Thumb

Biological: Behavioural genetics · Evolutionary psychology · Neuroanatomy · Neurochemistry · Neuroendocrinology · Neuroscience · Psychoneuroimmunology · Physiological Psychology · Psychopharmacology (Index, Outline)

The Thumb
File:Thumbs up.JPG
The 'thumbs up' gesture is a sign of approval in many cultures, and an obscene gesture in many others.
Latin	pollex, digitus primus, digitus I
Gray's	subject #
System
MeSH	A01.378.800.667.430.705
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Template:Fingers

The thumb is the lateral-most digit of the hand. The English adjective for thumb is pollical.

Anatomy of the opposable thumb[]

Bones[]

The thumb consists of three bones:

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

Muscles[]

File:Gray417 color.PNG

Cross section - forearm

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.

File:Hitchhiker's thumbs.jpg

Hitchhiker's thumbs

Hitchhiker's thumb[]

The thumb when extended (as in a "thumbs-up") can extend 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. While most people have either "hitchhiker's thumb" on both thumbs or neither, in some people, the condition only presents itself on one thumb.

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:

The four digits, not including the thumb.
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 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 human thumb[]

The evolution of the opposable or prehensile thumb is usually associated with Homo habilis, the forerunner of Homo sapiens.^[1]^[2]^[3] 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.

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.^[4] Walking may have been a byproduct of busy hands and not vice versa.

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 opposable thumb ensured that important human functions such as writing were possible.^[5]^[6] The thumb, in conjunction with the other fingers make humans and other species with similar hands some of the most dexterous in the world.^[7]

Other animals with opposable thumbs[]

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

Chimpanzees, gorillas, orangutans and humans: opposable thumbs on hands and feet.
Lesser Apes have opposable thumbs on both hands and both feet.
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.