The bones heal very slowly compared with most other tissues. The process of repair consists in the throwing out of a sort of cement about the ends of the fragments of the injured bones, which forms what is known as a callus, which is deposited in such a way as to constitute a sort of splint for the bone. At first, the callus is somewhat cartilaginous; after a time it becomes changed to bone. In very rare cases, the bones fail to unite, though this does not, according to Prof. Hamilton, occur in more than one case in five hundred. More or less deformity remains even if the bones are exactly coapted to each other. If the bones are not accurately set, or if after being set they are not properly kept in place, a considerable degree of deformity may result.

In some cases union takes place with the bones at more or less of an angle with each other. A deformity may also result from a shortening of the fractured limb due to overlapping of the fragments. This is especially frequent in fractures of the thigh in which more or less shortening generally occurs, the amount varying from a small fraction of an inch to two or three inches. If the shortening is not more than an inch, it will scarcely be noticed by the individual himself, and will not be observed in his walk.

Stiffness of joints in the vicinity of fractures is often found after recovery from the injury, being due either to interference with the motion of the joint by the callus, or to long-continued disuse of the joint.