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Occurrence and Ease of Articulation of Sign Language Handshapes: The Taiwanese
Next, lay a pencil across the open flat hand, and make a fist holding the pencil. Notice the difference between the way the index and middle fingers grip the pencil and the way the ring and pinky grip the pencil. The index and middle simply wrap around the pencil, steadying it on the radial (thumb) side. The ring and pinky cup around the pencil steadying it on the ulnar (pinky) side. This cupping action occurs because the ring and pinky metacarpals are mobile at the carpometacarpal joint; if they were not, they could not grip the pencil as tightly and the pencil would be loose on the ulnar side of the hand. It should be clear by now that the different behavior of the second and third metacarpals compared with the fourth and fifth metacarpals is attributed to their very different carpometacarpal joint structures.
To summarize, the wrist and hand have synovial, cartilagenous, and fibrous joints. The synovial joints are the distal interphalangeal joints, the proximal interphalangeal joints, the metacarpophalangeal joints, the radiocarpal joint, and all of the joints of the thumb. The cartilaginous joints are the ring and pinky carpometacarpal joints, the midcarpal joints, and the intercarpal joints. The fibrous joints are the index and middle carpometacarpal joints. The next section will consider what implications this anatomy has for sign language handshapes.
The Physiological Result of the Anatomy of the Carpometacarpal Joints
We have seen that the index and middle metacarpals are anatomically immobilized at the carpometacarpal joint. The ring and pinky metacarpals are permitted a small degree of mobility at the carpometacarpal joint. The physiological result is that an asymmetry exists between the radial fingers and the ulnar fingers at the carpometacarpal joint. This asymmetry renders the radial fingers more able to perform precision movements than the ulnar fingers. This fact may seem counterintuitive because, at the carpometacarpal joint, the index and middle are the fingers that are fixed whereas the ring and pinky are slightly moveable. In fact, the explanation for this capacity comes from the fact that, to perform a precision movement, the “proximal bones [must] be stabilized [i.e., held still] while the distal bones perform the movement” (Wells 1966, 54). If the fingers are to perform a precision movement, then the proximal bone involved in that movement needs to be immobilized. Where the action of fingers is concerned, the proximal bone is the metacarpal. Because the index and middle metacarpals are anatomically immobilized at the carpometacarpal joint, the index and middle fingers are capable of performing precision movements. In contrast, because the ring and pinky metacarpals are not anatomically immobilized at the carpometacarpal joint, the ulnar fingers do not meet the condition for performing precision movements (proximal bones stabilized while distal bone performs the movement). Thus, the ring and pinky fingers are not capable of performing precision movements.
For our purposes, a muscle is more accurately characterized as a muscle-tendon group such as the one pictured in figure 14. The muscle part of a muscle-tendon group is made up of contractile tissue (tissue that is able to contract). A muscle’s only ability is to contract and to return to its rest position. The tendon part of a muscle-tendon group can be conceptualized as a string coming off a muscle. Although a muscle has the ability to contract, its tendons, by themselves, have no such ability. Tendons, in other words, either are pulled or do nothing, depending on what is happening at the muscle origin.
14. In general, the radial fingers provide the precision and stability of the hand, and the ulnar fingers provide the power and stability of the hand (Galley and Forster 1987).