DISEASES OF THE HORSE. 263 



dust, must be set down among the local]}' acting causes. But exposure 

 to cold and wet, to rain and snow storms, to cold drafts and wet lairs 

 must also be accepted as causes of conjunctivitis, the general disorder 

 which the}' produce affecting the eye, if that happens to bo the weak- 

 est 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 injElammation 

 of the contiguous and continuous mucous membi-anes, 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. 



Syrnptoim. — 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. 

 But except when resulting from wounds and actual extravasation of 

 blood, the redness is seen to be superficial, and if the opacity is con- 

 lined to the edges, and does not involve the entire cornea, the aque- 

 ous humor behind is seen to be still clear and limpid. 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 thi>s 

 is due to the pain only and not to any excessive sensibility to light, as 

 shovvu by the comparatively Avideh^ dilated pupil. In internal oph- 

 thalmia, on the contrary, the narroY/ 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 tliat the point of greatest opacity, forming a white speck or fleecy 

 cloud, but too often blood vessels begin to extend from the adjacent 

 A'ascular covering of the eye (sclerotic) to the white spot, and that 

 portion of the cornea is rendered permanent]}^ opaque. Again, if the 

 AYOund has been severe, though still short of cutting into the anterior 

 layers of the cornea, the injury may lead to ulceration that may pene- 

 trate more or less deeply and leave a breach in the tissue which, if 

 filled up at all, is repaired by opaque fibrous tissue in place of the 

 transparent cellular structure. Pus may form, and the cornea assumes 

 a yelloAvish tinge and bursts, giving rise to a deep sore which is liable 

 to extend as an ulcer, and may be in its turn followed by bulging of 

 the cornea at that point (staphyloma). This inflammation of the con- 

 junctiva may be simpl}^ catarrhal, with profuse muco-purulent dis- 

 charge; it may be granular, the surface being covered with minute 

 reddish elevations, or it may become the seat of a false membrane 

 (diphtheria). 



