24. Luxations of the humeral extremity of the clavicle, take place, according to Petit, in two ways, 1st, under, and 2dly, over the acromion. If we attend to the disposition of the articulating surfaces, the superior of which rests obliquely on the inferior; if we examine, in particular, the relative position of the corocoid apophysis with respect to the clavicle, it will be difficult to conceive how the first kind of luxation can occur, without being accompanied by a fracture. Yet some facts added by Desault to the doctrine of Petit, on this point, seem to demonstrate the possibility of the clavicle sliding under the acromion. As to luxations forward and backward, the mobility of the shoulder, the facility with which it yields to motions impressed on it in these two directions, and the want of a resisting power, make the two bones that compose it, move together, still preserving their relative position. The luxation upward, then, is that which ought chiefly to occupy the attention of the practitioner. Yet even this is less frequent than the luxation of the sternal extremity, on account of the very great strength of the retaining ligaments, which, when this luxation occurs, must be, if not lacerated, at least very much distended. § VI. OF THE MECHANISM AND THE APPEARANCES. 25. A fall on the point of the shoulder is the most frequent cause of this luxation. The two articulating surfaces, representing an inclined plain, slide along each other, in such a manner, that that which belongs to the acromion is pushed inward, while that of the clavicle is directed outward. The capsule being stretched, gives way, and then the displacement is manifested by a preternatural protuberance over the acromion; by a stiffness in the motion of the shoulder; by the direction of this part, which is evidently drawn inward and downward; by the inclination of the head of the patient to the side affected; by a bending of the body; and by severe pains in the luxated part. These characters are essentially distinct, and ought to have prevented the error of Galen, who mistook a case of this kind for a luxation of the os humeri downward. Hippocrates and Ambrose Pare have foreseen the possibility of this mistake and even warned young practitioners to be on their guard against it. But, as citizen Sabattier judiciously observes, the position of the head of the humerus, under the arm, in a luxation of that bone, will remove all uncertainty respecting the nature of the injury. § VII. OF THE REDUCTION AND THE MEANS OF RETAINING IT.