308 STRUCTURE OF THE VERTEBRATES 



cavity of the eye gives the black appearance to the pupil. The 

 pink eyes of albinos are caused by the lack of pigment in the 

 iris, which permits the minute blood vessels to show and give 

 their color to the eye. 



3. Sclerotic layer. The sclerotic, or hard, layer is continuous 

 over the entire eye, and forms both the proximal tough mem- 

 branous covering and the cornea. The latter is very similar in 

 all vertebrates. The sclerotic coat varies more widely in struc- 

 ture. In a number of reptiles and some birds the sclera becomes 

 ossified into bony plates; but in all cases the eye remains freely 

 movable. 



4. Cavities. The attachment of the lens to the ciliary processes 

 of the choroid layer separates the cavity of the eye into two 

 distinct regions. The distal chamber between the cornea and 

 the lens is filled with a refracting fluid, the aqueous humor. 

 The larger space between the lens and retina is filled with a 

 semi-solid vitreous body or humor. The origin of the vitreous 

 body is in dispute; its function is evidently to maintain the 

 almost spherical shape of the eye. 



5. Function. Light rays passing through the cornea strike the 

 lens, and its biconvex shape concentrates them on the retina. 

 Each point of light and shade that is reflected to the eye is 

 focussed separately so that the retina receives a mosaic picture. 

 These stimuli are transmitted over the optic nerve and coordi- 

 nated in the brain. If the rays come to a focus in front of or 

 behind the retina the point of light becomes circular and the 

 image is blurred. This is near-sightedness or far-sightedness. 



Accommodation to distance is cared for by the movement of 

 the lens and the slight ability to vary the shape of the focussing 

 structure so that its convexity becomes greater or less. Light 

 accommodation is adjusted by several structures. The function 

 of the pigment layer has been mentioned. The major part of 

 adjustment is cared for by the constriction of the iris. In the 

 normal eye this reflex action admits only sufficient light to make 

 a clear picture and prevent the blurring from very intense 

 radiation. 



Bifocal vision appears first in the mammals. In the lower 

 groups the eyes are placed on either side of the head, and there 

 is a complete optic chiasma. In the mammals, however, decus- 



