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Overview

The thumb is one of the five fingers.

Anatomy of the thumb

Bones

The thumb consists of 3 bones:

• distal phalanx (of the first digit)
• proximal phalanx (of the first digit)
• first metacarpal

Muscles

Cross section - forearm

Cross section - hand

Its movements are controlled by eight muscles (each with "pollicis" in the name):

Name	Location	Nerve
extensor pollicis longus	forearm	posterior interosseous nerve
abductor pollicis longus	forearm	posterior interosseous nerve
flexor pollicis longus	forearm	anterior interosseous nerve
extensor pollicis brevis	forearm	posterior interosseous nerve
abductor pollicis brevis	hand	median nerve
flexor pollicis brevis	hand	median nerve
opponens pollicis	hand	median nerve
adductor pollicis	hand	ulnar nerve (deep branch)

The extensor pollicis longus tendon and extensor pollicis brevis tendon form what is known as the anatomical snuff box (an indentation on the lateral aspect of the thumb at its base) The radial artery can be palpated anteriorly at the wrist(not in the snuffbox) In the hand, the abductor pollicis brevis, adductor pollicis, flexor pollicis brevis, and opponens pollicis form the thenar eminence.

Hitchhiker's thumb

The thumb when extended (as in a "thumbs-up") can also appear to bend backwards toward the nail and outwards, a recessive congenital condition known as "hitchhiker's thumb", whereas for other people it will extend straight out with little backward bending. Having either condition appears to have no effect on the thumb's function.

As one of five fingers, and as companion to four fingers

The English word "finger" has two senses, even in the context of appendages of a single typical human hand:

1. Any of the five digits.
2. The four digits, not including the thumb.

Linguistically, it appears that the original sense was the broader 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 flow 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 "back" surface that features hair and a nail, and a hairless palm-of-the-hand side with fingerprint ridges instead

The thumb contrasts with each of the (other) four by being the only finger that:

• Is opposable
• Has two phalanges rather than three
• Has its inmost phalanx so close to the wrist
• Has much greater breadth and stubby proportions
• Is attached to such a mobile metacarpus (which produces most of the opposability)

Grips

Typical interdigital grips include the tips of thumb and second finger (forefinger/index finger) holding a pill or other small item, or thumb and sides of second and third fingers holding a pen or pencil.

Origin of the thumb

The evolution of the opposable or prehensile thumb is usually associated with Homo habilis, the forerunner of Homo sapiens. This, however, is the suggested result of evolution from Homo erectus (around 1 mya) via a series of intermediate anthropoid stages, and is therefore a much more complicated link.

The most important factors leading to the habile hand (and its thumb) are:

• the freeing of the hands from their walking requirements—still so crucial for apes today, as they have hands for feet, which in its turn was one of the consequences of the gradual pithecanthropoid and anthropoid adoption of the erect bipedal walking gait, and
• the simultaneous development of a larger anthropoid brain in the later stages.

Importance of the opposable thumb

The thumb, unlike other fingers, is opposable, in that it is the only digit on the human hand which is able to oppose or turn back against the other four fingers, and thus enables the hand to refine its grip to hold objects which it would be unable to do otherwise. The opposable thumb has helped the human species develop more accurate fine motor skills. It is also thought to have directly led to the development of tools, not just in humans or their evolutionary ancestors, but other primates as well . The thumb, in conjunction with the other fingers make humans and other species with similar hands some of the most dexterous in the world.

In addition, the opposable thumb has given rise to a popular gesture referred to as the "Thumbs-Up", a symbol of approval in western culture.

Other animals with thumbs

Many animals, primates and others, also have some kind of opposable thumb or toe:

• Bornean Orangutan - opposable thumbs on all four hands. The interdigital grip gives them the ability to pick fruit.
• Gorillas-opposable on all four hands.
• Chimpanzees have opposable thumbs on all four hands.
• Lesser Apes have opposable thumbs on all four hands.
• Old World Monkeys, with some exceptions, such as the genera, Piliocolobus and Colobus.
• Cebids (New World primates of Central and South America) - some have opposable thumbs
• Koala - opposable toe on each foot, plus two opposable digits on each hand
• Opossum - opposable thumb on rear feet
• Giant Panda - Panda paws have five clawed fingers plus an extra bone that works like an opposable thumb. This "thumb" is not really a finger (like the human thumb is), but an extra-long sesamoid bone that works like a thumb.
• Troodon - a birdlike dinosaur with partially opposable thumbs.
• Raccoon - a common mammal with thumbs, which are not opposables.

See also

• Adducted thumb syndrome
• Clubbed thumb
• Thumb sucking
• List of Mendelian traits in humans