PERIODIC OPHTHALMIA. 341 



disease, the other will, as a rule, become affected sooner or later. 

 This should be especially remembered, when one eye having cleared 

 up ajid become apparently sound, a month or two later the other 

 eye may be found to be affected more or less severely from sym- 

 pathy, direct nerve influence, or infection. Allow an escape of 

 aqueous humour sufficient to give the cornea a flattened appear- 

 ance. Care must be exercised that the flow of the fluid is very 

 gradual, so that the lens may not be torn from its attachments, or 

 the iris involved. In operating on the apparently sound eye, a 

 smaller quantity of aqueous humour is allowed to escape. Out of 

 100 cases, 80 recovered, 10 showed an improvement, and 10 did 

 badly. Other excellent results from this operation in periodic 

 ophthalmia have also been recorded. 



Cataract. 



This is an opaque condition of the crystalline lens, of its capsule, 

 or of both structures at the same time, by reason of which the light 

 that enters the pupil is obstructed on its way to the retina ; blind- 

 ness, total or partial, being the natural result. A cataract may 

 consist of only a small white or bluish-white spot which slightly 

 obscures the vision, and it may then be the cause of shying in the 

 animal ; or it may completely cover the affected structures. In 

 slight cataract, if the horse be taken into a dark room and the 

 eye be examined by the light of a candle, the speck may be seen 

 through the pupil, which will, more or less perfectly, contract or 

 dilate on the approach or removal of the taper ; but complete 

 catai-act will appear like a white curtain drawn across the opening 

 of the pupil, which will probably then be quite insensible to the 

 action of light. A careful comparison of the behaviour of both 

 eyes under the influence of the light of a candle in a room in 

 which there is no other source of light, will aid the correctness of 

 the examination. White specks on the cornea should not be con- 

 founded with cataract. 



An examination of the eyes in daylight is apt to lead an inex- 

 perienced observer into error, owing to the fact that light reflected 

 into the eye from white objects, such as white-washed walls, white 

 clothing, etc., causes the formation of white images within the 

 interior of the eye. 



The presence of cataract may be tested — under ordinary circum- 

 stances — by holding upright a lighted candle in front of the sus- 

 pected eye, in which, Ji it be healthy, three vertical reflections of 

 the candle will be seen, namely, one on the cornea, a second on 

 the front of the crystalline lens, and a third, turned upside down, 

 on the back of the lens. The inverted position of the last-named 



