The thumb is the first digit of the
hand
A hand is a prehensile, multi-fingered appendage located at the end of the forearm or forelimb of primates such as humans, chimpanzees, monkeys, and lemurs. A few other vertebrates such as the koala (which has two opposable thumbs on each "h ...
, next to the
index finger. When a person is standing in the medical
anatomical position
The standard anatomical position, or standard anatomical model, is the scientifically agreed upon reference position for anatomical location terms. Standard anatomical positions are used to standardise the position of appendages of animals with r ...
(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''.
Definition
Thumb and fingers
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: (also rendered as ) was, in the inferred
Proto-Indo-European language, a suffixed form of (or ), 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
The phalanges (singular: ''phalanx'' ) are digital bones in the hands and feet of most vertebrates. In primates, the thumbs and big toes have two phalanges while the other digits have three phalanges. The phalanges are classed as long bones.
...
, 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
The phalanges (singular: ''phalanx'' ) are digital bones in the hands and feet of most vertebrates. In primates, the thumbs and big toes have two phalanges while the other digits have three phalanges. The phalanges are classed as long bones.
...
rather than three
* Has greater breadth in the
distal
Standard anatomical terms of location are used to unambiguously descri