|View Our Catalog||
Occurrence and Ease of Articulation of Sign Language Handshapes: The Taiwanese
Muscle-tendon groups have two ends—the origin and the insertion. The origin is where the action of the muscle originates. Typically, at the origin, the tendons come together in one mass of contractile tissue (called a muscle head). However, it is possible for the origin of a muscle to have more than one head; in this case, each head controls different tendons. The insertion is usually distal to the origin of a muscle-tendon group. At the insertion, the tendons, which have split off from a muscle head, are attached to bone (Wells 1966). Knowing the location of the origin and insertion of muscles at every joint is important because their location is what determines which joints a muscle acts on and what the action of the muscle will be on the joint. Other things being equal, a muscle-tendon group manipulates every joint it crosses; the joints in the hand are no exception.
If we consider the possible configurations that hands can perform, it becomes clear that a joint in the hand might flex or extend to myriad points along an axis. So a metacarpophalangeal joint might be flexed or extended to any point in between the points in figure 15. However, linguistic distinctions do not seem to be made between, for example, a handshape flexed at the metacarpophalangeal joint at 45 degrees and a handshape flexed at the metacarpophalangeal joint at 55 degrees. Therefore, I follow other researchers (e.g., Corina and Sagey 1988; Sandler 1989) in isolating four configurations of the hand for examination: extension, flexion (including curving, bending, opposition, and full flexion), adduction, and abduction. Each discussion begins with an explication of the anatomy and then continues with an examination of the physiological implications of that anatomy.
When the fingers are extended, they are not flexed at any joint (see figure 16). The musculature responsible for this action involves the extensor muscles with assistance from the juncturae tendinum, the intrinsic muscles, and the abductors. We will examine the structural and functional properties of the anatomy that are responsible for extension of the fingers.
Extension of the thumb. The thumb is well supplied with the following muscles that help it achieve full extension: the extensor pollicis brevis, the abductor pollicis longus, and the extensor pollicis longus. The origins of all three muscles are in the forearm. Because of the location of its insertion at the interphalangeal joint of the thumb, the extensor pollicis brevis extends the thumb at the metacarpophalangeal joint. The interphalangeal joint is brought into full extension by the combined actions of two other muscles, the abductor pollicis longus and the extensor pollicis longus, whose insertions lie at more distal locations in the thumb.
Extension of the fingers. The index and pinky each have an independent extensor (Brand 1985) whose function is to extend only that finger primarily at the metacarpophalangeal joint. These extensors are the extensor indicis proprius (for the index finger) and extensor digiti minimi (for the pinky). The origins of the extensor indicis proprius and the extensor digiti minimi lie in the forearm. The insertion of each is just distal to the metacarpophalangeal joint.
Extension of all four fingers by the “common extensor,” extensor digitorum communis. The origin of the extensor digitorum communis, pictured in figure 17, lies in the forearm. It has four tendons that have two insertions each. The proximal insertions are at the wrist, and these will be discussed later in this chapter in the section about abduction and adduction of the fingers. The distal insertions, relevant here, are between the medial and distal phalanges of each of the four fingers.
15. In principle, the discussion could involve five configurations or three configurations. Perhaps perceptual considerations account for why four configurations have traditionally been isolated.