The hand as a concept: digital differences and their importance.
Digits of the hand vary from each other with respect to bone and joint architecture, muscles, tendons, and degree of functional independence. The hand may be conceived as having a central rigid framework moved at the wrist by flexors, extensors, abductors, and adductors. But on each side of this central structure lies a mobile (hinged) segment necessary for pinch and to increase the strength of grasp. At the distal end of this triplex framework lie multijointed appendages essential for any type of pinch or grasp more sophisticated than that of a lobster claw. The thumb has a highly mobile carpometacarpal joint but lacks a middle phalanx. Therefore, it has no proximal interphalangeal (PIP) joint and consequently no mechanical need for a lumbrical. Since the thumb has excellent coordination without a lumbrical, the lumbrical's function as a sense organ cannot be simply to coordinate finger movement. Mechanically the lumbrical is a deflexor of the PIP joint. Because lumbricals are richly endowed with muscle spindles, their passive stretch by contraction of the flexor digitorum profundus might both inhibit finger extensors and facilitate wrist extensors. Since each digit is unique, accurate mathematical and computer modelling of the hand must take into consideration this uniqueness.
Ranney
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School of Anatomy, Department of Kinesiology, University of Waterloo, Ontario, Canada.
D001842:Bone and Bones / Q000033:anatomy & histology; D003198:Computer Simulation; D006225:Hand / Q000033:anatomy & histology* / Q000502:physiology; D006801:Humans; D007596:Joints / Q000033:anatomy & histology; D018482:Muscle, Skeletal / Q000033:anatomy & histology; D013710:Tendons / Q000033:anatomy & histology