Fig. 149.—The usual pattern of hand. The grip is too small.
Fig. 150.—The index finger is the same length as the middle finger. The thumb and index fingers are furnished with nails. A small ball can be picked up.
Fig. 151.—The middle finger being longer than the index, the latter does not reach the surface of the table and the ball cannot be picked up.
If the fingers are rigid and in semiflexion it is possible to articulate all the metacarpo-phalangeal joints, fitting them with a spring, which keeps them flexed, and arranging for active extension as already described for the thumb. All that is necessary is to terminate the cord by five separate strings instead of one. In certain special cases this arrangement may be useful ([Figs. 155 to 157]). It seems to us useless to render the interphalangeal joints automatic.
As to the attempt which Beaufort appears to have made to give movement to the wrist also, we do not believe that any practical result has as yet been attained.
For the relative length of the fingers and the utility of a nail on the thumb and on the index finger see figures [150 and 151].
Shape of the Hand.
In the usual pattern ([Figs. 149 and 151]) the fingers are semiflexed and the thumb grips against the index finger, which is shorter than the middle finger as in the natural hand. If it is desired to pick up a ball, for example ([Fig. 151]), it will be seen that the middle finger projects and gets in the way. For this reason it is advisable that the index finger be longer than the middle, and in addition it is useful to furnish the thumb and index finger with a little projection representing the nail ([Fig. 150]).
In figure [148] will be seen an arrangement which allows the thumb to grip not by the tip, but by the whole length of its palmar surface (to hold, for example, a notebook). The fingers of this hand have ball and socket joints constructed in the way shown in figures 152 to 154. The joints keep passively the position in which they are placed. The attachment of the ball of the joint on an intermediate tenon is similar to that of the thumb shown on [page 98]. The articulation of the index finger prevents the sufficient excavation of the thenar eminence for the insertion of the Beaufort thumb with its powerful spring. The wrist rotates upon a bayonet joint.