4. The assistants still continuing the extension, the surgeon places the bolster between the arm and the trunk, taking care that the thick end be uppermost, if the displacement be in an inward direction, but lowermost, if it be in an outward one, as is most commonly the case (7). The bolster is to be fastened at top by two pins to a cast of the roller. 5. The arm is now pressed towards the trunk, and fixed against the bolster, by means of the second roller. This roller is applied like that which, in fractures of the clavicle, fastens down the arm to the bolster, by the oblique turns _c. c._ (Fig. 3. plate I.), with this difference, that in the present case, the turns ought to be very tight below, and looser above, if the displacement be in an inward direction. But, on the other hand, if it be outwardly, they must be loose below, and tight above. 6. The fore-arm is now to be suspended in a sling, and the whole apparatus afterwards surrounded by a piece of linen, which, by protecting the casts of the roller from friction, prevents them from being disturbed. 21. If we now compare the action of this apparatus with the indications of cure formerly laid down (17), it will be easy to perceive, that, by it, they are extremely well fulfilled. Indeed, the arm, being firmly fixed against the trunk, cannot move, otherwise than by motions common to it and the trunk, and nothing can derange the lower fragment, which is equally immoveable. Nor can the shoulder communicate any motion to the superior fragment. The bolster being differently disposed, according to the direction in which the lower fragment is displaced, will serve to move it in an opposite direction. Should this fragment be forced inwards, the thick head of the bolster will separate it to a distance from the thorax. It will be maintained in this state of separation, by the casts of the roller, which, being very tight below, will act on it as on a lever of the first kind, of which the bolster, forms the fulcrum, while the resistance to be overcome is the action of the latissimus dorsi, the pectoralis major and the teres major. The casts of the roller, by pressing the elbow to the body, will draw the fractured end of the bone in a contrary direction; and, in this respect, the bandage may be considered as an artificial muscle, forming a perfect antagonist to the natural ones.