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The thumb is the first digit of the
hand A hand is a prehensile, multi- fingered appendage located at the end of the forearm The forearm is the region of the upper limb between the Elbow-joint, elbow and the wrist. The term forearm is used in anatomy to distinguish it from the arm, ...

hand
, next to the
index finger The index finger (also referred to as forefinger, first finger, pointer finger, trigger finger, digitus secundus, digitus II, and many other terms) is the second finger, digit of a human hand. It is located between the thumb and the middle finge ...

index finger
. When a person is standing in the medical
anatomical position The standard anatomical position, or standard anatomical model, is the scientifically agreed upon reference position for anatomical location terms. Standard anatomical positions are used to standardise the position of appendages An appendage ...

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 Toes are the s (fingers) of the of a . species such as s that walk on their toes are described as being '. s, and other animals that walk on the soles of their feet, are described as being '; ' animals are those that walk on at the tips of th ...

hallux
'' for big toe), 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: # 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: (also rendered as ) was, in the inferred
Proto-Indo-European language Proto-Indo-European (PIE) is the theorized common ancestor of the Indo-European language family The Indo-European languages are a language family A language is a structured system of communication used by humans, including speech ( ...
, a suffixed form of (or ), which has given rise to many
Indo-European The Indo-European languages are a language family A language family is a group of language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be in relation ...
-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 The phalanges (singular: ''phalanx'' ) are digital Digital usually refers to something using digits, particularly binary digits. Technology and computing Hardware *Digital electronics Digital electronics is a field of electronics Elec ...

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 A fingerprint is an impression left by the friction ridge The dermis or corium is a layer of skin between the epidermis (skin), epidermis (with which it makes up the cutis (anatomy), cutis) and subcutaneous tissues, that primarily consists ...

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 The phalanges (singular: ''phalanx'' ) are digital Digital usually refers to something using digits, particularly binary digits. Technology and computing Hardware *Digital electronics Digital electronics is a field of electronics Elec ...

phalanges
rather than three * Has greater breadth in the
distal Standard anatomical terms of location deal unambiguously with the anatomy of animals, including humans. Terms used generally derive from Latin or Greek language, Greek roots and used to describe something in its standard anatomical position. This ...
phalanx than in the
proximal Standard anatomical terms of location deal unambiguously with the anatomy of animals, including humans. Terms used generally derive from Latin or Greek language, Greek roots and used to describe something in its standard anatomical position. This ...
phalanx * Is attached to such a mobile
metacarpus In human anatomy, the metacarpal bones or metacarpus form the intermediate part of the skeletal The skeleton refers to the frames of support of animal bodies. There are several different skeletal types: the exoskeleton An exoskeleton (from G ...
(which produces most of the opposability) *Curls horizontally instead of vertically and hence the etymology of the word: is Proto-Indo-European for 'swelling' (cf 'tumour' 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 The opponens pollicis is a small, triangular muscle in the hand, which functions to oppose the thumb. It is one of the three thenar muscles The thenar eminence is the mound formed at the base of the thumb on the palm of the hand by the muscles o ...
and
opponens digiti minimi The opponens digiti minimi (opponens digiti quinti in older texts) is a muscle Skeletal muscles (commonly referred to as muscles) are organs An organ is a group of tissues with similar functions. Plant life and animal life rely on many or ...
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 Circumduction may refer to: * Circumduction (anatomy), the circular movement of a limb * Circumduction (rhetoric), an ambiguous or roundabout figure of speech {{Disambiguation ...
. Primatologists and hand research pioneers
John John is a common English name and surname: * John (given name) John is a common English name and surname: * John (given name) * John (surname), including a list of people who have the name John John may also refer to: New Testament Works ...
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 The carpometacarpal (CMC) joints are five joint A joint or articulation (or articular surface) is the connection made between bones in the body which link the skeletal system into a functional whole.Saladin, Ken. Anatomy & Physiology. 7th ed. McG ...
). Arguably, this definition was chosen to underline what is unique to the human thumb.


Other primates

*
Primate A primate ( ) (from Latin , from 'prime, first rank') is a eutherian mammal constituting the Taxonomy (biology), taxonomic order (biology), order Primates (). Primates arose 85–55 million years ago first from small Terrestrial animal, ...

Primate
s fall into one of six groups: ** Thumbless:
spider monkey Spider monkeys are New World monkey New World monkeys are the five families of primate A primate ( ) (from Latin , from 'prime, first rank') is a eutherian mammal constituting the Taxonomy (biology), taxonomic order (biology), order P ...
and
colobus Black-and-white colobuses (or colobi) are Old World monkey Old World monkey is the common English name for a family In , family (from la, familia) is a of people related either by (by recognized birth) or (by marriage or other relat ...
** Nonopposable thumbs:
tarsier Tarsiers are haplorrhine primate A primate ( ) (from Latin , from 'prime, first rank') is a eutherian mammal constituting the Taxonomy (biology), taxonomic order (biology), order Primates (). Primates arose 85–55 million years ago ...

tarsier
s (which are found in the islands of Southeast Asia),
marmoset The marmosets (), also known as zaris or sagoin, are 22 New World monkey New World monkeys are the five families of primate A primate ( ) (from Latin , from 'prime, first rank') is a eutheria Eutheria (; from Greek , 'good, right' a ...
s (which are
New World monkeys New World monkeys are the five families of primates that are found in the tropical regions of Mexico, Central America, Central and South America: Callitrichidae, Cebidae, night monkey, Aotidae, Pitheciidae, and Atelidae. The five families are ran ...
) ** Pseudo-opposable thumbs: all
strepsirrhine Strepsirrhini or Strepsirhini (; ) is a Order (biology), suborder of primates that includes the Lemuriformes, lemuriform primates, which consist of the lemurs of Fauna of Madagascar, Madagascar, galagos ("bushbabies") and pottos from Fauna of A ...
s (lemurs, pottos and lorises) and
Cebidae The Cebidae are one of the five family (biology), families of New World monkeys now recognised. Extant members are the capuchin monkey, capuchin and squirrel monkeys. These species are found throughout tropical and subtropical South America, South ...
(capuchin and squirrel monkeys, which are
New World monkeys New World monkeys are the five families of primates that are found in the tropical regions of Mexico, Central America, Central and South America: Callitrichidae, Cebidae, night monkey, Aotidae, Pitheciidae, and Atelidae. The five families are ran ...
) ** Opposable thumbs:
Old World monkey Old World monkey is the common English name for a family In , family (from la, familia) is a of people related either by (by recognized birth) or (by marriage or other relationship). The purpose of families is to maintain the well-bei ...
s (Circopithecidae) except colobus, and all
great apes The Hominidae (), whose members are known as great apes or hominids (), are a taxonomic family In , family (from la, familia) is a of people related either by (by recognized birth) or (by marriage or other relationship). The purpose ...
** Opposable with comparatively long thumbs:
gibbon Gibbons () are ape Apes (Hominoidea ) are a branch of Old World tailless simians native to Africa and Southeast Asia Southeast Asia or Southeastern Asia is the United Nations geoscheme for Asia#South-eastern Asia, southeastern sub ...

gibbon
s (or lesser apes) ** Yet to be classified: other
New World monkeys New World monkeys are the five families of primates that are found in the tropical regions of Mexico, Central America, Central and South America: Callitrichidae, Cebidae, night monkey, Aotidae, Pitheciidae, and Atelidae. The five families are ran ...
(
tamarin The tamarins are squirrel-sized New World monkeys from the family (biology), family Callitrichidae in the genus ''Saguinus''. They are the first offshoot in the Callitrichidae tree, and therefore are sister group of a clade formed by the saddle- ...

tamarin
s, 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
ape Apes (Hominoidea ) are a branch A branch ( or , ) or tree branch (sometimes referred to in botany Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientis ...

ape
s and
Old World monkeys Old World monkey is the common English name for a family In human society A society is a Social group, group of individuals involved in persistent Social relation, social interaction, or a large social group sharing the same spati ...
, 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 ''Darwinius'' is a genus within the Taxonomic rank, infraorder Adapiformes, a group of Basal (phylogenetics), basal Strepsirrhini, strepsirrhine primates from the middle Eocene geologic time scale, epoch. Its only known species, ''Darwinius masill ...
'', an
Eocene The Eocene ( ) Epoch is a geological epoch In chronology 222px, Joseph Scaliger's ''De emendatione temporum'' (1583) began the modern science of chronology Chronology (from Latin Latin (, or , ) is a classical language belonging to the ...
primate
transitional fossil A transitional fossil is any fossil A fossil (from Classical Latin Classical Latin is the form of Latin language Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languag ...
between
prosimian Prosimians are a group of primate A primate ( ) (from Latin , from 'prime, first rank') is a eutherian mammal constituting the Taxonomy (biology), taxonomic order (biology), order Primates (). Primates arose 85–55 million years ago ...
and
simian The simians, anthropoids or higher primates are an infraorder In biological classification In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemi ...

simian
, had hands and feet with highly flexible digits featuring opposable thumbs and halluces.


Other placental mammals

*
Giant panda The giant panda (''Ailuropoda melanoleuca''; ), also known as the panda bear (or simply the panda), is a bear Bears are carnivoran mammals of the family (biology), family Ursidae. They are classified as caniforms, or doglike carnivoran ...

Giant panda
s — five clawed fingers plus an extra-long
sesamoid bone In anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organisms and their parts. Anatomy is a branch of natural science which deals with the structural organization of liv ...
beside the true first finger that, though not a true finger, works like an opposable thumb. * In some ''
Muridae The Muridae, or murids, are the largest family of rodents Rodents (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the a ...
'' the hallux is clawless and fully opposable, including
arboreal Arboreal locomotion is the locomotion Locomotion means the act or ability of an entity or person to transport or move oneself from place to place. Locomotion or Loco-Motion may refer to: Motion * Motion (physics) *Specific types of motion ** A ...
species such as ''
Hapalomys ''Hapalomys'' is a genus of rodent in the family Muridae endemic to Southeast Asia. It contains the following species: * Delacour's marmoset rat (''Hapalomys delacouri'') * marmoset rat (''Hapalomys longicaudatus'') * Suntsov's marmoset rat (''Ha ...
'', ''
Chiropodomys ''Chiropodomys'' (or pencil-tailed tree mice) is a genus of Old World rats and mice native to Southeast Asia and northeast India. They are tree-dwelling, very small mice, mostly found in tropical rainforest. In total six extant species have been ...
'', '' Vandeleuria'', and ''
Chiromyscus Fea's tree rat or Indochinese chiromyscus (''Chiromyscus chiropus'') is a species of rodent Rodents (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Lati ...
''; and
saltatorial This glossary of entomology describes terms used in the formal study of insect species by entomology, entomologists. A–C A synthetic chlorinated hydrocarbon insecticide, toxic to vertebrate ...
, bipedal species such as ''
Notomys A hopping mouse is any of about ten different Australia Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of ...
'' and possibly some
Gerbillinae Gerbillinae is one of the subfamilies of the rodent Rodents (from Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, kn ...
. * 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 Rodents (from Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Roman Republi ...

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 () may become opposable, allowing the cat to perform more complex tasks.


Marsupials

* In most phalangerid
marsupial Marsupials are any members of the mammal Mammals (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ' ...
s (a family of
possums Possum may refer to: Animals * Opossum, or possum, an order (Didelphimorphia) of marsupials native to the Americas ** Common opossum, native to Central and South America ** Virginia opossum, native to North America * Phalangeriformes, or possums, ...

possums
) except species '' Trichosurus'' and ''
Wyulda ''Wyulda'' is a genus of phalanger ''Phalanger'' (from the Greek ''phalangion'', meaning spider's web, from their webbed (fused) toesChambers English Dictionary) is a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxon ...
'' 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. * Similar to phalangerids though in a different order,
koala The koala or, inaccurately, koala bear (''Phascolarctos cinereus''), is an arboreal herbivorous marsupial native to Australia. It is the only Extant taxon, extant representative of the family Phascolarctidae and its closest living relative ...

koala
s 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. *
Opossum Opossums () are members of the marsupial Order (biology), order Didelphimorphia () Endemism, endemic to the Americas. The largest order of marsupials in the Western Hemisphere, it comprises 120+ species in 19 Genus, genera. Opossums originated i ...

Opossum
s 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 water opossum (''Chironectes minimus''), also locally known as the yapok (), is a marsupial Marsupials are any members of the mammalian Class (biology), infraclass Marsupialia. All extant marsupials are endemic to Australasia, Wallacea an ...
, 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 Chameleons or chamaeleons (family In human society, family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of families is to mai ...

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

* The bird-like dinosaur ''
Troodon ''Troodon'' ( ; ''Troödon'' in older sources) is a former wastebasket taxon Wastebasket taxon (also called a wastebin taxon, dustbin taxon or catch-all taxon) is a term used by some taxonomists In biology Biology is the natural sci ...

Troodon
'' 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 ''Bambiraptor'' is a Late Cretaceous The Late Cretaceous (100.5–66 Year#SI prefix multipliers, Ma) is the younger of two epoch (geology), epochs into which the Cretaceous geological period is divided in the geologic time scale. Stratum, Rock str ...

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 ''Nqwebasaurus'' (; anglicized as or ) is a Basal (phylogenetics), basal Coelurosauria, coelurosaur and is the basal-most member of the coelurosaurian clade Ornithomimosauria from the Early Cretaceous of South Africa. The name ''Nqwebasaurus'' is ...

Nqwebasaurus
'' — a
coelurosaur Coelurosauria (; from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is ap ...
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

* Most
bird Birds are a group of warm-blooded vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With ...

bird
s 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
hallux Toes are the s (fingers) of the of a . species such as s that walk on their toes are described as being '. s, and other animals that walk on the soles of their feet, are described as being '; ' animals are those that walk on at the tips of th ...

hallux
es.


Pterosaurs

* The wukongopterid pterosaur '''' 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 ''Phyllomedusa'' is a genus of tree frogs in the family Phyllomedusidae from tropical and subtropical South America (south to northern Argentina) and Panama. It has 16 recognised species. Secretion Some ''Phyllomedusa'' species produce a waxy se ...
'', a genus of frogs native to South America.


Human anatomy


Skeleton

The skeleton of the thumb consists of the
first metacarpal bone The first metacarpal bone or the metacarpal bone In human anatomy, the metacarpal bones or metacarpus, form the intermediate part of the skeletal hand located between the phalanges of the fingers and the carpal bones of the wrist which forms the ...
which articulates
proximal Standard anatomical terms of location deal unambiguously with the anatomy of animals, including humans. Terms used generally derive from Latin or Greek language, Greek roots and used to describe something in its standard anatomical position. This ...
ly with the at the
carpometacarpal joint The carpometacarpal (CMC) joints are five joint A joint or articulation (or articular surface) is the connection made between bones in the body which link the skeletal system into a functional whole.Saladin, Ken. Anatomy & Physiology. 7th ed. McG ...
and
distal Standard anatomical terms of location deal unambiguously with the anatomy of animals, including humans. Terms used generally derive from Latin or Greek language, Greek roots and used to describe something in its standard anatomical position. This ...
ly with the
proximal phalanx The phalanges (singular: ''phalanx'' ) are digit (anatomy), digital bones in the hands and foot, feet of most vertebrates. In primates, the thumbs and big toes have two phalanges while the other digits have three phalanges. The phalanges are cla ...
at the
metacarpophalangeal joint The metacarpophalangeal joints (MCP) are situated between the metacarpal In human anatomy, the metacarpal bones or metacarpus form the intermediate part of the located between the of the fingers and the of the , which forms the connection to t ...
. This latter bone articulates with the
distal phalanx The phalanges (singular: ''phalanx'' ) are digital Digital usually refers to something using discrete digits, often binary digits. Technology and computing Hardware *Digital electronics Digital electronics is a field of electronics Th ...
at the interphalangeal joint. Additionally, there are two
sesamoid bone In anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organisms and their parts. Anatomy is a branch of natural science which deals with the structural organization of liv ...
s at the metacarpophalangeal joint.


Muscles

The muscles of the thumb can be compared to
guy-wire A guy-wire, guy-line, or guy-rope, also known as simply a guy, is a tensioned cable Cable may refer to: Mechanical * Nautical cable A nautical cable is a band of tightly woven and clamped ropes, of a defined cable length, used during the age ...
s 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 The flexor pollicis longus (; FPL, Latin ''flexor'', bender; ''pollicis'', of the thumb; ''longus'', long) is a muscle Muscle is a soft tissue found in most animals. Muscle cells contain protein Proteins are large biomolecules or macromo ...
(FPL) originates on the anterior side of the
radius In classical geometry Geometry (from the grc, γεωμετρία; ' "earth", ' "measurement") is, with , one of the oldest branches of . It is concerned with properties of space that are related with distance, shape, size, and relative ...
distal to the
radial tuberosity Beneath the neck of the radius In classical geometry Geometry (from the grc, γεωμετρία; ' "earth", ' "measurement") is, with , one of the oldest branches of . It is concerned with properties of space that are related with dist ...
and from the
interosseous membrane An interosseous membrane is a thick dense fibrous sheet of connective tissue that spans the space between two bones forming a type of syndesmosis joint. Interosseous membranes in the human body: * Interosseous membrane of forearm The interosseous ...
. It passes through the
carpal tunnel In the human body The human body is the structure of a human being Humans (''Homo sapiens'') are the most abundant and widespread species In biology, a species is the basic unit of biological classification, classification and a ...
in a separate
tendon sheath A tendon sheath is a layer of synovial membrane around a tendon A tendon or sinew is a tough, high-tensile-strength band of dense fibrous connective tissue that connects muscle Skeletal muscles (commonly referred to as muscles) are organs ...
, 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 The median nerve is a nerve A nerve is an enclosed, cable-like bundle of fibers (called axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see American and British English spelling differences#-re, -er, spe ...

median nerve
(C7-C8) 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 In human anatomy, the abductor pollicis longus (APL) is one of the extrinsic muscles of the hand A hand is a prehensile, multi- fingered appendage located at the end of the forearm The forearm is the region of the upper limb between the El ...
(APL) originates on the dorsal sides of both the
ulna The ulna (''pl''. ulnae or ulnas) is a long bone The long bones are those that are longer than they are wide. They are one of five types of bone A bone is a rigid tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of si ...

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 In human anatomy, the metacarpal bones or metacarpus form the intermediate part of the skeleton, skeletal hand located between the phalanges of the fingers and the carpal bones of the wrist, which forms the connection to the forearm. The metacarpal ...
. 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 radial nerve divides into a superficial (sensory) and deep (motor) branch at the cubital fossa. The deep branch of the radial nerve The radial nerve is a nerve A nerve is an enclosed, cable-like bundle of fibers (called axon An axon (fro ...
(C7-C8). The
extensor pollicis longus In human anatomy The human body is the structure of a human being Humans (''Homo sapiens'') are the most abundant and widespread species In biology, a species is the basic unit of biological classification, classification and a ...

extensor pollicis longus
(EPL) 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 A fulcrum is the support about which a lever A lever ( or ) is a simple machine A simple machine is a mechanical device that changes the direction or magnitude of a force In physics Physics (from grc, φυσική (ἐπισ ...

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 radial nerve divides into a superficial (sensory) and deep (motor) branch at the cubital fossa. The deep branch of the radial nerve The radial nerve is a nerve A nerve is an enclosed, cable-like bundle of fibers (called axon An axon (fro ...
(C7-C8). The
extensor pollicis brevis In human anatomy The human body is the structure of a human being Humans (''Homo sapiens'') are the most abundant and widespread species In biology, a species is the basic unit of biological classification, classification and ...
(EPB) 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 radial nerve divides into a superficial (sensory) and deep (motor) branch at the cubital fossa. The deep branch of the radial nerve The radial nerve is a nerve A nerve is an enclosed, cable-like bundle of fibers (called axon An axon (fro ...
(C7-T1). The tendons of the extensor pollicis longus and extensor pollicis brevis form what is known as the
anatomical snuff box Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, ...
(an indentation on the lateral aspect of the thumb at its base) The radial artery can be palpated anteriorly at the wrist (not in the snuffbox).


Intrinsic

There are three
thenar muscles The thenar eminence is the mound formed at the base of the thumb on the palm of the hand by the muscles of the thumb#Intrinsic muscles, intrinsic group of muscles of the thumb. The skin overlying this region is the area stimulated when trying to ...
: The
abductor pollicis brevis The abductor pollicis brevis is a muscle in the hand that functions as an Anatomical terms of motion, abductor of the thumb. Structure The abductor pollicis brevis is a flat, thin muscle located just under the skin. It is a thenar muscle, and ther ...
(APB) originates on the scaphoid tubercle and the flexor retinaculum. It inserts to the radial sesamoid bone and the proximal phalanx of the thumb. It is innervated by the
median nerve The median nerve is a nerve A nerve is an enclosed, cable-like bundle of fibers (called axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see American and British English spelling differences#-re, -er, spe ...

median nerve
(C8-T1). The
flexor pollicis brevis The flexor pollicis brevis is a muscle in the hand that flexes the thumb. It is one of three thenar muscles. It has both a superficial part and a deep part. Origin and insertion The muscle's superficial head arises from the distal edge of the flex ...
(FPB) has two heads. The superficial head arises on the flexor retinaculum, while the deep head originates on three carpal bones: the trapezium,
trapezoid In Euclidean geometry Euclidean geometry is a mathematical system attributed to Alexandrian Greek mathematics , Greek mathematician Euclid, which he described in his textbook on geometry: the ''Euclid's Elements, Elements''. Euclid's meth ...

trapezoid
, and
capitate The capitate bone is found in the center of the carpal bone region, colloquially known as the wrist, which is at the distal end of the radius and ulna bones. It articulates with the third metacarpal bone (the middle finger) and forms the third car ...

capitate
. The muscle is inserted onto the radial sesamoid bone of the metacarpophalangeal joint. It acts to flex, adduct, and abduct the thumb, and is therefore also able to oppose the thumb. The superficial head is innervated by the
median nerve The median nerve is a nerve A nerve is an enclosed, cable-like bundle of fibers (called axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see American and British English spelling differences#-re, -er, spe ...

median nerve
, while the deep head is innervated by the ulnar nerve (C8-T1). The
opponens pollicis The opponens pollicis is a small, triangular muscle in the hand, which functions to oppose the thumb. It is one of the three thenar muscles The thenar eminence is the mound formed at the base of the thumb on the palm of the hand by the muscles o ...
originates on the tubercle of the trapezium and the flexor retinaculum. It is inserted onto the radial side of the first metacarpal. It opposes the thumb and assists in adduction. It is innervated by the
median nerve The median nerve is a nerve A nerve is an enclosed, cable-like bundle of fibers (called axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see American and British English spelling differences#-re, -er, spe ...

median nerve
. Other muscles involved are: The Adductor pollicis muscle, adductor pollicis also has two heads. The transversal head originates along the entire third metacarpal bone, while the oblique head originates on the carpal bones proximal to the third metacarpal. The muscle is inserted onto the ulnar sesamoid bone of the metacarpophalangeal joint. It adducts the thumb, and assists in opposition and flexion. It is innervated by the deep branch of the ulnar nerve (C8-T1). The first Dorsal interossei of the hand, dorsal interosseous, one of the central muscles of the hand, extends from the base of the thumb metacarpal to the radial side of the proximal phalanx of the index finger.


Variations

There is a variation of the human thumb where the angle between the first and second Distal phalanges, phalanges varies between 0° and almost 90° when the thumb is in a Thumbs Up, thumbs-up gesture. It has been suggested that the variation is an autosomal recessive trait, called a "Hitchhiker's thumb", with homozygous carriers having an angle close to 90°. However this theory has been disputed, since the variation in thumb angle is known to fall on a continuum and shows little evidence of the Bimodal distribution, bi-modality seen in other recessive genetic traits. Other formations of the thumb include a triphalangeal thumb and polydactyly.


Grips

One of the earlier significant contributors to the study of hand grips was orthopedic primatologist and paleoanthropologist John Napier (primatologist), John Napier, who proposed organizing the movements of the hand by their anatomical basis as opposed to work done earlier that had only used arbitrary classification. Most of this early work on hand grips had a pragmatic basis as it was intended to narrowly define compensable injuries to the hand, which required an understanding of the anatomical basis of hand movement. Napier proposed two primary prehensile grips: the ''precision grip'' and the ''power grip''. The precision and power grip are defined by the position of the thumb and fingers where: * The power grip is when the fingers (and sometimes palm) clamp down on an object with the thumb making counter pressure. Examples of the power grip are gripping a hammer, opening a jar using ''both your palm and fingers'', and during pullups. * The precision grip is when the intermediate and distal phalanges ("fingertips") and the thumb press against each other. Examples of a precision grip are writing with a pencil, opening a jar ''with the fingertips alone'', and gripping a ball (only if the ball is not tight against the palm). Opposability of the thumb should not be confused with a precision grip as some animals possess semi-opposable thumbs yet are known to have extensive precision grips (Cebus apella, Tufted Capuchins for example). Nevertheless, precision grips are usually only found in higher apes, and only in degrees significantly more restricted than in humans. The pad-to-pad pinch between the thumb and index finger is made possible because of the human ability to passively hyperextend the
distal phalanx The phalanges (singular: ''phalanx'' ) are digital Digital usually refers to something using discrete digits, often binary digits. Technology and computing Hardware *Digital electronics Digital electronics is a field of electronics Th ...
of the index finger. Most non-human primates have to flex their long fingers in order for the small thumb to reach them. In humans, the distal pads are wider than in other primates because the soft tissues of the finger tip are attached to a horseshoe-shaped edge on the underlying bone, and, in the grasping hand, the distal pads can therefore conform to uneven surfaces while pressure is distributed more evenly in the finger tips. The distal pad of the human thumb is divided into a proximal and a distal compartment, the former more deformable than the latter, which allows the thumb pad to mold around an object. In robotics, almost all robotic hands have a long and strong opposable thumb. Like human hands, the thumb of a robotic hand also plays a key role in gripping an object. One inspiring approach of robotic grip planning is to mimic human thumb placement. In a sense, human thumb placement indicates which surface or part of the object is good for grip. Then the robot places its thumb to the same location and plans the other fingers based on the thumb placement. The function of the thumb declines physiologically with aging. This can be demonstrated by assessing the motor sequencing of the thumb.


Human evolution

A primitive autonomization of the first
carpometacarpal joint The carpometacarpal (CMC) joints are five joint A joint or articulation (or articular surface) is the connection made between bones in the body which link the skeletal system into a functional whole.Saladin, Ken. Anatomy & Physiology. 7th ed. McG ...
(CMC) may have occurred in dinosaurs. A real differentiation appeared an estimated 70 mya in early primates, while the shape of the human thumb CMC finally appears about 5 mya. The result of this evolutionary process is a human CMC joint positioned at 80° of pronation, 40 of abduction, and 50° of flexion in relation to an axis passing through the second and third CMC joints. Opposable thumbs are shared by some primates, including most Catarrhini, catarrhines. The climbing and suspensory behaviour in Orthograde posture, orthograde apes, such as Common chimpanzee, chimpanzees, has resulted in elongated hands while the thumb has remained short. As a result, these primates are unable to perform the pad-to-pad grip associated with opposability. However, in pronograde monkeys such as baboons, an adaptation to a terrestrial lifestyle has led to reduced finger length and thus hand proportions similar to those of humans. Consequently, these primates have dexterous hands and are able to grasp objects using a pad-to-pad grip. It can thus be difficult to identify hand adaptations to manipulation-related tasks based solely on thumb proportions. The evolution of the fully opposable thumb is usually associated with ''Homo habilis'', a forerunner of ''Homo sapiens''. This, however, is the suggested result of evolution from ''Homo erectus'' (around 1 mya (unit), mya) via a series of intermediate simian, anthropoid stages, and is therefore a much more complicated link. Modern humans are unique in the musculature of their forearm and hand. Yet, they remain autapomorphic, meaning each muscle is found in one or more non-human primates. The extensor pollicis brevis and flexor pollicis longus allow modern humans to have great manipulative skills and strong flexion in the thumb. However, a more likely scenario may be that the specialized precision gripping hand (equipped with opposable thumb) of ''Homo habilis'' preceded walking, with the specialized adaptation of the spine, pelvis, and lower extremities preceding a more advanced hand. And, it is logical that a conservative, highly functional adaptation be followed by a series of more complex ones that complement it. With ''Homo habilis'', an advanced grasping-capable hand was accompanied by facultative bipedalism, possibly implying, assuming a co-opted evolutionary relationship exists, that the latter resulted from the former as obligate bipedalism was yet to follow. Walking may have been a by-product of busy hands and not vice versa. HACNS1 (also known as Human accelerated regions, Human Accelerated Region 2) is a Enhancer (genetics), gene enhancer "that may have contributed to the evolution of the uniquely opposable human thumb, and possibly also modifications in the ankle or foot that allow humans to walk on two legs". Evidence to date shows that of the 110,000 gene enhancer sequences identified in the human genome, HACNS1 has undergone the most change during the human evolution since the chimpanzee-human last common ancestor.


See also

* Prehensility * Thumb signal * Thumb twiddling * Thumb war * Pollicization


Notes


References

* * * * (National Center for Biotechnology Information, NCBI) * * * * * * * * * * * * * * * * * * * * * *


External links

* * {{Authority control Fingers