286 DISEASES OF THE HOESE. 



tutes a phase of external opththalmia which demands a special notice. 

 These have accordingly been already treated of. 



Causes. — The causes of external opththalmia are mainly those that 

 act locally — ^blows with whips, clubs, and twigs, the presence of for- 

 eign -bodies, like hayseed, chaff, dust, lime, sand, snuff, pollen of 

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

 bridle, the migration of the scabies (mange) insect into the eye, 

 smoke, ammonia arising from the excretions, irritant emanations 

 from drying marshes, etc. Road dust containing infecting microbes 

 is a common factor. A very dry air is alleged to act injuriously by 

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

 the undue exposure to bright sunshine through a window in front of 

 the stall, or to the reflection from snow or water, also 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, must be set down among the locally acting causes. 

 Exposure to cold and wet, to wet and snow storms, to cold drafts and 

 wet lairs must also be accepted as causes of conjunctivitis, the gen- 

 eral 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 opththalmia is a constant concom- 

 itant of inflammation of the contiguous and continuous mucous mem- 

 branes, as those of the nose and throat — hence the red, watery eyes 

 that attend on nasal catarrh, sore throat, influenza, strangles, nasal 

 glanders, and the like. In such cases, however, the affection of the 

 eye is subsidiary and is manifestly overshadowed by the primary and 

 predominating disease. 



Symptoms. — The symptoms are watering of the eye, 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 red- 

 ness, 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. 

 Except when resulting from wounds and actual extravasation of 

 blood, however, 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 limpid. The fever 

 is always less severe than in internal ophthalmia, and runs high only 

 in the worst cases. The eyelids may be kept closed, the eyeball re- 

 tracted, 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 in- 

 ternal ophthalmia, on the contrary, the narrow, contracted pupil is 



