As pointing out the part that concussion plays in its causation, we may mention that navicular disease is a disease of the middle-aged and the worked animal.  It is interesting to note, too, that it occurs in animals with well developed frogs—­in feet in which frog-pressure with the ground is most marked.  This at first sight appears to flatly contradict what we have said with regard to frog-pressure in other portions of this work.  With this, however, must be reckoned other predisposing causes.  In this case it is not to frog-pressure alone we must look, but to the condition of the frog itself, and that of the neighbouring parts.  It is when we have a frog which, though well developed and apparently satisfying all demands as to size and build, is at the same time composed of a hard, dry, and non-yielding horn that we must look for trouble.

The foot predisposed to navicular disease is the strong, round, short-toed or clubby foot, open at the heels, with a sound frog jutting prominently out between them.  Here is a frog exposed to all the pressure that might be desired for it, bounded at its sides by heels thick and strong, and indisposed to yield, and itself liable, from its very exposure, to become, in the warm stable, hard and dry, and incompressible’ (Percival).

Here, instead of acting, as normally 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