40. We may throw into two classes the numerous means, under all their variety of modifications, proposed for the reduction of luxations of the humerus. The one consists in forcing, by some mechanical power, the head of the bone into the cavity from which it had escaped, whether extension has been previously employed or not. The other is confined to disengaging it from the situation, into which it has been accidentally driven, leaving to the action of the muscles the care of its replacement. In the first of these, art does every thing; in the second, it only gives the proper direction to the powers of nature. These latter give but one course or direction to the action of powers externally applied: in the former, the head of the bone always moves in the diagonal of two powers opposed to each other at an angle more or less acute. 41. A history of the means destined to act in the first mode, would be too tedious to be introduced in this place, would throw no light on the processes about to be proposed, and are detailed at full length in many authors, to which the reader is referred. It will be sufficient to observe, that they all act somewhat in the following manner. Some body, placed under the arm-pit, serves as a fulcrum, on which the arm is made to move like a lever of the first kind, the resistance to which is constituted by the displaced head of the humerus, while the power is applied either at the lower part of this bone, or at the wrist. The extremity of the humerus being directed upwards and inwards, moves its head in the opposite directions, towards the glenoid cavity, where it replaces it with more or less facility. In this manner acted that machine so celebrated among the ancients and moderns, under the name of “_Ambi Hippocratis_,” whether it was employed in the precise form described by that prince of physicians, or with the additions and corrections, infinitely varied, which it has received from Paul of Egina, Ambrose Pare, Duverney, Freke, &c. By this, a double motion is communicated to the head of the humerus, which is, at the same time, directed, 1st, in the course above mentioned; and, 2dly, in such a manner, as to disengage it from the unnatural situation which it occupies. 42. Extension by the arm produces, in common, the second effect, and this is made in different modes. Sometimes the weight of the body on one side, and pulling by the luxated limb on the other, serve to produce it; and such was the mode of action of the ladder, the door, and the club, described in the treatise on fractures by Hippocrates, and revived in many modern works. At other times, the body was immoveably fixed, and a powerful extension made by the arm. This was the mode in which the machines of Oribasus operated, and also one of the methods formerly consecrated in the public places where the athletæ held their combats.