it should, as a resilient body, and an
aid to the absorption of concussion, it seems rather to play the part of
a foreign body, and to bring concussion about. Seeing, then, that the
navicular bursa is in very near contact with it, it is conceivable that
this joint-like apparatus should suffer, and the pedal articulation be left
unaffected, the more so when we take into consideration the compression
theory just described.

4. _A Weak Navicular Bone_.--When the disease commences first in the
bone--and there is no denying the fact that sometimes, although not
invariably, it does--it may be explained by attributing to the structure of
the bone an abnormal weakness in build.

The navicular bone consists normally of compact and cancellated tissue
arranged in certain proportions, the compact tissue without, and the
cancellated within. These proportions can only be judged of by the
examinations of sections of the bone, and when it is found in any case that
the cancellated tissue bulks more largely in the formation of the bone than
normally it should, we have what we may term a weak navicular bone. In this
connection Colonel Smith says: 'Though it is far from present in every case
of the disease, still I consider it a factor of great importance.'

5. _A Defective or Irregular Blood-supply to the Bone_.--This, Colonel
Smith considers, is brought about by excessive and irregular work, and by
the opposite condition--rest. The author points out that the bloodvessels
passing to and from the navicular bone run in the substance of the
interosseous ligaments, or in such proximity to them that it is conceivable
that under certain circumstances mechanical interference may occur to the
navicular circulation. He further points out a fact that is, of course,
well known to every veterinarian, that in periods of work the circulation
of the foot is hurried, and that in rest there is always a tendency to
congestion; and he says in conclusion: 'I cannot help thinking that
irregularities in the blood-supply in a naturally weak bone must be a
factor of some importance, especially when the kind of work the horse is
performing is a series of vigorous efforts followed by rest.'

6. _Senile Decay_.--With approaching age the various tissues lose their
vigour, and are prone to disease. The navicular bone and surrounding
structures are not exempt. With the other and more active causes we have
described acting at the same time it is not surprising that n