4. Subject to the influence of these different predisposing causes, the humerus would be constantly liable to luxations, did not the scapula, moveable like itself, furnish it, by accompanying its motions, with a point of support, differently disposed, according to the different position of its superior extremity; so that, to this two-fold mobility of the articulating surfaces, is to be attributed, in a great measure, the stability of their connexion. § II. OF THE KINDS OF THIS LUXATION. 5. The upper articulation of the humerus, though predisposed, in general, to luxations, is not equally so in every direction. There is a point at which luxation cannot take place. There are others, where, though possible, this accident has never been observed. It is necessary, therefore, before examining the mechanism of this luxation, to mention with precision, the directions in which it may occur. On this point, writers have differed in a very singular manner. Sometimes, to express the same thing, they have used a different language; and, at other times, have, by the same words, expressed things widely different. Always agreeing as to certain modes of dislocation, they have been divided as to others; while, in the midst of those contrarieties, the surgeon being embarrassed, is at a loss on what ground to found his practice. 6. The ancients, knowing but little of the natural relation of the parts surrounding the joint, were ignorant of the accidental ones, which these several parts assume, in the case now under our consideration. Hence, without doubt, arises the confusion, of their opinions on the subject. Many admitted of four kinds of luxations; a great number acknowledged only three; some subscribed to but two; while others believed in the possibility of none but one. 7. The first divided differently the directions in which the bone might be luxated. Some contended for luxations upward, downward, forward, and backward; and such was the opinion of the first Greek physicians, predecessors to the father of medicine, who has transmitted the opinion to us accompanied with a demonstration of its fallacy. Others have divided them into those that take place downward, upward, outward, and forward. This division is adopted by Galen, who yet produces only an example of a forward luxation, and does not give us to understand what he means by a luxation upward and inward. 8. The second distinguished the modes of this luxation, sometimes into downward, forward, and backward; as was the case with Oribazes; at other times, into downward, outward, and inward, according to the opinion of Paul of Egina, who, no doubt, adopted exactly the preceding division, expressing it only in different words; sometimes into downward, forward, and upward; such was the sentiment of Albucasis, who, notwithstanding, considered a luxation upward, as a very difficult, and very rare occurrence.