53 



Close or near objects can be seen distinctly but distant ob- 

 jects may be distorted or become very indistinct. Concave 

 glasses are used by farsighted persons; but since the use of 

 glasses is impractical for animals, shortsightedness, there- 

 fore, becomes a permanent cause of shying and fright. 



In the Normal or Emmetropic eye, the principal focus 

 falls on the retina, and distinct images. of all objects, at near 

 or far distances, form on the retina (fig. 15 — E). The cornea, 

 the aqueous humor, the lens and the vitreous humor take 

 part in the formation of the image — the refraction and col- 

 lection of the rays of light. The cornea is the principal 

 refracting medium when the eye is at rest; but the changes 

 in the convexity of the lens (caused by the contractions of 

 the ciliary muscle) are the means by which the eye is ad- 

 justed, or accommodates itself, to different distances. 



In the far-sighted, short-sigted and normal eye the curva- 

 ture of the cornea and of the lens is regular; but some- 

 times the curvature of the cornea may be so irregular that 

 one part or meridian may produce short sighteduess, anoth- 

 er part produce far-sightedness while still another meridian 

 may be normal. This condition produces a very much dis- 

 torted image and is a fruitful source of shying or the cause 

 of fear and fright. Irregularities in the meridians of the 

 cornea produce the condition known as astigmatism. This 

 defective vision may also be caused by an oblique position of 

 the lens. There are several kinds and deo^rees of astigma- 

 tism, all of which are very difficult to distinguish and can 

 only be relieved by the use of proper glasses which are in- 

 applicable to animals. 



Slight cloudiness or opaque spots in the cornea, weak 

 cloudiness of the aqueous humor, beginning cataract, be- 

 ginning amaurosis or beginning glaucoma are accompanied 

 by indistinct vision, and consequently produce frequent shy- 

 ing. In fact, partial blindness from any cause is always 

 attended by indistinct vision and shying, fear or fright. 



