A fracture may be rendered compound _from without_, the soft parts being damaged by the object which breaks the bone--as, for example, a cart wheel, a piece of machinery, or a bullet. Sloughing of soft parts resulting from the pressure of improperly applied splints, also, may convert a simple into a compound fracture. On the other hand, a simple fracture may be rendered compound _from within_--for example, a sharp fragment of bone may penetrate the skin; this is the least serious variety of compound fracture. As a rule, it is easy to recognise that the fracture is compound, as the bone can either be seen or felt. The _prognosis_ depends on the success which attends the efforts to make and to keep the wound aseptic, as well as on the extent of damage to the tissues. When asepsis is secured, repair takes place as in simple fracture, only it usually takes a little longer; sometimes the reason for the delay is obvious, as when the compound fracture is the result of a more severe form of violence and where there is comminution and loss of one or more portions of bone that would have contributed to the repair. Sometimes the delay cannot be so explained; Bier suggested that it is due to the escape of blood at the wound, whereas in simple fractures the blood is retained and assists in repair. If sepsis gains the upper hand in a compound fracture there is, firstly, the risk of infection of the marrow--osteomyelitis--which in former times was liable to result in pyæmia; in the second place, not only do loose fragments tend to die and be thrown off as sequestra, but the ends of the fragments themselves may undergo necrosis; involving as this does the dense cortical bone of the shaft, the dead bone is slow in being separated, and until it is separated and thrown off, no actual repair can take place. The sepsis stimulates the bone-forming tissues and new bone is formed in considerable amount, especially on the surface of the shaft in the vicinity of the fracture; in macerated specimens it presents a porous, crumbling texture. Sometimes the new bone--which corresponds to the involucrum of an osteomyelitis--imprisons a sequestrum and prevents its extrusion, in which case one or more sinuses may persist indefinitely. Cases are met with where such sinuses have existed for the best part of a long life and have ultimately become the seat of epithelioma. It should be noted that all the above changes can be followed in skiagrams. _Treatment._--The leading indication is to ensure asepsis. Even in the case of a small punctured wound caused by a pointed fragment coming through the skin it is never wise to assume that the wound is not infected. It is much safer to enlarge such a wound, pare away the bruised edges, and disinfect the raw surfaces.