RETINAL STRUCTURE AND VISUAL ACUITY 103 



extend beyond either macular region (e.g. sparrow hawk). 

 In the other, the band-like area encloses a nasal macula, 

 whereas the temporal macula lies above and apart from these 

 (e.g. tern). 



It is seen from the above, that in birds macular specializa- 

 tion reaches the highest degree and in no other animals do 

 we encounter such highly developed visual acuity. In these 

 macular regions in birds the cones are attenuated structures 

 closely packed. According to Rochon-Duvigneaud (1919) 

 there are 1,000,000 cones per sq. mm. at the fovea in Buteo. 

 This would necessitate practically a solid aggregation of 

 cones 1 M in diameter. In humans, the distance between 

 the centers of two adjacent cones is said to be about 2.5 /x, 

 which if correct, would mean about 160,000 cones per sq. mm. 

 at the fovea (v. Chapter III, p. 40). Based upon the fine- 

 ness and average distance apart of the photosensitive ele- 

 ments (as pointed out in the beginning of the chapter), we 

 would expect the visual acuity of hawks to be far superior 

 to that in man, and I think experience bears this out. Just 

 how much greater the resolving power is in a unit area 1 fx 

 in diameter as compared with 2.5 /x, I am not in a position 

 to say. Apparently the diffraction pattern limits the reso- 

 lution and this depends upon such factors as size of aperture 

 and distance from the lens to the receiving surface. This 

 is a matter for the physicist to evaluate. However, it is 

 apparent from the discussion and the illustrations that the 

 fovea of reptiles and birds is a more highly developed struc- 

 ture than in anthropoids and man. 



The eyes of diurnal reptiles and birds possess a peculiar 

 structure known as the pecten, which projects from the 

 optic disc into the vitreous. In the reptiles it exists typically 

 in the form of a cone (Detwiler, 1920). In most birds it 

 is present as a highly vascular comb-like structure containing 

 abundant pigment (Figures 70-77). This structure has been 

 assumed by many to play a nutritive role for the vitreous. 

 It is present in most elaborate form in birds which are noted 

 for their high visual acuity. What role, if any, this structure 



