no VERTEBRATE PHOTORECEPTORS 



To bring this about the cerebral layers of the retina are 

 spread apart and the blood vessels which accompany them 

 are lacking so that the light can travel without hindrance 

 to the receptive elements; in this way the optical disad- 

 vantages of an inverted retina are neutralized. Moreover, 

 the neuro-epithelium itself becomes eminently specialized 

 and contains cones only." He divides this region into three 

 concentric rings, viz., 



1) The macula lutea — characterized by yellow pigment. 



2) The rod-free area within the above, containing cones 



only. 



3) The fovea centralis (central pit) in the center. 



Wolff (op. cit.) defines the macula lutea as follows: '^ . . . a 

 shallow oval depression about the same size as the disc. 

 Its center lies 3.5 mm. lateral to the edge of the disc and 

 just below its middle. The side wall of the depression is 

 called the Clivus, and slopes gradually towards the fovea 

 centralis." He says further, ''It will thus be seen that at 

 the fovea centralis the layers of the retina are spread aside, 

 so that light may fall directly on the true percipient elements, 

 namely the cones." Since no maculae are described without 

 the presence of a fovea centralis, it is difficult to understand 

 the importance which is attached to this structure in anthro- 

 poids and man as a true macula. The same essential con- 

 figuration occurs in reptiles and birds. Are we to call this 

 an 'untrue' or pseudo-macula and fovea? There seems to 

 be no apparent reason for speaking of this as a true macula 

 and fovea in monkeys and man just because it happens to 

 be associated with such features as partial optic decussation, 

 binocular vision, and conjugate eye movements. 



The area centralis retinae is typically a localized thicken- 

 ing of the retina produced by crowding of the visual elements. 

 It is usually circular and is present in some amphibia and 

 in many reptiles (Figure 62). It may be in the form of a 

 band or stripe as in the alligator (Chievitz, 1891). The 

 typical area does not have a central pit or fovea. 



