The want of union is evidently due to defective activity of the bone-forming cells in the vicinity of the fracture. This may result from constitutional dyscrasia, or may be associated with a defective blood supply, as when the nutrient artery is injured. Interference with the trophic nerve supply may play a part, as cases are recorded by Bognaud in which union of fractures of the leg failed to take place after injuries of the spinal medulla causing paraplegia. The condition has been attributed to local causes, such as the interposition of muscle or other soft tissue between the fragments, or to the presence of a separated fragment of bone or of a sequestrum following suppuration. In our experience such factors are seldom present. If the treatment recommended for delayed union fails, recourse must be had to operation, the most satisfactory procedure being to insert a bone graft in the form of an intra-medullary splint. In certain cases met with in the bones of the leg in children, the degree of atrophy of the bones is such that it has been found necessary to amputate after repeated attempts to obtain union by operative measures have failed. In the tibia we have found that with the double electric saw a rod of bone can be rapidly and accurately cut, extending well above as well as below the site of fracture but unequally in the two directions; the rod is then reinserted into the trough from which it was taken _with the ends reversed_, so that a strong bridge of bone is provided at the seat of non-union. CLINICAL FEATURES OF SIMPLE FRACTURES In the first place, the _history of the accident_ should be investigated, attention being paid to the nature of the violence--whether a blow, a twist, a wrench, or a crush, and whether the violence was directly or indirectly applied. The degree of the violence may often be judged approximately from the instrument inflicting it--whether, for example, a fist, a stick, a cart wheel, or a piece of heavy machinery. The position of the limb at the time of the injury; whether the muscles were braced to meet the blow or were lax and taken unawares; and the patient's sensations at the moment, such as his feeling something snap or tear, may all furnish information useful for purposes of diagnosis. _Signs of Fracture._--The most characteristic signs of fracture are unnatural mobility, deformity, and crepitus.