354 HORSE, SWINE AND POULTRY DISEASES 



Causes. The causes of external ophthalmia are mainly those 

 that act locally blows with whips, clubs, and twigs, the presence of 

 foreign bodies like hay seed, chaff, dust, lime, sand, snuff, pollen of 

 plants, flies attracted by the brilliancy of the eye, wounds of the 

 bridle, the migration of mange insect into the eye, smoke, ammonia 

 rising from the excretions, irritant emanations from drying 

 marshes, etc. Road dust containing infecting microbes is a com- 

 mon factor. A very dry air is alleged to act injuriously by drying 

 the eye as well as by favoring the production of irritant dust ; and 

 the undue exposure to bright sunshine through a window in front 

 of the stall, or to the reflection from snow or water, is undoubtedly 

 injurious. The unprotected exposure of the eyes to sunshine through 

 the use of a very short overdraw check is to be condemned, and the 

 keeping of the horse in a very dark stall from which it is habitually 

 led into the glare of full sunlight, intensified by reflection from snow 

 or white limestone dust, must be set down among the locally acting 

 causes. But exposure to cold and wet, to wet and snow storms, to 

 cold drafts and wet lairs must also be accepted as causes of conjunc- 

 tivitis, the general disorder which they produce affecting the eye, if 

 that happens to be the weakest and most susceptible organ of the 

 body, or if it has been subjected to any special local injury, like dust, 

 irritant gases, or excess of light. Again, external ophthalmia is a 

 constant concomitant of inflammation of the contiguous and con- 

 tinuous mucous membranes, as those of the nose and throat. Hence 

 the red, watery eyes that attend on nasal catarrh, sore throat, influ- 

 enza, strangles, nasal glanders, and the like. In such cases, how- 

 ever, the affection of me eye is subsidiary and is manifestly over- 

 shadowed by the primary and predominating disease. 



Symptoms. The symptoms are watering of the eyes, swollen 

 lids, redness of the mucous membrane exposed by the separation of 

 the lids it may be a mere pink blush with more or less branching 

 redness, or it may be a deep, dark red, as from effusion of blood 

 and a bluish opacity of the cornea, which is normally clear and 

 translucent. But except when resulting from wounds and actual 

 extravasation of blood, the redness is seen to be superficial, and if 

 the opacity is confined to the edges, and does not involve the entire 

 cornea, the aqueous humor behind is seen to be still clear and lim- 

 pid. The fever is always less severe than in internal ophthalmia, 

 and only runs high in the worst cases. The eyelids may be kept 

 closed, the eyeball retracted, and the haw protruded over one-third 

 or one-half of the ball, but this is due to the pain only and not to 

 any excessive sensibility to light, as shown by the comparatively 

 widely dilated pupil. In internal ophthalmia, on the contrary, the 

 narrow contracted pupil is the measure of the pain caused by the 

 falling of light on the inflamed and sensitive optic nerve (retina) 

 and choroid. 



If the affection has resulted from a wound of the cornea, not 

 only is that the point of greatest opacity, forming a white speck or 

 fleecy cloud, but too often blood vessels begin to extend from the ad- 

 jacent vascular covering of the eye (sclerotic) to the white spot, and 



