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Thumb

The thumb is the first digit of the hand, next to the index finger. When a person is standing in the medical anatomical position (where the palm is facing to the front), the thumb is the outermost digit. The Medical Latin English noun for thumb is pollex (compare hallux for big toe), and the corresponding adjective for thumb is pollical.

The English word finger has two senses, even in the context of appendages of a single typical human hand: 1) Any of the five terminal members of the hand. 2) Any of the four terminal members of the hand, other than the thumb.

Linguistically, it appears that the original sense was the first of these two: *penkwe-ros (also rendered as *penqrós) was, in the inferred Proto-Indo-European language, a suffixed form of *penkwe (or *penqe), which has given rise to many Indo-European-family words (tens of them defined in English dictionaries) that involve, or stem from, concepts of fiveness.

The thumb shares the following with each of the other four fingers:

The thumb contrasts with each of the other four fingers by being the only one that:

and hence the etymology of the word: *tum is Proto-Indo-European for 'swelling' (cf 'tumor' and 'thigh') since the thumb is the stoutest of the fingers.

Anatomists and other researchers focused on human anatomy have hundreds of definitions of opposition. Some anatomists restrict opposition to when the thumb is approximated to the fifth finger (little finger) and refer to other approximations between the thumb and other fingers 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 fingers during opposition.

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