its fracture. In progression the hind part of the foot comes to the
ground first, and upon the hinder portion of the articulation would fall
the first effects of concussion, together with the greater part of the
body-weight. A yielding joint was in this position necessary, and that
formed by the navicular bone fills all requirements.

In this connection one next considers the part played by the front limbs
during progression. As Zundel expresses it, they are columns of support
rather than of impulsion, and, as the body-weight is thrown forward by the
hind-limbs, it is the duty of the fore-limbs to receive it. The shock or
concussion of the body-weight thus thrown forwards is first received by
the muscles uniting the limb to the trunk, and a great part of it there
minimized by their sling-like attachment. It is further absorbed by the
shoulder-joint, and from there passed on to the almost vertical bony column
represented by the radius and ulna, the knee, and the metacarpus. On
reaching the first phalanx, a portion of the remaining force is passed on
to the front of the phalanges and loses itself in front of the hoof, while
the other portion is transmitted to the flexor tendons, finally to the
perforans, and to the posterior parts of the foot. During progression,
therefore, the navicular bone is constantly pushed downwards and backwards
by the bony column, and is just as constantly pushed forwards and upwards
by the resistance of the perforans tendon. This means, of course, that
the navicular bone is more or less constantly subject to compression,
and constant pressure, as we know full well, is a pretty sure factor
in bringing about malnutrition of the parts, with atrophy or chronic
inflammatory changes as an end result.

Even with the limb at rest the pressure on both sides of the navicular bone
is still constant. The only circumstances under which we can conceive of
it being entirely absent, in fact, are when the tension on the tendon is
relaxed, and the body-weight altogether removed by the animal adopting the
recumbent position.

The compression theory as to the causation of navicular disease was, we
believe, first originated by Colonel Smith. He, at any rate, has laid much
stress on it in his writings. If we accept it, and we see every reason
that we should, then we must, with the author, admit the possibility of
navicular disease arising from long standing in one position.

3. _Concussion_.--This we are bound to a