286 DISEASES OF THE HORSE. 
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 
