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Occurrence and Ease of Articulation of Sign Language Handshapes: The Taiwanese
The extensor tendons of both the index and pinky fingers are tethered by only one juncturae tendinum. This structure contrasts the situation for both the middle and ring fingers, which are tethered on either side by two juncturae tendinum: the ring finger by (a) and (b) and the middle finger by (b) and (c) (see figure 21). I make two predictions based on this structure: First, fingers that are tethered by one juncturae tendinum are slightly freer than any fingers tethered by two. Thus, the index and pinky fingers are each a bit freer than the middle and ring fingers. Second, in extension, when the juncturae tendinum come into play, pairs of adjacent fingers have some effect on each other.
Because of their musculature, some fingers (the thumb, index, and pinky) can fully extend, and others (the middle and ring) cannot. Clearly, however, the abilities of both the middle and ring fingers change when they act with other fingers. This discussion centers on the conditions under which the middle and ring fingers can fully extend in concert with other fingers. I suggest, first, that when two fingers act together, any finger can extend fully if (a) it has an independent extensor or (b) it is connected by a juncturae tendinum to a finger with an independent extensor. Second, when three or four fingers act together, any finger can fully extend if each extended finger is directly connected by a juncturae tendinum to another extended finger and if one of the group has an independent extensor. The finger combinations that can extend fully and those that cannot are listed in table 1.