One of the simplest mechanisms--and one of the oldest, because it was designed by Gripouilleau--is that of the _agricultural labourer's ring_, intended to grip and manipulate the handle of a wheelbarrow or a plough. The ring is mounted on a transverse axis and moves in a horseshoe which in its turn revolves on a shank which is screwed into the forearm. The ring is provided with a screw, which may be tightened upon the handle if desired, but which is, however, rarely used (Figs. 188 to 190). The _tram-driver's bell_, represented in figure 192, is devised on the same principle. It moves on a transverse axis, and in figures 193 and 194 its utility in managing levers in driving a tram or a motor car will be easily seen. The rotation of the horseshoe on the axis of the forearm is not required. _The "cardan" joint_ also allows movement in every direction: it consists of two semicircles of metal, placed at right angles, each working around a transverse axis, these axes being united in the form of a cross. The construction and working will probably be understood without further explanation by a study of figures 195 to 198. The first two represent a system with a ball in the centre, which is well known commercially. The last two represent the simple universal joint generally used in orthopædic surgery. It is somewhat more cumbersome than the previous model. [Illustration: FIGS. 195 and 196.--Universal joint with ball.] [Illustration: FIGS. 197 and 198.--Simple universal joint.] [Illustration: FIG. 199.--Gardener's cylindrical spade holder. A universal joint permits movements in every direction.] The spade holder used at the agricultural centre at Limonest is mounted on a cardan (Fig. 199). The joint can be fixed by a compression screw which is easily and quickly adjusted. Where several tools are necessary each should be complete with its own universal joint. Other methods of terminal passive articulations are-- (1) Ball joints which have already been described in connection with the fingers and which may be applied to the wrist. (2) Bayonet joints which are only applicable to certain artificial hands which are not subjected to any great strain. [Illustration: FIG. 200.] Figure 200 explains this mechanism as it is applied to the artificial hand represented in figure 148. _CHAPTER VIII_ ARTIFICIAL LIMBS FOR AMPUTATION THROUGH THE ARM In this chapter we shall deal only with amputation of the arm below the upper third, _i.e._ with cases in which the stump is long enough to transmit movements to the artificial limb. Amputation through the deltoid muscle must be considered in association with disarticulation of the shoulder. Below the arm socket is attached an artificial limb which represents the elbow joint, forearm, and hand. There are two types to be described:-- 1. The artificial arm proper, which has the external shape of the natural limb.