Digit (anatomy)

A digit is one of several most distal parts of a limb, such as fingers or toes, present in many vertebrates.

Names

Some languages have different names for hand and foot digits (English: respectively "finger" and "toe", French: "doigts" and "orteils").

In other languages, e.g. Russian, Polish, Spanish, Portuguese, Italian, Czech, Tagalog, Turkish and Persian, there are no specific one-word names for fingers and toes; these are called "digit of the hand" or "digit of the foot" instead.

Human digits

Humans normally have five digits on each extremity. Each digit is formed by several bones called phalanges, surrounded by soft tissue. Human fingers normally have a nail at the distal phalanx. The phenomenon of polydactyly occurs when extra digits are present; the utility of such a mutation is case-dependent. ^[1]

Brain representation

Each finger has an orderly somatotopic representation on the cerebral cortex in the somatosensory cortex area 3b, ^[2] part of area 1^[3] and a distributed, overlapping representation in the supplementary motor area and primary motor area.^[4]

The somatosensory cortex representation of the hand is a dynamic reflection of the fingers on the external hand: in syndactyly people have a clubhand of webbed, shortened fingers. However, not only are the fingers of their hands fused, but the cortical maps of their individual fingers also form a club hand. The fingers can be surgically divided to make a more useful hand. Surgeons did this at the Institute of Reconstructive Plastic Surgery in New York to a 32-year-old man with the initials O. G.. They touched O. G.’s fingers before and after surgery while using MRI brain scans. Before the surgery, the fingers mapped onto his brain were fused close together; afterward, the maps of his individual fingers did indeed separate and take the layout corresponding to a normal hand.^[5]

Evolution

A reconstruction of a Panderichthys

Two ideas about the homology of arms, hands and digits exist. First that digits are unique to tetrapods^[6][7] and second that antecedents were present in the fins of early sarcopterygian fish.^[8] Until recently it was concluded that "Both genetic and fossil data support the hypothesis that digits are evolutionary novelties".^{[9]p. 640.}

However new research that has created a three-dimensional reconstruction of a Panderichthys, a coastal fish from the Devonian period 385 million years ago, shows that these animals already had many of the homologous bones present in the forelimbs of limbed vertebrates.^[10] For example, they had fin radials, bones similar to rudimentary fingers but positioned in the arm-like base of their fins.^[10] Thus there was in the evolution of tetrapods a shift such that the outermost part of the fins were lost and came to be replaced by early digits. This change is consistent with additional evidence from the study of actinopterygians, sharks and lungfish that the digits of tetrapods arose from pre-existing distal radials present in more primitive fish.^[10][11]

Controversy still exists since Tiktaalik, a vertebrate often considered to be the missing link between fishes and land-living animals, had stubby leg-like limbs that lacked the finger-like radial bones found in the Panderichthys. The researchers of the paper commented that "It is difficult to say whether this character distribution implies that Tiktaalik is autapomorphic, that Panderichthys and tetrapods are convergent, or that Panderichthys is closer to tetrapods than Tiktaalik. At any rate, it demonstrates that the fish–tetrapod transition was accompanied by significant character incongruence in functionally important structures.".^{[10]p. 638.}

Bird and theropod dinosaur digits

Birds and theropod dinosaurs (from which birds evolved) have three digits on their hands. Paradoxically the two digits that are missing are different: the bird hand (embedded in the wing) is thought to derive from the second, third and fourth digits of the ancestral five-digit hand. In contrast, the theropod dinosaurs seem to have the first, second and third digits. Recently a Jurassic theropod intermediate fossil Limusaurus has been found in the Junggar Basin in western China that has a complex mix: it has a first digit stub and full second, third and fourth digits but its wrist bones are like those that are associated with the second, third and fourth digits while its finger bones are those of the first, second and third digits.^[12] This suggests the evolution of digits in birds resulted from a "shift in digit identity characterized early stages of theropod evolution"^[12]

Notes

See also

• Polydactyly in early tetrapods
• Polydactyly

Fins, limbs and wings Fins Aquatic locomotion Cephalopod fin Fish locomotion Fin and flipper locomotion Caudal fin Dorsal fin Fish fin flipper Lobe-finned fish Ray-finned fish Pectoral fins Limbs Limb development Limb morphology digitigrade plantigrade unguligrade uniped biped facultative biped triped quadruped Arthropod Cephalopod Tetrapod dactyly Digit Wings Flying and gliding animals Bat wing Bird wing keel skeleton feathers Insect wing Pterosaur wing Wingspan Evolution Evolution of fish Evolution of tetrapods Evolution of birds Origin of birds Origin of avian flight Evolution of cetaceans Comparative anatomy Convergent evolution Analogous structures Homologous structures Related Animal locomotion Gait Robot locomotion Samara Terrestrial locomotion Tradeoffs for locomotion in air and water Rotating locomotion Undulatory locomotion