3. These numerous connexions secure the solidity of the joint, which is still farther strengthened before by the coronoid apophysis, together with the fleshy and tendinous extremities of the biceps, and brachialis muscles, and by the olecranon behind; at the inferior part, by the anconeus; on the sides by two ligaments which descend from the two tuberosities, and strong muscular fasciæ running from the same parts. The whole articulation is also surrounded by a thick capsule, strengthened by numerous accessory fibres. With such powers of resistance, how can this joint suffer a luxation? Yet next to that of the os humeri, it is perhaps most frequently subject to this accident. § II. OF THE KINDS AND CAUSES OF DISPLACEMENT. 4. Writers have admitted in general four kinds of displacement; backward, forward, outward, and inward. But all these are not alike frequent, as is proved by experience, and demonstrated by the relative situation of the parts. 5. In a backward direction the olecranon and the radius may pass up behind the humerus, as the coronoid apophysis offers but little resistance in consequence of its slight curvature. On the other hand, the kind of hook formed by the olecranon, prevents it and the radius from passing before the lower articulating extremity of the humerus, and therefore, without a fracture of the olecranon, a luxation in that direction is impracticable: at the sides, the two lateral ligaments, but more particularly the reciprocal joining or interlocking of the uneven articulating surfaces, present almost insurmountable obstacles to luxations laterally. Whence it follows, that luxation backward is much more frequent than the others; compared to lateral luxations, it is, at least, in the proportion of ten to one: with luxations forward, no comparison can be made; neither Petit nor Desault having ever met with such. 6. An external force produces the whole of these luxations, but according to each, this force must vary. In a fall sideways, suppose the hand be applied to the ground, with the arm extended, to save the body. It is evident that the resistance of the ground will tend to make the bones of the fore-arm pass upwards over the humerus, while the weight of the body pushing that bone downward and forward, will make it glide over the coronoid apophysis. Thus, the capsule, being distended before by the humerus, and behind by the bones of the fore-arm, will give way in one or the other place, or in both, as Desault observed in a man, who fell on his side, as he was carrying a heavy load: the weight of the body increased by the load, had such an effect, that the bones overlapped each other nearly two inches.