THE VERTEBRATE EYE 



much smaller than in many animals, particularly nocturnal 

 forms (e.g. birds, rodents, lemurs) as will be pointed out later. 



Whereas the dimensions of the bulb are relatively constant 

 for individuals of the same species, the vertebrate eye varies 

 enormously in its size. Fishes and birds have large eyes. 

 In amphibians and reptiles 

 they are small. Nocturnal 

 lemurs have characteristi- 

 cally large eyes as have also 

 deep sea fishes, and the eyes 

 of the owl are said to occupy 

 one-third the size of the head 

 (Figure 3). 



Whereas mammals vary 

 greatly in size, there is no 

 absolute correlation between 

 body and eye size. Duke- 

 Elder (1939) points out that 

 size seems to be correlated 

 to a greater degree with the 

 nimbleness of the animal's 

 movements and its necessity 

 for acute vision rather than 

 its bulk — giving as an ex- 

 ample the antelope whose eye is larger than that of the 

 elephant. It is also interesting to note that a meridional 

 section of the hawk eye (unidentified species) in my own 

 collection is almost as large as a corresponding section of an 

 adult human eye — illustrating again the lack of correlation 

 between the size of the eye and that of the body. 



Vertebrate eyes vary but little in general shape, although 

 there are some deviations. Deep sea fishes possess tubular 

 eyes which are extremely long in the antero-posterior 

 dimensions. The owl's eye is also of this general shape but 

 not so extreme. Many nocturnal animals have somewhat 

 elongated eyes by reason of their large and greatly curved 

 corneas (e.g. nocturnal lemurs, Figures 48, 49). 



Fig. 3. Section through the head of 

 the great horned owl (Bubo virgin- 

 ianus). FT, visual axis of the single, 

 temporal fovea; P, pecten. (Redrawn 

 from figure 4, Casey Albert Wood, The 

 Fundus Oculi of Birds, The Lakeside 

 Press, Chicago.) 



