PEEIODIC OPHTHALMIA. 373 



district. A knowledge of the causes of the malady is thus 

 of much more importance than that of any system of treat- 

 ment that has been suggested. The causes are predisposing 

 and exciting. Among the predisposing causes may be men- 

 tioned the following : 



1st. Soils of a clayey and humid character have a de- 

 leterious influence on horses raised on them. These are 

 soft and flabby, with a predominance of areolar tissue, thick 

 skins, long hair, flat feet, and a general lymphatic tempera- 

 ment. Such horses seem more susceptible to morbid influ- 

 ences, and especially to those producing this disease. If 

 removed to dry calcareous soils, the predisposition may re- 

 main latent throughout life ; but, on the other hand, horses 

 bred on the latter soils, and afterwards removed to the for- 

 mer, are very liable to contract the malady. 



2nd. Soils naturally damp from their own character, or 

 from that of the subsoil, and which have not been ameliorated 

 by drainage, have the same influence on the constitution of 

 the horse, and predispose to this affection in the same way as 

 the argillaceous. 



3rd. Excessive humidity of the atmosphere exerts a similar 

 determining tendency, and, accordingly, periodic ophthalmia 

 is a common disease on the banks of large rivers and lakes, 

 and, in some cases, in the vicinity of the sea. So great is 

 the influence of the peculiarities of soil and climate on the 

 development of this disease, that Spanish dealers are said to 

 buy up affected animals in the south-western departments of 

 France, being convinced that, if they are transported to cer- 

 tain regions beyond the Pyrennees, the malady will dis- 

 appear. Many parts of Ireland afford excellent examples 

 of the truth of the above remarks. 



4ith. Fodder of inferior quality, which contains a larger 

 amount of aqueous and few nutritive principles, has a debili- 



