17. It sometimes happens, that, having escaped either from the internal part, or from the inferior part of the capsule, the head passes behind the clavicle, and forms there a secondary luxation upwards, as has been observed by Ambrose Pare, and by Gallien, and of which an example or specimen was preserved in the cabinet of Desault. But, here, the secondary displacement must take place in a slow manner, and when it has taken place, art can seldom remove it, on account of the strong adhesions formed by the bony surfaces. Thus, in the example mentioned, there was a new cavity formed behind the clavicle, and the humerus adhered to the surrounding parts by a kind of new ligaments. 18. It appears from what has been said, that the humerus is subject to four different kinds of displacement. 1st, _Downward_: 2dly, _outward_, in both of which directions the luxation is always primitive; 3dly, _inwards_, where it is sometimes primitive, and sometimes consecutive; 4thly, _upwards_, where it can never be otherwise than consecutive. The second and fourth are very rare occurrences, and bear so small a proportion to the others, that the latter alone ought to command the attention of the practitioner. § III. OF THE CAUSES AND THE MECHANISM. 19. The causes and the mechanism of luxations of the humerus, vary according as the displacement is primitive or consecutive. The action of external bodies, directed against the arm, but more particularly falls, where this part strikes forcibly against a resisting body, give rise in general to primitive luxations, and, according as it is differently situated at the time of the fall, the humerus determines, by its position, the different kinds. 20. If it be separated from the trunk, without being carried either before or behind it, if the elbow be elevated, and the fall be on the side, the weight of the body, being almost entirely supported by this bone, pushes its upper extremity downward, distends the inferior part of the capsule, lacerates it, and produces a luxation downward, in which it may even be favoured by the action of the pectoralis major, the latissimus dorsi, and the teres major, as has been judiciously observed by Fabre. In such a case, these muscles, involuntarily contracting to support the trunk, act like the power or force in a lever of the second kind, the resistance to which is formed by the head of the bone, which they draw downward, while the lower extremity of the humerus, resting on the ground, constitutes the fulcrum. Some authors even regard, as an immediate cause of luxation, the powerful contraction of the deltoid muscle, which depresses the head of the bone, and forces it through the lower side of the capsule, a mode of displacement, the existence of which observation incontestibly establishes. The case of a scrivener, so often cited, is well known, who in lifting a book of records luxated his humerus in a downward direction.