562 A MANUAL OF VETERINARY PHYSIOLOGY 



night vision to be determined ; it is fair to suppose that it enables 

 an animal to see better in a dim light, though, as stated at p. 537, 

 it cannot enable it to see in the dark. 



Pathological. 



Diseases of the eye in the lower animals are not numerous, and 

 are only of great importance in those required for work, particularly 

 work of a special kind requiring acute vision, and in horses used for 

 riding purposes. A horse with defective vision may do good service 

 in draught, but with a riding horse it may prove a serious matter. 

 Injuries to the eyes of horses are frequent — lacerated eyelids from 

 projecting nails and hooks in the stable ; injuries to the cornea and 

 punctures of the eyeball caused in the hunting-field, and more com- 

 monly in the stable. The animal can offer no protection to its eyes 

 other than by the use of the retractor muscle. As might be expected, 

 the most serious injuries to the globe of the eye are produced by the 

 smallest objects. A blow on the eye from a large object may not 

 leave a mark on the eyeball, while a blow from a stick or the end 

 of a lash may destroy vision. Errors of accommodation have been 

 discussed on p. 545, and need not be dealt with further ; correcting 

 lenses have been employed, but are never likely to prove popular. 

 Shying is not necessarily due to defective vision, though there can be 

 no doubt that both astigmatism and short sight are responsible for 

 many cases. The most destructive eye disease is the so-called 

 Specific Ophthalmia, an attack of which affects almost every tissue 

 in the globe of the eye. Something resembling it has been observed 

 in cattle in South Africa, but, speaking broadly, there is no eye 

 disease in other animals or man which for intensity and destructive- 

 ness can approach it. When horses were taken less care of than 

 they are at the present day, and the laws of health were less under- 

 stood, the disease was rampant throughout Europe. In this country 

 it is now relatively rare, but is still common both on the Continent 

 of Europe and in North America. It always leads to destruction of 

 one or both eyes, extending over a series of attacks, each of which 

 leaves the eye worse than before. Cataract is the most common 

 internal disease of the eye in horses, and a serious cause of unsound- 

 ness. Unlike cataract in the human subject, the lens may not 

 become entirely opaque — in fact, total opacity is the least frequent 

 condition met with. Spots of opacity, one or more, sometimes no 

 larger than the head of a pin, and generally situated in the line of 

 vision, represent the bulk of cases of cataract. There are some forms 

 of cataract which escape detection when the eye is merely illuminated 

 from the front, and can only be seen when the lens is illuminated 

 from the back by means of the ophthalmoscopic mirror. In fact, no 

 one by the ordinary method of eye examination can, without risk 

 of error, declare an eye free from cataract without using the mirror 

 of the ophthalmoscope. There are many cataracts concealed by 

 the margin of the pupillary opening which only become visible when 

 the eye is examined with this instrument. 



