21. The mechanism of a primitive luxation inwards, differs a little from the preceding. The elbow, being at once separated from the trunk, and carried backward, the person falls: the weight of the body rests on the humerus; the capsule is lacerated in its fore-part; and a displacement in the same direction supervenes. 22. In a luxation outwards, the elbow is carried forward, towards the opposite shoulder; the capsule being stretched outwardly, gives way in that part, provided the humerus be acted on by a sufficient power. But what can this power be? In a fall, the arm being pushed against the trunk, and stopped by it, cannot carry its motion to a sufficient extent to produce a laceration of the capsule. Hence a luxation outwards must be extremely rare. Indeed no instance of it is to be found in books of surgery. Desault, in particular, never witnessed it. Besides, when in a fall, the arm, separated from the body, is carried backward or forward, the weight of the body acts obliquely on it, and it is but partially subjected to the action of the latissimus dorsi, the pectoralis major, and the teres major. So that no kind of luxation ought from these considerations, to be very frequent, except that in a downward direction, where the influence of both causes is direct. Yet luxation inwards is common enough, and in many instances Desault has observed this primitive mode of displacement, though many modern authors doubt the fact, believing, with Hippocrates, that, primitively, all luxations are downwards. 23. It may so happen that in a primitive luxation, the capsule is only greatly stretched, in which case, the articulating surfaces are but partially displaced; but this membrane more frequently suffers a rupture, through which the head of the bone escapes. To this phenomenon writers, in general, have paid too little attention, notwithstanding the opening of dead bodies has oftentimes demonstrated its existence to practitioners, particularly to Desault, who has given two examples of it modelled in wax; one, of a luxation inward, and the other downward, both found in subjects that died in the Hotel-Dieu. Bell relates some analogous facts, and another English surgeon has also had occasion to meet with them. 24. Oftentimes, in compound fractures, one of the fragments passes through the integuments. In the dislocation of the humerus something similar to this occurs. The capsule is sufficiently lacerated to allow the head to escape; but the opening, being then too narrow, forms around the neck of the bone a noose or kind of collar, which prevents it from re-entering the place which it originally occupied. Thus, in the fractures of which I have just spoken, the aperture in the skin does not, at times, admit of the reduction of the fragment, without a previous dilatation.