590 
HEREDITARY DISEASES OF HORSES. 
sometimes a speck scarcely perceptible, and interfering 
slightly with vision ; at other times large, with white lines 
passing outwards in every direction, and causing nearly total 
blindness. Eyes having any of these appearances must be 
regarded as unsound, and specially susceptible of inflam- 
mation, which is apt to be excited in them by such causes, 
as exposure to cold, high feeding, over-work, or debility, and 
is liable to return again and again, until the animal is totally 
blind. But before the occurrence of an acute attack it is 
scarcely possible, without an examination of its progenitors, 
to determine 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 confor- 
mation or of minute texture differing from health, and which, 
although generally unobservable, is yet capable, under 
favouring circumstances, of fostering serious and irreme- 
diable disease. 
Ample evidence can be adduced in support of the here- 
ditary 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 decidely marked even in the fourth and 
fifth generations, often appearing, and sometimes speedily 
causing blindness, very early in 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 
hereditary tendency; and this two-fold mode of production 
has given rise to much contrariety of opinion concerning the 
hereditary nature of the complaint. It is sometimes pro- 
duced even in its worst form by over-work and injudicious 
feeding, but such accidental cases are seldom hereditary, for, 
