The callous ulcer is most frequently found on the shoulders and backs of old horses as the result of ill-fitting collars and saddles, or on the withers, or in the heels. It is usually preceded by a succession of abrasions, during which the skin and subcutaneous tissues become infiltrated and thickened, and the vessels surrounded and compressed by a contracting undergrowth. As a consequence the blood-supply to the part is diminished, its vitality is weakened, and the skin is predisposed to ulcerate.
The callous ulcer is more or less hollowed and like a saucer. The edges are thickened, and raised above the general surface, which is usually smooth, and of a pale-yellowish colour. It discharges a small quantity of thin, sero-purulent fluid, and shows little or no disposition to throw up granulations. The skin round and about the wound, and the tissue beneath it, are hard and thickened, and firmly adherent to the underlying parts.