The Senses. 305 



bundles of connective tissue, giving a metallic lustre to the 

 eye. The colour is not due to pigment, but is iridescence 

 caused by interference of waves of light' (M'Kendrick). 

 Over this region there is an entire absence of the pigment 

 so characteristic of the inner surface of the choroid, and 

 the appearance of the fundus of the horse when examined 

 by the ophthalmoscope is an extremely beautiful object. 



The Ophthalmoscope. — We may here describe in outline 

 the theory of this instrument, and the appearance of the 

 picture presented by it. 



To examine the eye, a mirror with a hole in the centre 

 is applied to the eye of the observer; from a suitable 

 source of light rays are thrown through the pupil on to the 

 retina to be examined. 



When a light is thrown into the eye, the rays are re- 

 flected back through the pupil in the direction in which 

 they entered, and pass through the hole in the mirror into 

 the eye of the observer. 



This is the principle of the ophthalmoscope. On looking 

 through it at the retina of the horse, a remarkable golden- 

 yellow or greenish-yellow surface is illuminated studded 

 with minute dots ; this is the tapetum. Examination shows 

 this surface to be situated above the optic papilla, and to 

 be half-moon in shape ; below it the optic papilla comes out 

 of a reddish or pink colour, with a slightly raised whitish 

 margin. It is so difficult to study the eye, owing to its 

 frequent movement, that only occasional glimpses of the 

 papilla can be obtained. From the optic papilla several 

 vessels may be seen radiating, but extending no great dis- 

 tance from it. This is characteristic of the retina of the 

 horse. The remainder of the fundus is purple or brown, 

 the shading in it resembling that employed for representing 

 mountains in a map. 



Owing to the presence of the tapetum in the horse, a per- 

 fect examination of the lens, and a fair examination of the 

 fundus may be made without the aid of artificial light; 

 under the influence of artificial light the pupil dilates so 

 much that there is no need for the use of atropine. 



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