Is a thumb a finger?

The definition for finger is as follows: ‘each of the four slender jointed parts attached to either hand (or five, if the thumb is included)’. This wording implies that, while the thumb isn’t typically regarded as a finger, there is enough evidence of this use to include it in the definition. Although thumbs have certain similarities to fingers, there are some key differences. It’s therefore more accurate to describe a thumb as one of five digits that we have on each hand, rather than as a finger.

‘Digit’ is the technical term which covers fingers, thumbs, and toes in humans, and similar appendages in some animals. The thumb is the short, thick first digit of the human hand. The other four digits are our fingers.  

What are the similarities and distinctions between the thumb and the four fingers? Some are obvious: all five digits on our hands are jointed and have nails on one surface of the uppermost joint, and a unique pattern of fingerprints (or thumbprints) on the other surface of that joint. However, the thumb only consists of one joint and two phalanges(bones), whereas the other four digits have two joints and three phalanges. The thumb is set lower on the hand and apart from the fingers.  The thumbs of humans and other primates (such as chimpanzees) are opposable: we can move our thumb to face and touch the other digits of the same hand, which enables us to carry out a wide range of manual actions that animals with non-opposable digits are unable to perform.

The word ‘thumb’ is from Old English thūma and is related to Dutch duim and German Daumen. All these words can be traced back to anIndo-European root shared by the Latin verb tumere, meaning 'to swell', which gave us the English words tumescent, tumulus, tumour, and tumid, among others.

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