15. The first of these powers, namely, that which depresses the point of the shoulder, appears to have escaped the notice of the ancient physicians of Greece, who attributed the apparent depression of this part, to the elevation of the sternal fragment, and, accordingly, endeavoured by making compression on the latter, to restore it to a level with the other. Hippocrates, more judicious than those who had preceded him, demonstrated that their doctrine, false in its principles, was still more dangerous in its consequences, and that the sternal fragment being immoveable, lost its relative position with respect to the humeral, only because the latter was depressed by the weight of the arm. This doctrine of the father of physic is satisfactorily proved, by a comparison of the sound shoulder with the diseased one, and has since been admitted by all practitioners. Indeed, the mere recollection that one of the uses of the clavicle is to support the shoulder at that level necessary for the performance of its functions, is alone sufficient to convince us, that, in case of its ceasing to fulfil that office, the shoulder must obey the laws of its own gravity, increased by that of the hand and arm. 16. The illustrious Petit, and with him Duverney, in acknowledging this cause of displacement, have added to it as another the action of the deltoid muscle on the external end of the bone; in this action, the end of the clavicle is the moveable point, while the humerus affords the fixed point. But how can we admit this cause, when the humeral fragment, in passing under the sternal, moves in a backward direction? So far is the deltoid muscle from drawing the bone downward, that here the bone rather draws the muscle in part backward, and yet, in such a case, the displacement is as perceptible as in any other. Besides, when the sternal fragment, broken obliquely upwards, supports the humeral and prevents a displacement, why does not the deltoid produce this displacement? It is then in the weight of the arm and shoulder alone, that we must look for the passive power, which depresses them, and which produces a displacement in the direction of the transverse diameter or thickness of the clavicle. 17. A second power, highly active, co-operates with this. I allude to the permanent contraction of the muscles, that extend from the breast to the clavicle and shoulder: from this cause arises the displacement in the longitudinal direction of the bone. The pectoralis major, the pectoralis minor, the subclavius, the serratus major, and the trapezius, unite their efforts in producing this displacement. These muscles are, in certain respects, antagonists to each other, but they all unite in drawing the shoulder forward and inward. None of them appears to act with more effect than the pectoralis major. To this, in particular, is to be attributed the displacement in a forward direction.