DISEASES OF THE EYE AND ITS APPEiNDAGES 119 



CATARACT 



As the term cataract is usually understood, it includes changes in the 

 crystalline lens, which convert it from a transparent body into an opaque 

 mass (fig. 256), which may be compared to a small biconvex piece of chalk; 

 the appearance, in fact, is the same as that which is constantly noticed 

 in the crystalline lens of fishes when cooked, and seen in the dish as a 

 small globular body of a dense white colour. The professional man, 

 however, recognizes the fact that the term cataract applies to any opaque 

 speck, or number of specks, however minute they may be, which appear 

 in any part of the lens or its capsule; indeed, a cataract may be so ex- 

 tremely minute as to be absolutely invisible to the naked eye, and the 

 use of an ophthalmoscope by an expert is indispensable for the detection 

 of the disease in its initial stage. 



One of the old veterinary writers, Dr. Bracken, in 1737, describes 



Fig. 256.— Cataract 



four varieties of cataract, i.e. white, pearl colour, yellow and black or 

 green cataract. All these varieties are said to have been met with recently 

 in different animals, but they are only to be detected on post-mortem 

 examination, so far, at least, as the peculiarity of colour is concerned, and 

 as a rule, post-mortem examinations are not common in connection with 

 the existence of this disease. If the cataract is perfectly visible there is 

 no particular object in making a special examination of the lens after the 

 death of the animal, and if there is no evidence of its existence, it is not 

 suspected, and therefore not sought for; but it must be clear that in all 

 cases cataract at the commencement may be discovered by the use of 

 optical instruments, and their use is undoubtedly called for in all cases 

 where a horse is in the constant habit of shying, or in any way gives reason 

 for a suspicion that the sight is defective. 



Besides the division of cataracts as to colour, there is another which 



