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 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