DISEASES OF THE JOINTS 433 
articulation often participate in the inflammatory changes, 
and in many cases become completely ossified. The true 
articulatory surface of the bone, that articulating with the 
os pedis and with the os corone, is never affected. 
Causes.—In enumerating the causes of navicular disease, 
we shall follow the example of Colonel Smith and classify 
them under certain headings—namely, (1) Hereditary Pre- 
disposition ; (2) Compression ; (8) Concussion ; (4) A Weak 
Navicular Bone; (5) A Defective or Irregular Blood-supply 
to the Bone ; and (6) Senile Decay. 
Fic, 163.—Tur Navicutar Bone FROM A CASE oF LONG-STANDING 
NAVICULAR DISEASE. 
The erosion of the cartilage on its central ridge is most marked, and 
the porous appearance of the bone thus uncovered points to the 
existence within it of a rarefactive ostitis. Along its edges large 
osteophytic outgrowths speak of the effects of an osteoplastic 
periostitis. 
1. Hereditary Predisposition.—That navicular disease is 
hereditary is a fact that has for a long time been insisted 
on, and has come to be so generally admitted that we do 
not intend to dwell on it here. As we have said before, it 
is found in the lighter breeds of horses (and, according to 
Zundel, especially in the English breeds), and is there seen 
to be frequently transmitted from parent to offspring. 
2. Compression.—By this is meant the compression of 
the navicular bone between the os pedis and the os corone 
in front, and the perforans tendon behind. 
In order to appreciate this explanation of the causation 
of navicular disease at its true value, it will be well to con- 
sider briefly the physiology of the parts in question. 
28 
