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Opposition and apposition
A bonobo "fishing" for termites, an example of incomplete/"untrue" opposition.
In humans, opposition and apposition are two movements unique to the thumb, but these words are not synonyms:
Primatologists and hand research pioneers J. Napier and P. Napier defined opposition as: "A movement by which the pulp surface of the thumb is placed squarely in contact with - or diametrically opposite to - the terminal pads of one or all of the remaining digits." For this true, pulp-to-pulp opposition to be possible, the thumb must rotate about its long axis (at the carpometacarpal joint). Arguably, this definition was chosen to underline what is unique to the human thumb.
Anatomists and other researchers focused exclusively on human anatomy, on the other hand, tend to elaborate this definition in various ways and, consequently, there are hundreds of definitions. Some anatomists restrict opposition to when the thumb is approximated to the fifth digit (little finger) and refer to other approximations between the thumb and other digits as apposition. To anatomists, this makes sense as two intrinsic hand muscles are named for this specific movement (the opponens pollicis and opponens digiti minimi respectively).
Other researchers use another definition, referring to opposition-apposition as the transition between flexion-abduction and extension-adduction; the side of the distal thumb phalanx thus approximated to the palm or the hand's radial side (side of index finger) during apposition and the pulp or "palmar" side of the distal thumb phalanx approximated to either the palm or other digits during opposition.
Moving a limb back to its neutral position is called reposition and a rotary movement is referred to as circumduction.
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