Callus, any unnatural hardness in the body, particularly of the skin, as on the hands or feet, from friction or pressure. When these excrescences are of such a size or so situated as to produce pain or inconvenience, relief is obtained by paring away the projecting non-vascular parts, and by protecting the skin from a continuance of the friction or pressure which caused the induration. The most common application of the word, however, is to the new growth of osseous matter around and between the extremities of fractured bones, serving to unite them. The mode of reparation is attended by the following changes: 1. Extravasation of blood where the bone is fractured. After this is absorbed, liquor sanguinis is effused, and assumes the position which the blood had occupied. 2. This consolidates, and the watery portion being absorbed, the rest becomes organized.

3. This period of plastic exudation lasts eight or ten days, and then becomes quasi-cartilaginous.

4. This mass contracts, increases in density, and gradually becomes very hard and strong. 5. The ossification or solidification advances from the periphery, and the fractured extremities are now surrounded by a bony case termed the provisional callus. 6. After this is formed, continuity is truly restored by the formation of what is called definitive callus or true bone, which takes place between the fractured extremities. 7. Finally, the provisional callus is absorbed and disappears. It was formed merely to serve as a natural case or splint to maintain the broken extremities in their position, while the osseous reparation was proceeding to restore the natural unity and continuity of structure.