320 



THE BIOLOGY OF BIRDS 



portion in owls, Golden-Eagle, and some other predatory 

 birds. The eye is adapted for horizontal vision, and every 

 one has seen a hen turn her head through a right angle to 

 get a glimpse of something overhead. 



Some of the detailed peculiarities of the eye are of much 

 interest. The cornea, which is usually very convex, is 

 in some cases particularly strong, probably a protective 

 adaptation. This is very marked in some diving birds, 

 and also in crows and diurnal birds of prey. The eye may 



CON 

 COR 



SCL CHO 



Fig. 47. — Diagrammate median vertical section through a bird's eye. 

 CON., conjunctiva ; cor., cornea; A. H., aqueous humour in the anterior 

 chamber of the eye ; p., the pupil, surrounded by the iris (ir.) ; l., the 

 lens; c.p., ciliary processes fastened to the lens; cm., ciliary muscle; 

 SCL.R., sclerotic ring of bone ; v.h., vitreous humour in posterior chamber 

 of the eye; R., retina or percipient layer, innermost ; CHO., the black 

 vascular choroid ; scL., the firm sclerotic, outermost ; pe., the pecten, 

 projecting into the vitreous humour; o.N., the optic nerve. 



thus be strengthened against the pressure of deep water or 

 against the concussion of branches. Strengthening of the 

 eyeball is also effected by a ring of bones at the anterior 

 margin of the sclerotic where it joins the cornea — an interest- 

 ing ring because it occurs also in some reptiles, and is a 

 tell-tale evidence of the reptilian origin of birds. In many 

 there is a narrow posterior embrasure of small bones where 

 the optic nerve enters the eye. 



