120 



Hereditary Diseases of Horses. 



possible, without an examination of its progenitors, to deter- 

 mine positively whether an animal is predisposed to periodic 

 ophthalmia. Horses with small dark eyes, large, coarse heads, 

 and of dull and phlegmatic disposition, are, however, generally 

 considered to be specially subject to the disease. There is 

 seldom any very apparent defect of the eyes, either in structure 

 or function ; still it cannot be doubted that there exists in them 

 some peculiarity of conformation or of minute texture differing 

 from health, and which, although generally unobservable, is yet 

 capable, under favouring circumstances, of fostering serious and 

 irremediable disease. 



Ample evidence can be adduced in support of the hereditary 

 nature of ophthalmia. Cases of congenital blindness in stock 

 subject to it are recorded. These, however, are rare ; but 

 opacities of the cornea and cataracts are not uncommon. The 

 tendency to the disease frequently shows itself before the animal 

 has been stabled or worked ; but more commonly, such changes 

 in the mode of life appear to be the immediate cause of the 

 attack. A very large number of the stock of the celebrated 

 Irish horse " Cregan " have become affected by ophthalmia of the 

 worst kind. I am told by a gentleman well acquainted with this 

 stock that the tendency is still decidedly marked even in the 

 fourth and fifth generations, often appearing, and sometimes 

 speedily causing blindness, very early in j^life, as at two or 

 three years of age, and even before the animals have been 

 exposed to what are considered the ordinary exciting causes of 

 ophthalmia. 



Specific ophthalmia affords a good illustration of a malady 

 which, although usually hereditary, is occasionally produced by 

 accidental causes, and to all appearance independently of here- 

 ditary tendency ; and this two-fold mode of production has 

 given rise to much contrariety of opinion concerning the here- 

 ditary nature of the complaint. It is sometimes produced even 

 in its worst form by over-work and injudicious feeding, but such 

 accidental cases are seldom hereditary, for, as we have above 

 remarked, acquired peculiarities are less likely to be hereditary 

 than inherent ones. From this it is obvious, that all blind 

 animals are not at once to be condemned as unfit for getting 

 sound and perfect stock. The cause of their blindness must be 

 inquired into ; and when it can be shown that they have lost their 

 sight from accidental causes, and that the stock from which they 

 sprang was free from all diseases of the eyes, they may be safely 

 used for breeding purposes. If, however, on the other hand, 

 the blindness cannot be traced to any adequate extraneous cause, 

 or if the sire or dam, or any other relatives of the animal, 

 be also blind, or affected with cataracts, the animal must be 



