528 HEREDITARY DISEASES OF HORSES. 
patible with its being in other cases decidedly hereditary. 
Such cases as we have just adduced only serve to show that 
the same disease is not always referable to the same causes, 
and that causes very different in their nature occasionally 
produce the same effects. 
Diseases accidently produced during the lifetime of an 
individual occasionally become hereditary, but not usually so. 
Blindness produced by injury or ordinary external causes, and 
roaring produced by phlebitis or even by bronchitis, are sel- 
dom hereditary ; and it appears as a general rule, admitting, 
however, of some expections, that a local injury or disease 
produced by accidental causes is not likely to be hereditary, 
although a generally deteriorated state of health, however 
produced, is very apt to be so. 
There are various maladies which, from their simulating 
some of the characters of hereditary diseases, have been 
thought by many to be truly hereditary. Abortion affords 
an apt illustration of such a mistake. This disorder fre- 
quently prevails in a stock for a long series of years, and^ 
sometimes even during several generations. But although 
corresponding in these respects to may hereditary diseases, 
it differs essentially from them, inasmuch as it attacks all 
animals alike when exposed to the same exciting causes, 
shows no special preference for those bred from a stock in 
which abortion has been prevalent, does not affect those 
removed to a distance from the locality in which the disease 
prevails, and may sometimes be effectually and immediately 
arrested by a radical change in the system of management. 
These conditions are quite sufficient to disprove the here- 
ditary nature of abortion; and when such conditions occur 
in connection with any other disease, they may safely be 
accepted as ample evidence of its being produced by external 
or extraneous circumstances, independently altogether of any 
hereditary predisposition. 
There are some maladies in which it is comparatively easy 
to trace the connection between conformation and disease. 
In the horse certain sorts of limbs notoriously predispose 
to certain diseases. Thus, bone spavins are most usually 
seen where there is a disproportion in the size of the limb 
above and below the hock ; curbs, where the os calcis is 
small and the hock straight ; stains of the tendons of the fore- 
leg, where the limb is round and the tendons and ligaments 
confined at the knee ; and navicular disease, where the chest 
is narrow and the toes turned out. Amongst horses so formed, 
these diseases are unusually common, and are developed 
by causes which would be quite inadequate to produce them 
