thumb thumb the thumb is the first digit of the hand, next to the index finger. when a person is standing in the medical anatomical position (where the palm is facing to the front), the thumb is the outermost digit. the medical latin english noun for thumb is pollex (compare hallux for big toe), and the corresponding adjective for thumb is pollical. thumbbones of the thumb, visible at far leftdetailsarteryprinceps pollicis arteryveindorsal venous network of handnervedorsal digital nerves of radial nerve, proper palmar digital nerves of median nervelymphinfraclavicular lymph nodesidentifierslatinpollexdigitus i manusdigitus primus manusmeshd013933ta98a01.1.00.053ta2151fma24938anatomical terminology definition thumb and fingers the english word finger has two senses, even in the context of appendages of a single typical human hand: any of the five terminal members of the hand. any of the four terminal members of the hand, other than the thumb linguistically, it appears that the original sense was the first 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 stem 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 dorsal surface that features hair and a nail, and a hairless palmar aspect with fingerprint ridges the thumb contrasts with each of the other four fingers by being the only one that: is opposable to the other four fingers has two phalanges rather than three. however, recently there have been reports that the thumb, like other fingers, has three phalanges, but lacks a metacarpal bone. has greater breadth in the distal phalanx than in the proximal phalanx is attached to such a mobile metacarpus (which produces most of the opposability) curls horizontally instead of vertically and hence the etymology of the word: tum is proto-indo-european for 'swelling' (cf 'tumor' and 'thigh') since the thumb is the stoutest of the fingers. opposition and apposition humans anatomists and other researchers focused on human anatomy have hundreds of definitions of opposition. some anatomists restrict opposition to when the thumb is approximated to the fifth finger (little finger) and refer to other approximations between the thumb and other fingers as apposition. to anatomists, this makes sense as two intrinsic hand muscles are named for this specific movement (the opponens pollicis and opponens digiti minimi respectively). other researchers use another definition, referring to opposition-apposition as the transition between flexion-abduction and extension-adduction; the side of the distal thumb phalanx thus approximated to the palm or the hand's radial side (side of index finger) during apposition and the pulp or "palmar" side of the distal thumb phalanx approximated to either the palm or other fingers during opposition. moving a limb back to its neutral position is called reposition and a rotary movement is referred to as circumduction. primatologists and hand research pioneers john and prudence napier defined opposition as: "a movement by which the pulp surface of the thumb is placed squarely in contact with – or diametrically opposite to – the terminal pads of one or all of the remaining fingers." for this true, pulp-to-pulp opposition to be possible, the thumb must rotate about its long axis (at the carpometacarpal joint). arguably, this definition was chosen to underline what is unique to the human thumb. other primates a bonobo "fishing" for termites, an example of incomplete/"untrue" opposition primates fall into one of six groups: thumbless: spider monkey and colobus nonopposable thumbs: tarsiers (which are found in the islands of southeast asia), marmosets (which are new world monkeys) pseudo-opposable thumbs: all strepsirrhines (lemurs, pottos and lorises) and cebidae (capuchin and squirrel monkeys, which are new world monkeys) opposable thumbs: old world monkeys (circopithecidae) except colobus, and all great apes opposable with comparatively long thumbs: gibbons (or lesser apes) yet to be classified: other new world monkeys (tamarins, aotidae: night or owl monkeys, pitheciidae: titis, sakis and uakaris, atelidae: howler and woolly monkeys) the spider monkey compensates for being virtually thumbless by using the hairless part of its long, prehensile tail for grabbing objects. in apes and old world monkeys, the thumb can be rotated around its axis, but the extensive area of contact between the pulps of the thumb and index finger is a human characteristic. darwinius masillae, an eocene primate transitional fossil between prosimian and simian, had hands and feet with highly flexible digits featuring opposable thumbs and halluces. other placental mammals giant pandas — five clawed fingers plus an extra-long sesamoid bone beside the true first finger that, though not a true finger, works like an opposable thumb. in some muridae the hallux is clawless and fully opposable, including arboreal species such as hapalomys, chiropodomys, vandeleuria, and chiromyscus; and saltatorial, bipedal species such as notomys and possibly some gerbillinae. the east african maned rat (lophiomys imhausi), an arboreal, porcupine-like rodent, has four fingers on its hands and feet and a partially opposable thumb. most rodents have a partly opposable toe on each front paw, letting them grasp. additionally, in many polydactyl cats, both the innermost toe and outermost toe (pinky) may become opposable, allowing the cat to perform more complex tasks. marsupials left: opposable toes of the sulawesi bear cuscus forelimbright: opposable thumb on rear foot of an opossum in most phalangerid marsupials (a family of possums) except species trichosurus and wyulda the first and second toes of the forefoot are opposable to the other three. in the hind foot, the first toe is clawless but opposable and provides firm grip on branches. the second and third toes are partly syndactylous, united by skin at the top joint while the two separate nails serve as hair combs. the fourth and fifth toes are the largest of the hind foot. koalas have five toes on their fore and hind feet with sharp curved claws except for the first toe of the hind foot. the first and second toes of the forefeet are opposable to the other three, which enables the koala to grip smaller branches and search for fresh leaves in the outer canopy. similar to the phalangerids, the second and third toes of the hind foot are fused but have separate claws. opossums are new world marsupials with opposable thumbs in the hind feet giving these animals their characteristic grasping capability (with the exception of the water opossum, the webbed feet of which restrict opposability). the mouse-like microbiotheres were a group of south american marsupials most closely related to australian marsupials. the only extant member, dromiciops gliroides, is not closely related to opossums but has paws similar to these animals, each having opposable toes adapted for gripping. reptiles the front feet of chameleons are organized into a medial bundle of toes 1, 2 and 3, and a lateral bundle of toes 4 and 5, and the hind feet are organized into a medial bundle of toes 1 and 2, and a lateral bundle of toes 3, 4 and 5. dinosaurs dinosaurs belonging to the family of bird-like dinosaur troodontidae had a partially opposable finger. it is possible that this adaptation was used to better manipulate ground objects or moving undergrowth branches when searching for prey. the small predatory dinosaur bambiraptor may have had mutually opposable first and third fingers and a forelimb manoeuvrability that would allow the hand to reach its mouth. its forelimb morphology and range of motion enabled two-handed prehension, one-handed clutching of objects to the chest, and use of the hand as a hook. nqwebasaurus — a coelurosaur with a long, three-fingered hand which included a partially opposable thumb (a "killer claw"). in addition to these, some other dinosaurs may have had partially or completely opposed toes in order to manipulate food and/or grasp prey. birds four types of bird feet(right foot diagrams)