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Occurrence and Ease of Articulation of Sign Language Handshapes: The Taiwanese
At a joint, two bones meet. Understanding the nature of the joints in the hand will help us see their relevance for sign language handshapes. One classification organizes joints into those with a space between the two bones and those without a space between the two bones (Wells 1966). The former permit gliding movement; the latter permit very little movement or no movement. From these groupings, three types of joints can be described: (1) synovial (freely moving), (2) cartilaginous (slightly moveable), and (3) fibrous (fixed) (Galley and Forster 1987). The hand has examples of all three.
Fingers and Hand
The fingers and hand have three sets of joints, all labeled in figure 11. At the distal interphalangeal joint, the distal and medial phalanges meet. At the proximal interphalangeal joint, the medial and proximal phalanges meet. At the metacarpophalangeal joint, or knuckle, the proximal phalanx of each finger meets its respective metacarpal at the metacarpal heads. All of these joints are synovial, moving freely in flexion and extension. In addition, the metacarpophalangeal joints of the four fingers permit abduction and adduction (spreading apart and coming together).
Hand and Wrist
The wrist has four sets of joints. These are the carpometacarpal joints, the midcarpal joints, the intercarpal joints, and the radiocarpal joint. Proceeding from the distal to the proximal joints, the carpometacarpal joint is where the metacarpals meet the distal row of carpal bones. The midcarpal joints and the intercarpal joints are within the wrist itself. The midcarpal joints are those that connect the four carpal bones in the proximal row with the four carpal bones in the distal row. The intercarpal joints are between adjacent carpal bones in both rows (Wells and Luttgens 1976). The radiocarpal joint is where the bones of the forearm, the radius and the ulna, meet the proximal row of carpal bones. Most of these joints are cartilaginous, permitting only slight movements. Together, however, they allow wrist movements of flexion, extension, hyperextension, radial flexion (moving the wrist to the right), ulnar flexion (moving the wrist to the left), and circumduction.
For our purposes, the carpometacarpal joints (labeled in figure 11) are the most interesting. We can productively examine the five carpometacarpal joints in three groups: the carpometacarpal joint of the thumb, the carpometacarpal joints that distally attach to the index and the middle fingers, and those that distally attach to the ring and the pinky. The thumb’s carpometacarpal joint, sometimes called a “saddle joint,” is synovial. In fact, the first metacarpal is the most mobile of the five at the carpometacarpal joint and permits not only the movements necessary to produce sign language handshapes but also a number of other movements. The carpometacarpal joints of the four fingers are extremely interesting for our purposes. The construction of these joints varies in ways that have important implications for sign language handshapes. The carpometacarpal joints of the second and third metacarpals are fibrous or fixed, rendering the second and third metacarpals immobile. In contrast, the carpometacarpal joints of the fourth and fifth metacarpals are cartilagenous, making them slightly moveable. The fibrous parts of the hand are pictured in figure 13.
The following two exercises help demonstrate the difference between the attachments of the second and third metacarpals and the fourth and fifth metacarpals at the carpometacarpal joint. First, take a friend’s hand with fingers facing toward you and palm down. Using both of your hands, grasp the second and third metacarpal of your friend’s hand with your thumb on the dorsal side of the hand and your index finger on the palmar side. Attempt to push down (toward the floor) on one metacarpal while pulling upward (toward the ceiling) on the other. You will see that it is not possible to move the second and third metacarpals. Now, grasp the fourth and fifth metacarpals, and try the same thing. You will quickly see that, quite in contrast with the second and third metacarpals, these are permitted plenty of movement.