DISEASES OP THE EYE. 269 



ous, but a fuller inquiry contradicts this. If these are well grown 

 they invigorate and fortify the system, while, like any other fodder, 

 if grown rank, aqueous, and deficient in assimilable principles, they 

 tend to lower the health and open the way for the disease. 



The period of dentition and training is a fertile exciting cause, for 

 though the malady may appear at any time from birth to old age, yet 

 the great majority of victims are from two to six years old, and if a 

 horse escapes the affection till after six there is a reasonable hope that 

 he will continue to resist it. The irritation about the head during the 

 eruption of the teeth, and while fretting in the unwonted bridle and 

 collar, the stimulating grain diet and the close air of the stable all 

 combine to rouse the latent tendency to disease in the eye, while direct 

 injuries by bridle, whip, or hay seeds are not without their influence. 

 In the same way local irritants, like dust, severe rain and snow 

 storms, smoke, and acrid vapors are contributing causes. 



It is evident, however, that no one of these is sufficient of itself to 

 produce the disease, and it has been alleged that the true cause is a 

 microbe, or the irritant products of a microbe, which is harbored in 

 the marshy soil. The prevalence of the disease on the same damp 

 soils which produce ague in man and anthrax in cattle has been 

 quoted in support of this doctrine, as also the fact that the malady is 

 always more prevalent cceteris paribus in basins surounded by hills 

 where the air is still and such products are concentrated, and that a 

 forest or simple belt of trees will, as in ague, at times limit the area 

 of its prevalence. Another argument for the same view is found in 

 the fact that on certain farms irrigated by town sewage this malady 

 has become extremely prevalent, the sewage being assumed to form a 

 suitable nidus for the growth of the. germ. But on these sewage 

 farms a fresh crop may be cut every fortnight, and the product is 

 precisely that aqueous material which contributes to a lymphatic 

 structure and a low tone of health. The presence in the system of a 

 definite germ has not yet been proven, and in the present state of our 

 knowledge we are only warranted in charging the disease to the 

 deleterious emanations from the marshy soil in which bacterial fer- 

 ments are constantly producing them. 



Heredity is one of the most potent causes. The lymphatic constitu- 

 tion is of course transmitted and with it the proclivity to recurring 

 ophthalmia. This is notorious in the case of both parents, male and 

 female. The tendency appears to be stronger, however, if either 

 parent has already suffered. Thus a mare may have borne a number 

 of sound foals, and then fallen a victim to this malady, and all foals 

 subsequently borne have likewise suffered. So with the stallion. 

 Keynal even quotes the appearance of the disease in alternate gen- 

 erations, the stallion offspring of blind parents remaining sound 

 through life and yet producing foals which furnish numerous victims 

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