Except in the instances stated above, the action of the muscles is not immediate. They act only secondarily on the external fragment, which, being stedfastly attached to the scapula and humerus, is obedient to the motions impressed by the muscles on these two bones; motions which, in a sound state, the clavicle has a power of controlling. 18. To the weight of the lower extremity (15 and 16), and the spontaneous action of the muscles (17) must be added, as another cause of displacement, the motions which are communicated to the arm by external bodies, and which, being imparted ultimately to the clavicle, derange the fragments, by separating them, approximating them, or making them overlap each other, according to the direction in which they act. 19. When a fracture occurs at the extremity next the shoulder, no displacement of the fragments in general takes place. This circumstance is attributed to the action of the trapezius, which draws each fragment upwards with equal force. However this may be, it is doubtless to such cases that we must refer the complete cures, obtained without any retentive apparatus, by Gasparetti, Brown, and other writers. Hence also, without doubt, arise the difficulties experienced by certain practitioners, such as Duverney, with respect to the diagnosis of this disease. These fractures may be mistaken for fractures of the acromion, being situated so immediately in its vicinity. § VI. OF THE REDUCTION. 20. On looking into the causes of that displacement (15 ... 18), so common in fractures of the clavicle, it appears that in almost every case, the external extremity of the humeral fragment is drawn, by a double power, downward, inward, and forward. Hence it follows, 1st. That the resistance opposed to this power, by the means used for the purpose of reduction, and the retentive apparatus subsequently employed, ought to be directed upward, backward, and outward, these directions being the reverse of those in which the powers of displacement act: 2dly. That, in as much as these powers, viz. the weight of the parts and the action of the muscles, are in constant operation, and, besides, as the motions of the arm are continually disturbing the fragments of the bone, the apparatus ought to be equally constant in its action, and should keep up, without any remission, the effect produced, at first, by the means of reduction. This principle is applicable to every case, and ought to be the standard of comparison, for determining the advantages or disadvantages of different bandages, and processes for the reduction of fractures of the clavicle.