Vote My Thumb | Vote My Thumbnail | Vote My Site | VoteMyThumb.com

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

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 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.

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 writing was possible. 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:

• Bornean Orangutan - opposable thumbs on both hands and both feet. The interdigital grip gives them the ability to pick fruit.
• Gorillas-opposable on both hands and both feet.
• Chimpanzees have opposable thumbs on both hands and both feet.
• Lesser Apes have opposable thumbs on both hands and both feet.
• 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.