Beneath the hoof-horn extending round the coronet is a prominent band of fibrous tissue called the coronary cushion. Its surface is covered with a large number of closely-packed vascular villi - little projecting bodies which have been likened to the pile on velvet (fig. 400).
Each of these little processes fits into a small hole in the coronary groove of the hoof, and from them the crust is produced and renovated. This being the case, it is most important to the integrity and strength of the crust that the coronary cushion should be in a healthy condition; and so long as it is so, the horn which it forms is ample in quantity, and possesses the normal hardness, toughness, density, and thickness. When, however, inflammatory disease affects the secreting villi and the fibrous cushion, from which they proceed, the horn becomes altered in quality, and, losing the cohesive property by which its fibres are bound together, it becomes dry, loose in texture, and crumbles away. This changed condition is due to two factors - (1) the altered nature of the horn secreted; (2) the separation of the secreting villi from one another by the swelling and expansion of the coronary cushion, whereby the horn fibres are made to stand apart from each other, and fail to form that close compact mass of horn which the crust presents in a healthy condition.
Some horses inherit a dry and brittle state of the hoof-horn, and are specially liable to chronic villitis. Blows to the coronet are the chief exciting causes, and the writer has also seen it follow upon a severe blistering, quittor, and sand crack.
Chronic villitis most frequently affects the front region of the coronet. Sometimes it attacks but a small surface, at others it extends for some distance towards the quarters. It commences by swelling, with some heat and tenderness to pressure over the affected surface. Lameness may or may not exist at first; but where the disease continues to extend, and the swelling becomes considerable, the foot is brought to the ground with the bearing thrown upon the heel, as in laminitis, and lameness is pronounced. The horn in front of the coronet, instead of being smooth, now presents a rough and broken appearance. It is loose in texture, and flaky masses may be broken away with the finger. The crust at this point has a tendency to develop a ring-like character, to crack cross-ways, and to separate from its connection with the coronary cushion. When this takes place there is a slight oozing of serosity, and blood and matter may also be discharged.
Fig. 400. - Villi of the Coronary Cushion and Growth of Horn.
a, Villi of the Coronary Cushion, b, b, Horn of the hoof, with tubes in transverse section, c, Vessels of the villi.
It is usually the case that no attention is given to this disease in its primary stage, and it is not until considerable swelling appears at the coronet and serious changes have taken place in the horn that any notice is taken of it.
All injuries to this region, however slight, should receive prompt attention. A day or two in the stable, with cold, wet swabs to the part and a mild dose of physic, may be the means of preventing the injury from assuming a chronic character. Where this has already become established the toe should be shortened, the heel of the foot lowered, and the heels of the shoe thinned from the quarters backward. The object of this is to take the bearing from the front of the foot and relieve the affected part. A mild counter-irritant may then be applied to the coronet every three or four days, but no attempt should be made to blister, nor should an irritant of any kind be employed if there is oozing or discharge from the junction of horn and hair. Moderate pressure to the enlarged coronet is attended with benefit in some cases. This may be applied by means of tea-lead folded in such a way as to bear equally upon it, and fixed by means of a bandage.
The patient may be turned into a wet pasture wearing the lead compress, after a course of counter-irritation to the coronet.
If there is oozing from the injured coronet the horse should be confined to a loose-box, and after the wound has been thoroughly cleansed with warm carbolized water it should be freely dusted over with iodoform and covered with a pad of cotton-wool. This, secured by a bandage, will serve to keep the dirt out and encourage healing.
When again put to work, care should be taken to keep the heel down and the toe short, so that as little bearing as possible be made to fall in front of the foot.
Although an animal suffering from this affection may by judicious treatment be kept in work for some time, the liability of the hoof to crack at the coronet, or to be torn away from its connection with it, is always present, and may at any time call for rest and active treatment.