62 OPHTHALMOLOGY FOR VETERINARIANS 



perfect binocular vision; hence, refractive errors, causing 

 a greater effort to see, particularly close objects, tend to 

 produce a weakness of one or more of the extrinsic 

 muscles, resulting in a turning outward or inward of a 

 few degrees of one or both eyes. The condition is hardly 

 noticeable, though it is brought out by certain forms of 

 examination. 



This may go on and on until the position is easily 

 seen by a second person, and the eye assumes the ap- 

 pearance of being crossed. He may be able to see with 

 either eye singly and normally, but the eye that is cov- 

 ered, or not fixing, turns outward or inward, as the case 

 may be. He may be wholly dependent upon one eye, 

 and the eye that is not used becomes partially blind 

 (amblyopic) . 



This is not so in the animal, because of the position 

 of the eyes and numerous other reasons. A cross-eyed 

 animal is seldom ever seen. They may, however, be 

 subject to paralysis of the muscles, by reason of pressure 

 upon or disease of the nerves supplying those muscles, 

 the same as in man. 



One needs only remember the anatomic relations, ac- 

 tion, and nerve supply to determine which muscle and 

 nerve is involved. 



Ophthalmoplegia.— This is a condition in which all 

 the muscles are paralyzed. There being no resistance, 

 the eyeball stands out prominently from the orbit and 

 is immobile. The Hd may droop (ptosis) because of 



