3. Concussion. - This we are bound to admit as a cause, and in so doing partly explain the comparative, almost total, immunity of the hind-feet from the disease. The fore-limbs, as we have already pointed out, are little more than props of support, and the force of the propelled body-weight is transmitted largely down their almost vertical lines, to end largely in concussion in the foot. With the hind-limbs matters are different. 'These,' as Percival explains it, 'have their bones obliquely placed, so as to constitute, one with the other, so many obtuse angles, to the end, that by forming powerful levers, and affording every advantage for action to the muscles attached to them, they may be fitted for the purpose of propulsion of the body onward.'

The effect of these several obtuse-angled joints in the limb is to absorb the greater part of the force exerted by the body-weight before it reaches the foot. When with this we take the facts that the fore-limbs have to carry the head and neck, and that they have to bear this added weight, plus a propelling force from behind, we see why it is that they should be so subject to the disease, and the hind-limbs so exempt.

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 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 navicular disease is seen as a result.

In conclusion, it is well, perhaps, to say that, no matter to which particular theory of causation we may lean, we should make up our minds to consider them as a whole. While one cause may be exciting, the other may be predisposing, and the two must act together before evil results are noticed. It may be that even more than two are concerned in bringing on the disease, and to each the careful veterinarian will give due consideration.