9. The third were of opinion, that, in undergoing a displacement, the head of the humerus could be carried only downward, under the arm-pit, which is the most common direction, or forward, a course which it takes more rarely. Celsus is almost the only writer who has contended for this division. “_Humerus_, says he, _modo in alam excidit, modo in partem priorem_.” 10. Lastly, The fourth believe, with Hippocrates, in none but a displacement downwards, the only one which that physician has met with in his practice. “_At vero humerus, inferiorem in partem excidit; aliam in partem excidere non audivi._” 11. The moderns, in borrowing from the ancients their divisions of luxations, did not, like them, determine a priori and in a vague manner, the precise spot and direction of displacement; but ascertained these points by subsequent observation, with more precision, in proportion as a knowledge of anatomy shed light on them. They also paid particular attention to the essential difference between primitive and consecutive luxations. 12. Petit admitted of four kinds of luxations, 1st, downward, on the edge of the scapula: this is a very rare occurrence: 2dly, outward, under the spine of that bone, a kind very difficult to be primitively produced. 3dly, inwards, under the hollow of the armpit. 4thly, forward, between the corocoid apophysis and the clavicle. With this illustrious practitioner, Heister acknowledged four kinds of displacements; but, here again, was a new variety, both in expression, and in meaning. The one says, downward, under the arm-pit, the other forward, under the pectoralis major; the one, backward, under the scapula, the other outward, under its spine. According to Duverney, luxations are never primitively in any other direction than downward; the others being only the subsequent effect of muscular action. 13. In the midst of these very complicated modes of treating a very simple subject, it is necessary, first, in order to acquire definite ideas, to divide luxations of the humerus into primitive, which are the immediate effect of external violence, and consecutive, which succeed the primitive, through the influence of causes which I am about to consider. Let us suppose four lines to inscribe, in form of a parallelogram, the oval surface of the glenoid cavity, one representing the superior edge, another the inferior, a third the internal, and the fourth the external.