CHAPTER XII 



RECAPITULATION 



In the foregoing chapters an attempt has been made to 

 present a general account of the visual cells of vertebrates, 

 with particular emphasis upon their development, structure, 

 distribution, and certain aspects of their physiology. 



That the retina, in general, possesses two types of visual 

 elements, viz., rods and cones, has been known since its 

 discovery by Max Schultze in 1866. Elaborations of his 

 findings led to the so-called Duplicity Theory which implies 

 that colorless (scotopic) vision at low illuminations is medi- 

 ated by the rods, and that the cones are concerned primarily 

 with vision at high illuminations and with color (photopic) 

 vision. 



Despite certain objections, the duplicity theory has stood 

 the tests of physiologists in their quantitative measurements 

 of the major functions of vision, viz., dark-adaptation, in- 

 tensity discrimination, visual acuity, and flicker responses. 

 The existence of structural and functional duality of the 

 retina in the majority of forms is a fact of common knowl- 

 edge, and animals possessing both types of visual cells are 

 equipped for both diurnal and nocturnal vision. Many 

 animals, however, are exclusively cone-bearing and are 

 essentially diurnal in their behavior, whereas others are 

 exclusively rod-bearing, and are typically nocturnal in 

 their habits. Thus the mode of life of the animal appears 

 to be correlated closely with the structure of the retina. 

 Even from an histological examination of the retina, one can 

 predict with some assurance, something of the habits of 

 the animal as well as its visual ability. Many examples 

 demonstrating the correlation between retinal structure and 

 the mode of life of the animal are given in Chapter V. 



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