In human anatomy, the thumb is the first digit on a hand. The human thumb is fully opposable to the tips of the other fingers in that it may position itself, and be folded inward, toward the rest of the hand and fingers, if so required. It rotates at the carpometacarpal joint and so can complete the sometimes quite delicate task of grasping objects by pressing them against the rest of the hand or finger(s).

Anatomy of the thumb

In anatomy, the thumb is also called the first finger, pollex, digitus primus, or digitus I.

The thumb consists of 3 bones:

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

Its movements are controlled by eight muscles:

• opponens pollicis
• abductor pollicis brevis
• flexor pollicis brevis
• adductor pollicis
• flexor pollicis longus
• abductor pollicis longus
• extensor pollicis brevis
• extensor pollicis longus

The first four of these muscles are in the hand and the first three of these form the thenar eminence. The other muscles come from the forearm. The extensor pollicis longus tendon and extensor pollicis brevis tendon form what is know as the anatomical snuff box (an indentation on the lateral aspect of the thumb at its base) where one can usually palpate the radial artery.

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 million years ago ) via a series of intermediate anthropoid stages, and is therefore a much more complicated link.

The most important factor leading to the habile hand (and its thumb) is the freeing of the hands from their walking requirements - still so crucial for apes today, 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.

Other animals with opposable thumbs or digits

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

• 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 wrist bone that works like a thumb.
• Koala - opposable toe on each foot, plus two opposable digits on each hand
• Opossum - opposable thumb
• Cebids (New World primates of Central and South America) - some have opposable thumbs
• Bornean Orangutan - opposable thumbs so that its forefeet are really like hands. The interdigital grip gives them the ability to pick fruit. They also have an opposable big toe.

See also

• Thumb twiddling
• Rule of thumb