Later, however, changes in the form of the hoof and the coronet begin to appear.  The skin of the coronet, especially in the region of the toe, becomes more or less thickened and indurated, and the same remark applies to the subcutaneous tissues.  The most marked change, however, is the alteration in the shape of the hoof.  The wall protrudes at the toe in a manner that has been termed ‘buttress-like,’ and has given to the condition one of its names.  This, of course, entirely alters the contour of the horny box.  From being more or less U-shaped, it approaches nearer the formation of the letter V, the point of the V being at the toe.

In the later stages the coronary enlargement is plainly seen to be due to an extensive formation of bone.  It is, in fact, a reparative callus, and the reason it reaches so large a size is probably to be accounted for by the pull of the extensor pedis upon the detached pyramidal process.  As might be expected, this displacement of the fractured portion, with its effect of giving greater length to the extensor pedis, leads to a backward displacement of the os coronae upon the pedal bone.  As a result there is a marked depression at the coronet, the depression being heightened in effect by the exostosis in front.  Pyramidal disease is, as a rule, met with in the hind-feet, but occurs also in the fore.

Pathological Anatomy.—­When occurring without fracture, the first observable change is a thinning of the articular cartilage of the pyramidal process, through which the bone beneath appears abnormally white.  Later the thinning of the cartilage progresses until at last it becomes entirely obliterated.  This destruction of the cartilage commences first at the highest point of the articular surface of the pyramid, and gradually reaches further backward into the joint.  While this is taking place the new bone is being formed on the front of the os pedis, below and around the process, until, as we have already seen, an exostosis is formed, large enough to be noticeable at the coronet.  This, of course, partly implicates the joint and the points of the insertion of the extensor tendon.

Finally, fracture may, or may not, take place.  When it does, the exostosis is larger, and the general deformity of the hoof greater.

Treatment.—­Ordinary treatment, such as point or line firing, repeated blisters, or hoof section, each of which we have tried, appears to be utterly useless.  So far as we have been able to gather from the writings of other practitioners, however, neurectomy returns the animal for a time to usefulness.  If the fore-limb is the seat of trouble, either plantar or median neurectomy may be practised; if the hind, then the best results are obtained by section of the posterior tibial.