1. _The Arm Socket._--We have already said that this must be continued into a shoulder cap of considerable extent, which may be perforated in the region of the point of the shoulder in order to render the appliance lighter. Abduction at the shoulder is thus sacrificed. The arm socket is made of leather, open down the front and laced. By being laced it fits the stump more securely. It is strengthened by two steels which may be fixed in the frontal plane because, as we shall see, a passive rotation at the elbow joint is possible. These steels are directly continuous below with a hemispherical steel cap, which is pierced in the axis of the limb by a hole into which is bolted the connecting piece to which the rod which represents the forearm is attached. 2. _Articulation at the Elbow._--The forearm consists of simple metal tube, attached beneath the arm socket by methods which depend upon the following principles. [Illustration: FIGS. 208 and 209.--_Worker's arm and show arm._ The worker's arm consists of a metal rod which swings backwards and forwards at the elbow and also rotates upon the arm socket. To the end of this rod an appliance can be screwed (a ring and hook are here shown). Around the worker's arm a show arm with a hand (Fig. 209) can be fixed. They are shown in place in Figure 210.] The only movements that the stump can transmit to the arm socket are forward and backward movements hinging about the shoulder, and abduction. The first of these movements is the only really useful one for the workman. The downward pressure exerted by active extension of the elbow no longer exists; in order to press upon an object the sound hand must be used, for it is not practicable to make use of the weight of the body thrown forward for this purpose. [Illustration: FIG. 210.--Show arm in position.] In backward and forward movements--considering, for example, the use of the file--the angle at the elbow opens when the arm is thrust forward and closes when it is pulled backwards. These passive movements of the joint must not be impeded in any way, that is to say, the forearm must swing freely below the arm upon a transverse axis and it must also be able to rotate freely around a vertical axis. These movements are secured in the ploughman's hand which was designed sixty years ago by Gripouilleau and in which the joint which we have shown as a method of attaching the mobile ring to the wrist is utilised. The forearm rod attached by a strong transverse pin swings freely in a little stirrup-shaped cap, which itself rotates around a bolt by which it is firmly fixed into the metal or wooden hemisphere which terminates the arm socket.[14] [14] Wood, which was used by Gripouilleau, has been given up.