546 THE EYE IN EVOLUTION 



avoid obstacles and recognize food by sight ^ ; these latter more fundamental 

 aspects of vision, therefore, have subcortical integrations. In dogs the 

 incapacity is greater ; the animal retains the faculty of perception of light 

 and can discriminate differences of intensity unimpaired, but in imfamiliar 

 surroundings it gropes with its paws, moving cautiously as if blind (Goltz, 

 1892 ; Pavlov, 1927 ; Lashley, 1931 ; Marquis, 1934 ; Wing and Smith, 

 1942). Cats react similarly although they retain orientation and the 

 discrimination of objects to a considerable extent in the dark (Smith, 1937). 

 Monkeys suffer much more incapacity ; light perception remains but 

 discrimination between brightness is lost and performance is greatly impaired, 

 particularly in bright light compared with conditions involving dark 

 adaptation (Marquis and Hilgard, 1937 ; Kliiver, 1941). If part of the 

 visual cortex of rats is removed, deficiencies in the response to visual 

 conditioned reflexes are proportional to the amount removed irrespective 

 of the area mutilated (Lashley, 1922-34), while in monkeys some 

 responses which are lost can be relearned, presumably by a new area 

 (Ades, 1946). Similar substitute areas can be utilized for auditory responses 

 in dogs (Allen, 1945). 



It is thus apparent that although a considerable degree of specificity 

 of function appears for the first time in the neocortex of Mammals, it is 

 still largely plastic with imprecise localization ; only in man does ablation 

 of the occipital cortex lead to permanent blindness with complete loss of all 

 sensations of light. In him the only sub-cortical visual activity is pupillary, 

 and in him alone is vision in its entirety a cortical function. 



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^ It is resting that in amblyopia in squint, light perceptioia and spatial orientation 



remain norii. hile form vision suffers. 



