VERTEBRATE PHOTORECEPTORS 



CHAPTER I 

 THE VERTEBRATE EYE 



It is well recognized among biologists that the eyes of 

 vertebrates are constructed upon a common generalized plan. 

 Despite the architectural similarity which one meets when 

 studying the structure of eyes, one cannot help but be im- 

 pressed with the fact that there are many deviations from 

 the common scheme. This pertains not only to the sensory 

 receptor and nervous transmitting mechanisms, but to the 

 light-collecting and focusing devices as well. 



Perhaps the most common characteristics of the verte- 

 brate eye are the mode of origin of its essential parts, the 

 presence of three concentric layers or tunics (tunica fibrosa, 

 tunica vasculosa and tunica interna, and their respective 

 component modifications), and the existence of an inverted 

 retina, so that the photosensitive elements are turned away 

 from rather than towards the source of illumination. This 

 last condition has stimulated speculation among many stu- 

 dents of biology, and various theories have been advanced 

 which aim to account for the origin and inversion of the 

 retina during the course of phylogeny. These theories have 

 been reviewed recently in an article by Walls (19396) who 

 also presents his own theory. 



Although our chief interest in the following pages will be 

 concerned with the retina, and in particular with the visual 

 cells themselves, it seems expedient by way of introduction 

 to give a brief account of the bulb as a whole, which will be 

 based primarily upon the human eye. No attempt will be 



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