8. But it is different with respect to a displacement in the cross direction of the bone. Here the cause of the separation of the broken ends, may be the same with the cause of their fracture, as happens in the passage of a carriage wheel over the limb, or in the falling of some body against it; and then, 1st, the fragments are separated from before backward, or contrarywise, and hence, a protuberance on the one side of the limb, and a depression on the other; 2dly, or else they are pressed against each other laterally from without inwards. From this latter cause arises that inequality which the limb exhibits at the place of the fracture; the slight depression which it manifests on its sides; and the protrusion or bulging out of its anterior and posterior surfaces, by means of the mass of muscles which are pushed in these directions, by the approximation of the fragments to each other. 9. A proper reduction removes the first kind of displacement, namely, that which occurs in the cross direction of the bone backward or forward (8); and unless an external force be applied anew, it does not again return. On the contrary, how exact soever the reduction may be, in the second kind, namely, that which takes place laterally from without inwards, the fragments are soon found to have approached each other again. Above, the pronator teres presses the superior fragment of the radius against that of the ulna; below, the two fractured extremities are pressed against each other, by the contractions of the pronator quadratus. From this double cause arises, unless something prevent it, the contact of the four fractured ends, which have been sometimes found united together by a common callus, as is proven by several cases sent to Desault, and by the cases of different patients admitted into the Hotel-Dieu, after having undergone an improper treatment. In such cases, the movements of pronation and supination being entirely destroyed, are but imperfectly supplied, as Duverney remarks, by those of a rotation of the arm. But if the four broken ends should not even be joined together by a common callus, still the space between the bones being evidently diminished, impedes muscular action and the motions of the limb depending thereon. § V. OF THE REDUCTION. 10. It follows from what has been said on the displacement of the fragments (8), and on the causes which have a constant tendency to re-produce this displacement (9), that the extending forces, intended to remove it, should be, in general, less powerful than in most other fractures, because their principal object is, to restore to the limb its natural length, which is here but very little affected.