542 A MANUAL OF VETERINARY PHYSIOLOGY 



examined. When light is thrown into the eye, the rays are 

 reflected 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. On looking at the retina of the horse, a brilliantly 

 coloured surface is seen illuminated, the tints being a mixture of 

 yellow, green, and blue, studded with minute dots ; this coloured 

 area is the tapetum (Plate I. , Fig. 4) . Examination shows this sur- 

 face to be situated above the optic disc or papilla ; the optic papilla 

 appears of a pinkish colour, with a slightly raised whitish margin. 

 The fundus is a beautiful object, but very difficult to study in the 

 eye of the horse, owing to its frequent movement, so that only 

 occasional glimpses of the papilla can be obtained. From the 

 optic papilla a dense network of vessels may be seen radiating, 



g Fig. 161. — Direct Method of using the Ophthalmoscope (Stewart). 



Light falling on the perforated concave mirror M passes into the observed eye 

 E' ; and, both E' and the observing eye E being supposed emmetropic and 

 unaccommodated, an erect virtual image of the illuminated retina of E' is 

 seen by E. 



but extending no great distance from it ; this is characteristic 

 of the retina of the horse. The remainder of the fundus is purple 

 or brown, but owing to its extent very little of it can be seen. 

 In other animals the vessels radiating from the disc are wider 

 apart and more regular, and several of them have received names ; 

 moreover, the arteries can be distinguished from the veins, which 

 is not possible in the horse. It is to be borne in mind that the 

 view thus obtained of the fundus of the eye is a magnified image y 

 both the lens and vitreous humour making it appear about three 

 times larger than normal. Owing to the presence of the tapetum 

 in the horse, a perfect examination of the lens and fundus may 

 be made without the aid of artificial light ; while under the in- 

 fluence of artificial light the pupil dilates so much that there is 

 no need for the use of atropine. 



