Beneath the acromion, is discovered a depression, always situated lower down, than that which accompanies the fracture of that apophysis. If one hand be placed on the head of the bone, while the other is employed in moving the lower fragment in different directions, or, while an assistant, engaged in making the necessary extension, communicates to this fragment a rotatory motion, 1st, the head will be perceived to remain motionless; 2dly, the friction of the two divided ends will produce a crepitation more or less perceptible. This twofold sign is always decisive as to the existence of a fracture; but the swelling of the joint may occasionally prevent the practitioner from availing himself of it. The fragments remain sometimes in contact, without experiencing any displacement, in which case, most of the signs not manifesting themselves, the diagnosis is rendered more difficult. But most frequently a displacement occurs, and then it is the inferior fragment that is deranged, and not the superior one which is so short that it can be but little effected by the action of the muscles. 7. The displacement is in general but slightly perceptible in the longitudinal direction of the bone, unless when, in a very oblique fracture, the fragments present points which irritate the muscles, excite them to contraction, and augment their force; or, when a blow of great violence, continuing to act after the bone is broken, causes the fragments to overlap each other. Thus has the body of the bone been forcibly drawn upwards, or driven in the same direction, till having passed through the deltoid muscle, and the external integuments, it has even risen considerably above the level of its head. But in general, as Petit observes, the weight of the limb hanging down the side, opposes to the action of the muscles a sufficient degree of resistance; and it is in the direction of the cross-diameter or thickness of the bone, that the displacement most frequently occurs. It is to be observed, that the lower fragment is driven either inward or outward, rarely in any other direction. In the first case, which is by far the most common, the elbow is somewhat removed from the body, and cannot be brought near to it without pain; in the second, which is more rarely met with, it is moved in an opposite direction. In the one, the contractions of the deltoid muscle and the natural curve of the humerus, in the other, the united action of the pectoralis major, the latissimus dorsi, and the teres-major, appear to have an essential influence on the displacement. In each case, the displacement is facilitated by the mobility of the lower fragment, and of the shoulder, when an apparatus from being improperly constructed, fails to prevent the movements of the whole extremity.