158 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



be drawn upward over the eye, wiping it and protecting it 

 from injury without entirely excluding the light, for it is 

 semitransparent (Fig. 97). A remnant of this membrane, 

 in useless condition, is found even in man. 



Fovea, — Birds not only have a fovea, but in some there 

 are two in each eye. The most distinct of these is in the 

 axis of the eye, and therefore at the bottom of the retinal 

 concave. Now since the optic axes are not parallel, as 

 in man, but are widely divergent (Fig. 100, page 162), 

 the side of the head must be turned toward an object in 



Fig. 97. — Eye of a bird showing (nm) the nictitating membrane. 



order that its image shall fall on this fovea. We will 

 speak of this again under binocular vision in vertebrates. 

 Reptiles. — These are in many ways similar to birds. 

 The sclerotic bones are found in lizards and turtles (Fig. 

 98), though not in crocodiles and snakes. In some rep- 

 tiles — e. g., in snakes — the lids are absent. The dry, 

 horny epidermis passes directly over the cornea of the 

 eye, and in skin-shedding comes off with the rest of the 

 epiderm. Also some lizards — e. g., chameleon and 

 phrynosoma — have a distinct fovea. 



