162 VERTEBRATE PHOTORECEPTORS 



In others, the crowding is so great as to cause a local thicken- 

 ing in the retina. This is the area centralis retinae. Further 

 specialization for increased visual acuity constitutes the 

 foveas which are found in some salt water fishes, diurnal 

 lizards, birds, monkeys, the great apes, and man. These 

 various specializations of the fundus for increased visual 

 acuity are described in Chapter VIII. The generally held 

 view that the fovea has been developed to allow light to fall 

 directly upon the photoreceptors without passing through 

 all the layers of the retina, has been challenged by Walls 

 (1937), who has shown that this structure must be viewed 

 as a mechanism for increasing the resolving power of the 

 retina. This matter is taken up fully in Chapter IX, which 

 contains also a discussion of certain evolutionary relation- 

 ships of the fovea. It is pointed out that whereas foveal 

 development and the ability for extensive movements of the 

 eyes appear to go together, conjugate movements, binocular 

 vision, and partial decussion of the optic tracts are not neces- 

 sarily implied. A critique is offered of G. Elliott-Smith's 

 contention that marked evolutionary advances in the de- 

 velopment of the anthropoid brain are due to the presence 

 of the macula. 



It is recognized that both rod and cone vision is initiated 

 by the decomposition of photochemical substances in the 

 outer segments of the visual cells. In connection with rod 

 function, the photopigment rhodopsin (visual purple) has 

 been known since its discovery by Franz Boll in 1876. 

 Although this substance has been studied by many physiolo- 

 gists, in recent years it has been subjected to an exhaustive 

 spectrophotometric analysis by Wald, who has shown that 

 vitamin A is the precursor of visual purple as well as the 

 product of its decomposition. Visual purple is a conjugated 

 protein, the special properties of which involve principally 

 a colored prosthetic group. This is derived from the widely 

 distributed class of yellow to red highly saturated lipoidal 

 pigments known as the carotenoids. In fresh water fishes 

 as well as in those which spawn in fresh water, Wald found 



