2. RESISTANCE TO UPWARD PRESSURE.--The artificial limb should be capable of resisting upward pressure, when a thrusting force is exerted by the hand. This is secured in the following three ways, the hand being presumed to hang vertically with the elbow straight:-- (1) By pressure of the end of the stump in the socket (in amputations low down with a palmar flap--for example, in disarticulation at the wrist joint). (2) By pressure of the top of the forearm socket on the enlargement of the forearm below the elbow. (3) By pressure of the inner side of the upper edge of the armlet against the axilla. But, in actual work, thrusting movements are nearly always made with the elbow bent to a right angle or almost so, then the pressure transmitted through the forearm piece is borne almost entirely by the steels of the armlet. 3. RESISTANCE TO ROTATION.--A well-adjusted artificial arm cannot rotate on the limb because-- (1) The forearm is elliptical in section and not circular, this is specially so in the lower third. (2) Flexion of the elbow is only possible if the artificial joint is in the same plane as the axis of the elbow joint--that is, the sagittal plane. (3) The axillary strap of the shoulder attachment prevents rotation. [Illustration: FIG. 129.--The three regions used as points of resistance to upward pressure.] [Illustration: FIG. 130.--The three regions at which rotation of the apparatus may be prevented. ] 2. ELBOW JOINT 1. _The Concavity of the Armlet._--At the elbow joint the pinching of the anterior soft parts on flexion is liable to take place in just the same way as occurs at the back of the knee in amputations through the leg. To avoid this it is necessary-- (1) That the axis of the joint should lie in a prolongation of a line passing through the epicondyle and the epitrochlea. (2) That the armlet and the forearm socket should be cut away in front in crescent-shaped concavities. [Illustration: FIG. 131.--Limb for amputation in the middle third of the forearm.] The depth of these concavities is estimated when the limb is fitted. Both the arm and the forearm may be cut away freely without any resulting inconvenience, provided that the stump is long; but if the stump is short and includes only the upper third of the forearm, it is impossible to cut away the forearm socket sufficiently without depriving the stump of a proper hold in the socket, so that movements are not transmitted to the forearm lever with their proper force. Consequently the socket for the forearm must be cut away very little, and must be carried up to the level of the fold of the elbow when the joint is flexed. The flesh in front of the elbow will not be pinched if, the forearm being fitted very accurately, the muscles of the upper arm are allowed free play, by cutting away the front of the armlet to half its height, but in this case an indirect attachment to the shoulder is essential.[12]