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The thumb is one of the five fingers.
Anatomy of the thumb
The thumb consists of 3 bones:
Its movements are controlled by eight muscles (each with "pollicis" in the name):
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.
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:
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:
The thumb contrasts with each of the (other) four by being the only finger that:
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:
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 proceeding 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 later resulted from the former as obligate bipedalism was yet to follow. 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 thumb, in conjunction with the other fingers make humans and other species with similar hands some of the most dexterous in the world.
Other animals with thumbs
Many animals, primates and others, also have some kind of opposable thumb or toe:
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Thumb". A list of authors is available in Wikipedia.|