Sauropsida: Class Aves 



533 



Fig. 415. Eye of an owl; sec- 

 tion in optical axis. (Ch) Cho- 

 roid; (CM) ciliary muscle; (Co) 

 cornea; (Cv) vitreous humor; 

 (Ir) iris; (L) lens; (Op) optic 

 nerve; (OS) sheath of nerve; 

 (P) pecten; (RT) retina; (Sc) 

 sclerotic; (VK) aqueous humor; 

 (VN) junction of sclerotic and 

 cornea, (f) marks location of 

 bony plates in the sclerotic 

 layer. (Courtesy, Wiedersheim: 

 "Grundriss der vergleichenden 

 Anatomie der Wirbeltiere," 

 Jena, Gustav Fischer.) 



The eye is developed to a high degree of efficiency. It is extraor- 

 dinarily large relative to the size of the head. Whereas in Anamnia 

 and Reptilia the eyeball is approximately spherical or, in many fishes, 

 more nearly hemispherical with the flattened surface external, in birds 

 it is more or less elongated in the direction of the optic axis, and its 

 exposed surface is strongly convex. The elongation is especially marked 

 in nocturnal birds such as owls (Fig. 415). The sclerotic layer of the 

 eyeball is largely cartilaginous and a ring of bony sclerotic plates 

 surrounds the corneal region (Fig. 415f). 



A conspicuous but problematic feature of the bird's eye is a struc- 

 ture which lies in the midst of the vitreous humor (Fig. 415, P). It is a 

 folded membranous thing whose form is best likened to that of a half- 

 folded fan, the narrow end attached to the rear wall of the eyeball at 

 the region of entrance of the optic nerve. It projects freely into the 

 vitreous humor, the length varying greatly in different species and in 

 some cases extending even to the inner surface of the lens. Its tissue 

 is pigmented, highly vascular, and well supplied with nerves. The func- 

 tion of this structure, the pecten, is not definitely known. That it has 

 something to do with accommodation has been proposed and denied. 

 Its richness in blood-vessels suggests that it may serve to make nutri- 

 ment and oxygen more readily available to the internal tissues of the 

 eyeball. A similar structure, but much less well developed, occurs in 

 lizards, and appears in a very weakly developed condition in other 

 reptiles. 



In accommodation of the eye for vision at varying distances, the 

 convexity of the lens is altered, as in reptiles, but the effect of this 



