Much of the etiology of fractures, particularly from the teacher's point of view, may be presented as an application of the principles of practical mechanics—a part as it were of the subject of the strength of materials in engineering. With their characteristic thoroughness the Germans have given a good deal of attention to the mechanics of the etiology of fractures, particularly to flexion, torsion perforation fractures, but American surgeons have been too much engrossed in the more practical problems of therapy to devote much attention to the somewhat abstruse mathematical considerations as to how the different forms of fracture happen to be produced, and the text-books used by our students have little to say on the subject. In the multitudinous forms of accident causing fracture, the bones of the skeleton are subjected to stresses which in the ultimate analysis may be resolved into tensile and compressive stresses. These almost never occur singly but always in combination