re
seen to be torn away from their abnormal adhesion with the navicular bone,
while others are seen to be still attached thereto. The surface of the
navicular bone itself exhibits small defects in the bony substance, which
have been brought about by a rarefactive ostitis. _a_, The perforans tendon
cut through and reflected; _b_, the sole.]

The interosseous and postero-lateral ligaments of the 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 coronae, 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 Predisposition_; (2) _Compression_; (3)
_Concussion_; (4) _A Weak Navicular Bone_; (5) _A Defective or Irregular
Blood-supply to the Bone_; and (6) _Senile Decay_.

[Illustration: FIG. 163.--THE NAVICULAR 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 coronae 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 consider briefly the
physiology of the parts in question.

The navicular bone is what we may term a complement of the os pedis. It
exists, in fact, simply in order that the os coronae may have a sufficiently
large articulatory surface to play upon. One wonders at first that Nature
did not arrive at this by originally placing a larger bone below. Colonel
Smith explains this by suggesting that this would in all probability have
meant