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Concept
Thumb
Summary
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:
Any of the five terminal members of the hand.
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:
Having a skeleton of phalanges, joined by hinge-like joints that provide flexion toward the palm of the hand
Having a dorsal surface that features hair and a nail, and a hairless palmar aspect with fingerprint ridges
The thumb contrasts with each of the other four fingers by being the only one that:
Is opposable to the other four fingers
Has two phalanges rather than three. However, recently there have been reports that the thumb, like other fingers, has three phalanges, but lacks a metacarpal bone.
Has greater breadth in the distal phalanx than in the proximal phalanx
Is attached to such a mobile metacarpus (which produces most of the opposability)
Curls horizontally instead of vertically
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.
Official source
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