

 mentary. The inference to be drawn from these analogies is that the after 

 image and its allied phenomena are caused by changes in the conducting 

 paths of visual impressions which are similar to those found to exist in paths 

 belonging to the motor system. What the nature of these changes in the 

 conducting paths may be is at the present time undecided. McDougall has 

 suggested that they are fatigue effects in the synapses of the higher conduct- 

 ing paths. If this view is correct it would seem difficult to explain why after 

 images and contrast phenomena are best seen with a rested eye. 



UTILITY OF THE AFTER IMAGE. We have seen that the 

 effect of the after image is to inhibit the possible repetition of a 

 similar stimulus, and at the same time to favour the reception of one 

 of a different nature. The process is therefore one which favours change, 

 for not only is there a tendency to efface an old impression but also to welcome 

 a new one. Such effects must be of great value to an organ such as the eye 

 the function of which is to a considerable extent, in everyday life, to present 

 to consciousness the greatest number of impressions in a given time. For 

 example, by measuring the time taken to read a passage in which almost 

 every word was of importance, it was found that on an average eight words were 



