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Occurrence and Ease of Articulation of Sign Language Handshapes: The Taiwanese
This arrangement is slightly more complicated than it first appears. When the extensor tendons cross the metacarpophalangeal joint, they are located on the dorsal side of each finger. But just distal to the metacarpophalangeal joint, they start to cross around the finger to the palmar side. They cross the proximal interphalangeal joint midway between the dorsal and palmar sides of the fingers. Then, at the distal interphalangeal joint, the tendons of the common extensor are squarely on the palm side of the hand. This structure is schematically illustrated in figure 18.
The physiological result of the placement of the tendons of the extensor digitorum communis is that, when the extensor contracts, the metacarpophalangeal joint extends, the distal interphalangeal joint extends slightly, and the proximal interphalangeal joint remains flexed. This shape resembles the sign language handshape configuration known as curved. At this point, the lumbricals and interossei (known collectively as the intrinsic muscles because their origins and insertions are in the hand itself) come into play.
Extension by the intrinsic muscles. The seven interosseous muscles, that is, the three palmar interossei and the four dorsal interossei, are located on both the palmar and dorsal sides of the hand. The palmar and dorsal interossei have their origins between the metacarpal bones and their insertions at the distal interphalangeal joint. The four lumbricals are located on the radial (thumb) side of each of the four fingers. Similar to the interossei, their origins are on the palmar side of the metacarpus, and their insertions are on the dorsal side of the fingers at the distal interphalangeal joint (see figure 19).
The lumbricals and the interossei help accomplish full extension by pulling the medial and distal phalanges into full extension while they simultaneously flex the metacarpophalangeal joint (Wells 1966). The fact that the intrinsic muscles both flex and extend is a result of their positioning. At the metacarpophalangeal joint, the intrinsics are located on the palmar side of the finger. The intrinsics curve around the finger, and at the proximal interphalangeal joints, they are located toward the dorsal side of the finger. At the distal interphalangeal joints, they are clearly on the dorsal side of the finger. This structure is illustrated in figure 20. Contraction of these muscles results in simultaneous flexion (at the metacarpophalangeal joints) and extension (at the proximal interphalangeal and distal interphalangeal joints).
Juncturae tendinum. The final aspect of anatomy and physiology for extension of the fingers is the juncturae tendinum, a group of three ligaments located on the dorsal side of the hand. These ligamentous bands connect the tendons of the extensor digitorum communis. The juncturae tendinum are pictured in figure 21. The juncturae tendinum labeled (a) connects the extensor tendon of the pinky to that of the ring finger. The juncturae tendinum labeled (b) connects the extensor tendon of the ring finger to that of the middle finger. The juncturae tendinum labeled (c) connects the extensor tendon of the middle finger to that of the index finger.
The physiological relevance of the juncturae tendinum anatomy is not completely clear. However, it is expected that, as the fingers extend, the juncturae tendinum will pull on each other, which will cause the fingers to be dependent on one another to varying degrees. Although I leave the exact mechanics to be worked out by specialists in this field, I speculate here on the hypothesized physiological effects of the anatomical facts.
16. The difference between the actual curved configuration and the loosely curved configuration referred to here is that the actual curved configuration requires that the flexion at proximal interphalangeal and distal interphalangeal joints be greater. To see what contraction of the extensor digitorum communis accomplishes, first, hold the left hand in a position of rest (the fingers of the left hand should be loosely flexed). Next, place the right index finger on the dorsal side of the left hand at the metacarpophalangeal joint of the left index finger. Next, firmly move the right index finger toward the second (index) carpometacarpal joint, pulling the skin along. The left index finger will extend slightly at the metacarpophalangeal joint and will remain loosely flexed at the two more distal joints. In this demonstration, the skin on the dorsal side of the hand has been forced to function much as the extensor digitorum communis does.