12 OPHTHALMOLOGY FOR VETERINARIANS 



ered. The higher in the scale of animal hfe, the more 

 nearly is the eye like that of man; the only difference 

 is the addition of some conveniences of which man is 

 not in need. Most of the quadrupeds, for instance, 

 are supplied with a third eyeUd, or nictitans mem- 

 brane, and a retractor muscle. The former acts as a 



Fig. I. — Portion of skull of horse, showing bony orbit. 



finger to remove foreign bodies, dust, etc., that may 

 fall upon the cornea. The latter serves to draw the 

 eyeball backward into the orbit and protect it from 

 approaching harm. These, together with the varia- 

 tions in size, are the only practical differences from the 

 human eye. 



The eyes of quadrupeds present nearly laterally, 



