(b) Local Or Coronary Contraction.
Definition. - Contraction at the heels, confined to the horn immediately succeeding that occupied by the coronary cushion. Really, the condition is but a somewhat arbitrary subdivision of contracted hoof, as we have just described it in general. For that reason we shall give it but very brief mention.
Symptoms. - In this case the horn of the heels, instead of running down in a straight line from the coronary margin to the bearing surface of the wall, presents a more or less distinct concavity (See Fig. 79, a, a).
As is the case with contraction considered as a whole, this deformity may affect one or both heels; and during its first appearance, which is after the first few shoeings, the animal may go distinctly lame.
Causes. - Coronary contraction may occur in hoofs of normal shape immediately shoeing is commenced, and frog pressure with the ground removed. It is far more likely to ensue, however, if the hoof is flat, with the heels low, and the wall sloping. And with those predisposing circumstances it is that the horse goes lame, and not with the hoof of normal shape.
Seeing, then, that this condition is largely dependent upon the shape of the foot, we may, to some extent, regard it as hereditary. Seeing further, however, that it only appears when shoeing is commenced, we may in a greater degree also regard it as acquired. The lesson, therefore, that this and other forms of contraction should teach us is the carefulness with which the shoeing should be superintended in a large stud, or in any case where the animal is of more than ordinary value.
Fig. 79. - Hoof With Local Or Coronary Contraction (As Indicated At The Points A, A).
The explanation of the restricted nature of this form of contraction is simple enough. We have only to refer to the lessons taught by the experiments of Lungwitz, described in Chapter III (General Physiological And Anatomical Observations)., and the condition almost explains itself. We remember that, briefly, the coronary margin of the wall resembles a closed elastic ring, which yields and expands to local pressure, no matter how slight. We remember also that removal of the counter-pressure of the frog with the ground tended to contraction of the wall's solar edge when weight was applied. Connect these two facts with the experience that this form of contraction more often than not occurs in hoofs with sloping heels, and we arrive at the following:
1. The excessive slope of the heels tends to throw a more than usual part of the body-weight upon the posterior portion of the coronary margin of the wall, with a consequent expansion of that part of the coronary margin implicated.
2. That the shoeing, in removing the counter-pressure of the frog with the ground, is at the same time tending to bring about contraction of the lower portions of the wall at the heels and quarters.
3. That this tendency to contraction will at first appear in the thinner portion of the area of wall named - namely, in that immediately below the bulging coronary margin.
We thus get the appearance depicted in Fig. 79 - a contraction (a, a) of the heels in the horn below the coronary margin, with the coronary margin itself bulging above, and a hoof of apparently normal width below.
We say 'apparently' with a purpose, for, as actual measurements will show, the wall near the solar edge is really contracting, for reasons which we have just described connected with shoeing. Its 'appearance' of normal width is accounted for thus: The contraction at a, a is caused by the dragging inwards of the coronary cushion brought about by the sinking downwards of the plantar cushion, with which body it will be remembered the coronary cushion is continuous. With the constant dragging in and down of the coronary cushion there is given, to the horn-secreting papillae, studding both the lower third of its outer face and its lowermost surface, a distinct 'cant' outwards. Below the lowermost limit of the coronary cushion, then, by reason of the cant outwards of the coronary papillae in the situations mentioned, the horn of the wall takes a more outward direction than normal, a fact which lessens in effect the contraction as a whole really going on. It is interesting, too, to note that by this outward cant of the wall below, and the bulging of the coronary margin above it, the contraction (a, a) is heightened in effect, and caused to appear greater than really it is.
From what we have said it follows that contraction of the heels, excepting the extreme coronary margin, is existent generally, and not confined solely to a, a.
We have, then, in this condition, as we indicated at the commencement, but a phase in the evolution of ordinary contracted heels, for, with the progress of the contraction already existing at a, a, and below those points, it is only fair to assume that with it falling in of the at present bulging coronary margin must sooner or later occur, that, though expanded when compared with the wall below it, it will be really contracted as compared with what it was once in that same foot.
We may therefore conclude this section by remarking that factors tending to contraction of the heels in general are equally potent in the causation of contracted coronet alone.
Treatment. - Exactly that described for contracted heels. Bearing in mind that contracted coronary margin is but the onset of contracted heels, and that its first exciting cause is that of removal of the ground-pressure upon the frog, the most careful attention must be paid to the shoeing. The use of bar shoes, ordinary frog pads, or heelless shoes and bar pads, are especially indicated, together with abundant exercise. By these means the normal movements of expansion will be brought into play, and the condition quickly remedied.