(1) _According to the Degree of Damage done to the Bone._--A fracture may be incomplete, for example in _greenstick fractures_, which occur only in young persons--usually below the age of twelve--while the bones are still soft and flexible. They result from forcible bending of the bone, the osseous tissue on the convexity of the curve giving way, while that on the concavity is compressed. The clavicle and the bones of the forearm are those most frequently the seat of greenstick fracture (Fig. 41). _Fissures_ occur on the flat bones of the skull, the pelvic bones, and the scapula; or in association with other fractures in long bones, when they often run into joint surfaces. _Depressions_ or indentations are most common in the bones of the skull. The bone at the seat of fracture may be broken into several pieces, constituting a _comminuted_ fracture. This usually results from severe degrees of direct violence, such as are sustained in railway or machinery accidents, and in gun-shot injuries (Fig. 2). [Illustration: FIG. 2.--Radiogram of Comminuted Fracture of both Bones of Forearm.] _Sub-periosteal_ fractures are those in which, although the bone is completely broken across, the periosteum remains intact. These are common in children, and as the thick periosteum prevents displacement, the existence of a fracture may be overlooked, even in such a large bone as the femur. A bone may be broken at several places, constituting a _multiple_ fracture (Fig. 1). _Separation of bony processes_, such as the coracoid process, the epicondyle of the humerus, or the tuberosity of the calcaneus, may result from muscular action or from direct violence. _Separation of epiphyses_ will be considered later. (2) _According to the Direction of the Break._--_Transverse_ fractures are those in which the bone gives way more or less exactly at right angles to its long axis. These usually result from direct violence or from end-to-end pressure. _Longitudinal_ fractures extending the greater part of the length of a long bone are exceedingly rare. _Oblique_ fractures are common, and result usually from indirect violence, bending, or torsion (Fig. 3). _Spiral_ fractures result from forcible torsion of a long bone, and are met with most frequently in the tibia, femur, and humerus. [Illustration: FIG. 3.--Showing (1) Oblique fracture of Tibia; (2) Oblique fracture with partial separation of Epiphysis of upper end of Fibula; (3) Incomplete fracture of Fibula in upper third. Result of railway accident. Boy æt. 16.]