This narrow grip, between the tips of the thumb and index finger only, is not always convenient. A commercial traveller or a foreman could not easily hold with it the order book, in which he has to write. But if the thumb, held by a powerful spring, is parallel to the palm of the hand and grips against the other fingers, which are stretched out and not semiflexed, the grip will be strong and convenient, especially if a mechanism is introduced between the forearm and the hand, allowing the latter to be rotated at will into any position ([Fig. 148]).
As in the preceding case the thumb may have either a simple grip or an automatic grip opened voluntarily by a cord from the shoulder.
The following is a very interesting method which allows a fork or pen to be held, the automatic thumb being used. The fingers are half flexed, the index being separated from the middle finger, so that the handle of a pen can be inserted between them. The grip of the thumb is not against the tip of the index finger but against the outer side of the last phalanx of the middle finger, against which in consequence the handle of the object held will be pressed (Fig. 147).
Fig. 146—Appliance with automatic thumb. The cord is fixed to a loop which passes round the sound shoulder. Abduction and forward movement of the shoulder and flexion of the elbow open the thumb.
Fig. 147.—Hand with space between the index and middle fingers, wide enough to take the handle of a fork, which is held by pressure of the thumb against the side of the middle finger.
The extended fingers are better placed for gripping than the partially flexed fingers, although the latter are convenient to the patient in certain ways. Ball and socket joints are inserted at the interphalangeal joints. (Details are shown in figures [152 to 154].) These are so stiff that they maintain the position in which they are placed passively, as do the joints of an artist's lay figure.
Fig. 148.—Articulated hand for commercial travellers. The thumb, lying parallel to the palm of the hand, takes a secure hold of such an article as a memorandum book.