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 coronæ 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 coronæ 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 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 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.