The Thumb
The "thumbs up" gesture is a sign of approval in some cultures, but an obscene gesture in others.[1]
Latin	pollex, digitus primus, digitus I
Artery	princeps pollicis artery
Lymph	infraclavicular lymph nodes[2]
MeSH	Thumb

Fingers
Thumb · Index · Middle · Ring · Little

The thumb is the first digit of the hand. When a person is standing in the medical anatomical position (where the palm is stretched forward), the thumb is the lateral-most digit. The Medical Latin English adjective for thumb is pollical.

Contents 1 Anatomy 1.1 Bones 1.2 Opposable thumbs 1.3 Hitchhiker's thumb 2 As one of five digits, and as companion to four fingers 3 Grips 4 Origin of the human thumb 5 Importance 6 Other animals with opposable thumbs 7 References

[edit] Anatomy

[edit] Bones

The thumb consists of three bones:

• Distal phalanx (of the first digit)
• Proximal phalanx (of the first digit)
• First metacarpal

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.

[edit] Opposable thumbs

An animal species is said to have opposable thumbs if the thumb is capable of bending in such a way that it can touch all the other digits on the hand. Most species do not have opposable thumbs. Opposable thumbs are a signature feature of the primate family, and played a large role in the ancient humans inventing and using tools.

[edit] Hitchhiker's thumb

A recessive congenital condition known as "hitchhiker's thumb" exists if the thumb, when extended (as in a "thumbs-up") can extend backwards toward the nail and outwards. People who do not have this condition are able to extend the thumb straight out with little backward bending. Neither condition appears to have any ill-effect on the thumb's function^{[citation needed]}. While most people have either "hitchhiker's thumb" on both thumbs or neither, in some people the condition only presents itself on one thumb.

[edit] As one of five digits, 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. The four digits, not including the thumb.
2. Any of the five digits.

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 greater breadth in the distal phalanx than in the proximal phalanx
• Is attached to such a mobile metacarpus (which produces most of the opposability)

[edit] Grips

One of the earlier significant contributors to the study of hand grips was orthopedic surgeon and anthropologist John Russell Napier who proposed organizing the movements of the hand by their anatomical basis as opposed to work done earlier that had only used arbitrary classification ^[3][4]. Most of this early work on hand grips had a pragmatic basis as it was intended to narrowly define compensable injuries to the hand, which required an understanding of the anatomical basis of hand movement. Napier proposed two primary prehensile grips: the precision grip and the power grip,.^[5] The precision and power grip are defined by the position of the thumb and fingers where:

• The power grip is when the fingers (and sometimes palm) clamp down on an object with the thumb makes counter pressure. examples of the power grip are gripping a hammer, opening a jar using both your palm and fingers, and during pullups.
• The precision grip is when the intermediate and distal phalanges ("fingertips") and the thumb press against each other. examples of a precision grip are writing with a pencil, opening a jar with the fingertips alone, and gripping a ball (as long as its not tight against your palm).

Opposability of the thumb should not be confused with a precision grip as some animals possess semi-opposable thumbs yet are known to have extensive precision grips (Tufted Capuchins for example)^[6]. Nevertheless, precision grips are usually only found in higher apes, and only in degrees significantly more restricted than in humans^[7][8][9].

[edit] Origin of the human thumb

Opposable thumbs are shared by many primates, including most simians, and some prosimians. Darwinius masillae, a transitional form between simians and prosimians, living about 47 million years ago, had opposable thumbs, providing a grip useful for climbing and gathering fruit.^[10]

The evolution of the fully opposable or prehensile thumb is usually associated with Homo habilis, the forerunner of Homo sapiens.^[11][12] 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
• The simultaneous development of a larger anthropoid brain in the later stages

It is possible, though, that a more likely scenario may be that the specialized precision gripping hand (equipped with opposable thumb) of Homo habilis preceded walking, with the specialized adaptation of the spine, pelvis, and lower extremities preceding a more advanced hand. And, it is logical that a conservative, highly functional adaptation be followed by a series of more complex ones that complement it. With Homo habilis, an advanced grasping-capable hand was accompanied by facultative bipedalism, possibly implying, assuming a co-opted evolutionary relationship exists, that the latter resulted from the former as obligate bipedalism was yet to follow.^[13] Walking may have been a by-product of busy hands and not vice versa.

HACNS1 (also known as Human Accelerated Region 2) is a gene enhancer "that may have contributed to the evolution of the uniquely opposable human thumb, and possibly also modifications in the ankle or foot that allow humans to walk on two legs". Evidence to date shows that of the 110,000 gene enhancer sequences identified in the human genome, HACNS1 has undergone the most change during the evolution of humans following the split with the ancestors of chimpanzees.^[14]

[edit] Importance

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 opposable thumb ensured that important human functions such as writing were possible.^[15][16] The thumb, in conjunction with the other fingers, makes human hands and those of other species with similar hands some of the most dexterous in the world.^[17]

[edit] Other animals with opposable thumbs

Many animals also have some kind of opposable thumb or toe.

• Most primates
  • Apes, including the great apes (Bonobo, Chimpanzee, Gorilla, Orangutan, human) and the gibbons—opposable thumbs on both hands and feet.
  • Old World Monkeys, with some exceptions, such as the genera Piliocolobus and Colobus.
  • Some Cebids (New World primates of Central and South America).
  • Tarsiers
  • Many strepsirrhine primates, such as lemurs, lorises, and the Aye-aye.
• Giant Pandas - five clawed fingers plus an extra-long sesamoid bone that, though not really a finger (as the human thumb is), works like an opposable thumb.
• Most phalangerid marsupials (a family of possums) - an opposable toe on each foot, plus two opposable digits on each hand
• Koalas - similar to phalangerids, though they are in a different order.
• Opossums - opposable "thumbs" on the rear feet.
• Many polydactyl cats - when a cat has six toes, both the innermost and outermost ("pinky") toes may become opposable, allowing the cat to perform more complex tasks.
• Troodon, a birdlike dinosaur - partially opposable thumbs.
• Bambiraptor, a small, predatory dinosaur - it could touch the outer two of its three digits together in an opposable grip.
• Phyllomedusa, a genus of frogs native to South America.
• Lophiomys, a genus of african cricetid (rodent), with only one extant species (Lophiomys imhausi), but several extinct species.

Most birds have at least one opposable digit on the foot, in various configurations, but these are seldom called "thumbs".

[edit] References

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