THE VERTEBRATE EYE 5 



the cartilage may occur (e.g. elasmobranchs, teleosts). In 

 the anterior portion of the sclera of the hawk, bone is present. 

 A similar condition exists in the owl's eye (Figure 6). 



Fig. 6. Photomicrograph showhm |)i t^>ence of bone hi the anterior portion ot 

 the cartilaginous sclera of the owl eye. X 115. 



Cornea 



The transparent cornea forms the anterior portion of the 

 tunica fibrosa. In man it constitutes the anterior one-sixth 

 of the eye and is more curved than the scleral portion. 

 Viewed from in front it is elliptical — viewed from behind 

 it is circular. The anterior surface is not uniformly curved; 

 in the central portion it is spherical whereas towards the 

 periphery it becomes flattened, especially towards the nasal 

 side. Many measurements have been made of corneal 

 curvatures in man (Duke-Elder, 1939). It is also a note- 

 worthy fact that the refractive power of the human cornea 

 is about two and one-half times that of the lens so that most 

 of the refraction of light takes place at the corneal surface 

 rather than in the lens itself. 



The cornea varies in its relative size among vertebrates. 

 In general it may be said that it is large in fishes and mam- 

 mals, and small in reptiles and birds (excluding nocturnal 

 forms). Apparently all nocturnal forms have large corneas 

 which of course is an advantage for collecting the small 



