The artificial arm is often abducted from the trunk, which constitutes an inconvenience. This is sometimes due to a fault in the alignment, the arm piece not being at right angles to the shoulder cap. It is, however, more often due to the cylindrical shape given to the arm bucket which forces it away from the trunk. The inner side of the bucket should be flattened so that it may hang vertically close to the thorax. The details of construction are different for the true artificial arm and the worker's arm. 1. ARTIFICIAL ARM The arm and forearm pieces are both made of leather. There is no object in making them to lace, the stump is enclosed in a socket in which it need not fit very tightly, because, as we shall explain, this appliance is unsuitable for heavy work. These two parts are strengthened with steels, which are articulated by hinge joints at the level of the elbow. We have to study-- 1. The position of the steels and the direction of the axis of the joint. 2. The lock to fix the elbow joint in a flexed position. 1. _Position of the Steels._--The stump can transmit to the arm socket the various movements grouped under the name of circumduction, but its hold does not enable it to transmit rotation. It is therefore undesirable--although usual--to attach the steels on the arm and forearm to the inner and outer sides of the limb. If this is done, as rotation is impossible, flexion of the forearm at the elbow can only be carried out in the sagittal plane. But this movement is only exceptionally required; the elbow being flexed to the right angle and fixed in this position by a ratchet the limb forms a hook upon which an object may be hung, provided that the forearm lies transversely in contact with the abdomen and not antero-posteriorly. Flexion should therefore be in a plane which is almost the frontal plane (20° or 30° in front of this), and not in the sagittal plane. As there is no active rotation of the arm therefore the steels must be almost in the sagittal plane (the anterior a little external, the posterior a little internal). In certain carefully constructed appliances the arm bucket is cut transversely above the elbow and between the two parts a bayonet joint is fixed where the arm can be rotated by the sound hand, so that the direction of the elbow movement can be altered. 2. _Ratchet to fix the Elbow Joint in the Flexed Position._--When at rest the forearm should hang vertically. But the hand can only be used when the elbow is flexed to an obtuse angle or a right angle, the latter position being more often used. Therefore when the patient has bent the joint to the required angle with his sound hand, he must be able to fix it in this position. This fixation is effected by means of a ratchet attached to the outer side of the elbow, which can be locked or unlocked at will.