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, the thumb is the outermost digit. The Medical Latin English noun for thumb is pollex, and the corresponding adjective for thumb is pollical.

Definition

Thumb and fingers

The English word finger has two senses, even in the context of appendages of a single typical human hand:
1) Any of the five terminal members of the hand.
2) 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 was, in the inferred Proto-Indo-European language, a suffixed form of *penkwe, which has given rise to many Indo-European-family words that involve, or stem 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 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
  • Curls horizontally instead of vertically
and hence the etymology of the word: *tum is Proto-Indo-European for 'swelling' 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 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.
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 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. Arguably, this definition was chosen to underline what is unique to the human thumb.

Other primates

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.
  • Most rodents have a partly opposable toe on each front paw, letting them grasp.
  • * In some mice, 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, an arboreal, porcupine-like rodent, has four fingers on its hands and feet and a partially opposable thumb.
Additionally, in many polydactyl cats, both the innermost toe and outermost toe may become opposable, allowing the cat to perform more complex tasks.

Marsupials

  • In most phalangerid marsupials 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.
  • 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.
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

  • Most birds have at least one opposable toe on the foot, in various configurations, though these are seldom called "thumbs". They are more often known simply as halluxes.

    Pterosaurs

  • The wukongopterid pterosaur Kunpengopterus bore an opposable first toe on each wing. The presence of opposable thumbs in this taxon is thought to be an arboreal adaptation.

    Amphibians

  • Phyllomedusa, a genus of frogs native to South America.

    Human anatomy

Skeleton

The skeleton of the thumb consists of the first metacarpal bone which articulates proximally with the carpus at the carpometacarpal joint and distally with the proximal phalanx at the metacarpophalangeal joint. This latter bone articulates with the distal phalanx at the interphalangeal joint. Additionally, there are two sesamoid bones at the metacarpophalangeal joint.

Muscles

The muscles of the thumb can be compared to guy-wires supporting a flagpole; tension from these muscular guy-wires must be provided in all directions to maintain stability in the articulated column formed by the bones of the thumb. Because this stability is actively maintained by muscles rather than by articular constraints, most muscles attached to the thumb tend to be active during most thumb motions.
The muscles acting on the thumb can be divided into two groups: The extrinsic hand muscles, with their muscle bellies located in the forearm, and the intrinsic hand muscles, with their muscle bellies located in the hand proper.

Extrinsic

A ventral forearm muscle, the flexor pollicis longus originates on the anterior side of the radius distal to the radial tuberosity and from the interosseous membrane. It passes through the carpal tunnel in a separate tendon sheath, after which it lies between the heads of the flexor pollicis brevis. It finally attaches onto the base of the distal phalanx of the thumb. It is innervated by the anterior interosseus branch of the median nerve It is a persistence of one of the former contrahentes muscles that pulled the fingers or toes together.
Three dorsal forearm muscles act on the thumb:
The abductor pollicis longus originates on the dorsal sides of both the ulna and the radius, and from the interosseous membrane. Passing through the first tendon compartment, it inserts to the base of the first metacarpal bone. A part of the tendon reaches the trapezium, while another fuses with the tendons of the extensor pollicis brevis and the abductor pollicis brevis. Except for abducting the hand, it flexes the hand towards the palm and abducts it radially. It is innervated by the deep branch of the radial nerve.
The extensor pollicis longus originates on the dorsal side of the ulna and the interosseous membrane. Passing through the third tendon compartment, it is inserted onto the base of the distal phalanx of the thumb. It uses the dorsal tubercle on the lower extremity of the radius as a fulcrum to extend the thumb and also dorsiflexes and abducts the hand at the wrist. It is innervated by the deep branch of the radial nerve.
The extensor pollicis brevis originates on the ulna distal to the abductor pollicis longus, from the interosseus membrane, and from the dorsal side of the radius. Passing through the first tendon compartment together with the abductor pollicis longus, it is attached to the base of the proximal phalanx of the thumb. It extends the thumb and, because of its close relationship to the long abductor, also abducts the thumb. It is innervated by the deep branch of the radial nerve.
The tendons of the extensor pollicis longus and extensor pollicis brevis form what is known as the anatomical snuff box The radial artery can be palpated anteriorly at the wrist.