Figs. 158 and 159.
Fig. 160.
Below the leather forearm piece, which laces up, the lateral steels are continuous with each other in the form of an arch, to which the grip is riveted.
The latter consists of a strong semicircular piece of metal facing downwards, ending in a pair of wide and thick jaws, like those of a locksmith's pliers. When the apparatus is at rest, these are kept in contact by the pressure of two powerful fixed springs, attached to the semicircle on the forearm above and to the jaws below. The external and dorsal spring is attached to the tip of its jaw, the internal and palmar (the side on which the manipulating cord is attached) to the base of it.
The pliers are opened in the following way:—
Inside the semicircle to which the jaws are attached, lies a cylinder with its ends cut obliquely; this rotates about a transverse axis, and when at rest lies with its longer side upward. To the palmar edge of the shorter side is attached a transverse eccentric, to which is hooked a cord actuated as described in [figure 146]. When this is drawn upward the cylinder rotates so that the wider side comes between the jaws of the pliers and opens them; when the cord is relaxed the springs turn the cylinder back again find the jaws close. Figures 158 and 159 show the appliance at rest and with the jaws open.
This appliance is patented and is made in one piece. We demonstrate in [figure 160] that it would be very easy to make the pliers detachable from the forearm, with a screw connection, just as is done in the various other appliances which will be described.
The grip of the automatic thumb always lacks power, for two reasons. There is no room in the thenar eminence to fit a powerful spring and the grip has always a very narrow hold.