586 
HEREDITARY DISEASES OF HORSES. 
tendons perform the work required of them with ease and 
safety : without it, they are apt to suffer from the sudden and 
violent shocks to which they are subjected, especially when 
the horse is put to fast work. 
Navicular Disease depends upon strain or laceration of the 
tendo perforans just where it passes over the navicular bone. 
It causes pain and tenderness of the parts affected, a short, 
tripping, but cautious gait, a wiring in of the heels, and a 
wasting of the muscles of the shoulder, with all the other 
well-known symptoms of grogginess. The predisposition to 
this disease is especially great in horses with narrow chests, 
upright pasterns, and out-turned toes. Even with average 
work, horses in which this conformation is decided can 
scarcely fail to become groggy, for the distance between the 
point at which the tendo perforans is inserted into the os 
pedis and that at which it passes over the navicular bone is 
so short, and the angle it makes so acute, that the tendon 
acts at a mechanical disadvantage, and is constantly liable to 
strain. But defects like these rarely occur singly, there 
usually existing in addition a want of mutual adjustment 
between other parts of the limb. Navicular disease is, there- 
fore, to a certain extent hereditary, in so far as there are 
certain forms of limb especially subject to it. A tendency to 
it exists in several stocks that have come under my own 
observation ; and I am informed by a veterinary friend, 
Mr. Tuthill, long resident in Ireland, that he knows of the 
progeny of several Irish horses, in which navicular disease is 
so common, that they are always looked upon with suspicion, 
and bring in consequence lower prices than their general 
appearance would otherwise warrant. The progeny of 
( Young Musician , 5 for example, a thorough-bred horse, 
well known in Ireland, and especially in the western counties, 
all show a great tendency to this disease. 
Acute diseases are usually referable to some cause or 
causes which are often violent in their nature, but operate 
for a comparatively short time ; their special locality may be 
determined, or their type or intensity modified, by the 
particular constitution of different animals; they seldom, 
however, owe their existence to inherent hereditary causes. 
Chronic diseases, on the other hand, usually result from the 
continued operation of causes inadequate to induce acute 
maladies; they often occur as consequences of badly- treated or 
acute attacks, their development is greatly dependent upon 
the special constitution of the individual, and many of them 
are more or less hereditary. No diseases better illustrate 
