This is an exaggeration of the horny epidermis occurring where the parts are exposed to unusual friction. It is a true hypertrophy, being a direct provision of nature to protect the true skin from the excessive friction to which it is exposed. The best example of the callosity is in the horny hands of workmen, but it is often seen in the feet, at points which are peculiarly exposed to pressure. From the situations of these callosities, as Hebra pointed out, the occupation of the person may often be made out.

The Corn (Clams) is closely related to the callosity. Here, also, there is friction, but it is associated with compression. It occurs chiefly in the foot where there is a concentric pressure from ill-fitting boots, and friction against either the boot or a neighbouring toe. There is, as in the former case, an excessive new-formation of horny epidermis, but, by the concentric pressure, the layers of epidermis have their directions changed, and growing against each other, they are projected inwards towards the cutis. There is thus an ingrowing kernel of hard horny epidermis which has given rise to the name of corn. The cutis is irritated and undergoes atrophy. Around the kernel or "core" the epidermis is always thickened, the condition being like that of the callosity.