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.
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.
It is clear that this complete liberty of action has its disadvantages; the elbow joint can never be made to assume a fixed position against any passive resistance; moreover, in actual practice the useful range of either of these movements is small. For this reason attempts have been made to devise methods by which they can be limited in the various worker's arms which have been designed since the beginning of the war. In all these arms the mechanism of the elbow joint is derived from that of the ploughman's arm of Gripouilleau. Unfortunately none of these mechanisms in which a pressure screw is used for fixation possess any strength. At first sight, in a new appliance they appear attractive and work well, but it is well known to all mechanics that the thread of a screw which is in constant use quickly wears and then it is impossible to tighten it.