564 DISEASES OF THE EYES. 



horse's head away from a strong light, shading the eye with 

 a Hack hat, and obscuring any white article of clothing that 

 the examiner may be wearing at the time. But in doubtful 

 cases of soundness of the eyes, a more searching test is required, 

 and the mode of examination is by reflection of light, called 

 the catoptric test. For the application of this test, the horse 

 must be put in a dark place, and should the pupil be sensitive 

 to light, it must be dilated with belladonna. 



When a lighted candle is moved before the healthy eye, three 

 images of it may be observed. First, the erect image, that 

 moves in the same direction as the candle, is produced by 

 reflection from the surface of the cornea ; secondly, another erect 

 image, produced from the anterior surface of the crystalline 

 lens, which also moves in the same direction; and thirdly, a 

 small inverted image, situated between the other two, is reflected 

 from the posterior surface of the lens, and moves in the contrary 

 direction to the others and to the movements of the candle 



In cataract the inverted imag^e is rendered indistinct or 



CD 



abolished, and the second erect one sometimes abolished also. 

 In glaucoma, the deep (or second) erect image is rendered more 

 evident than in a sound eye, and it is only in a very advanced 

 stasre that the inverted ima2:e is obliterated. In amaurosis all 

 the images are as distinct as in the healthy eye. 



EXAMINATION BY THE OPHTHALMOSCOPE. 



Very small cataracts are discoverable only with the aid of 

 this instrument ; the catoptric test not being always sufficiently 

 satisfactory. 



In order to examine with the ophthalmoscope, much practice 

 with the instrument is required, and the veterinarian should 

 become an adept in its use before resorting to it in an important 

 case. 



A dark place is necessary, and the examination is much 

 facilitated by first dilating the pupil with belladonna. 



The examiner is to stand in front of the horse, with the 

 instrument in his hand. An assistant stands near the horse's 

 shoulder, holding a lighted candle (this is better than a stronger 

 light). The examiner is to manipulate his instrument until the 

 light of the candle is reflected into the horse's eye ; in many 



