The thumb is the first digit of the
hand
A hand is a prehensile, multi-fingered appendage located at the end of the forearm or forelimb of primates such as humans, chimpanzees, monkeys, and lemurs. A few other vertebrates such as the koala (which has two opposable thumbs on each "h ...
, next to the
index finger
The index finger (also referred to as forefinger, first finger, second finger, pointer finger, trigger finger, digitus secundus, digitus II, and many other terms) is the second digit of a human hand. It is located between the thumb and the mid ...
. 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 of animals with r ...
(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 digits (fingers) of the foot of a tetrapod. Animal species such as cats that walk on their toes are described as being '' digitigrade''. Humans, and other animals that walk on the soles of their feet, are described as being '' pl ...
'' 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 reconstructed common ancestor of the Indo-European language family. Its proposed features have been derived by linguistic reconstruction from documented Indo-European languages. No direct record of Proto-Indo-E ...
, a suffixed form of (or ), which has given rise to many
Indo-European
The Indo-European languages are a language family native to the overwhelming majority of Europe, the Iranian plateau, and the northern Indian subcontinent. Some European languages of this family, English, French, Portuguese, Russian, Dutc ...
-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 bones in the hands and feet of most vertebrates. In primates, the thumbs and big toes have two phalanges while the other digits have three phalanges. The phalanges are classed as long bones.
...
, 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 ridges of a human finger. The recovery of partial fingerprints from a crime scene is an important method of forensic science. Moisture and grease on a finger result in fingerprints on surfac ...
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 bones in the hands and feet of most vertebrates. In primates, the thumbs and big toes have two phalanges while the other digits have three phalanges. The phalanges are classed as long bones.
...
rather than three
* Has greater breadth in the
distal
Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...
phalanx than in the
proximal
Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...
phalanx
* Is attached to such a mobile
metacarpus
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 connection to the forearm. The metacarpal bones ...
(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. It lies deep to the abductor pollicis brevis and lateral to the flexor pollicis brevis.
Structure
The opp ...
and
opponens digiti minimi
The opponens digiti minimi (opponens digiti quinti in older texts) is a muscle in the hand. It is of a triangular form, and placed immediately beneath the palmaris brevis, abductor digiti minimi and flexor digiti minimi brevis. It is one of t ...
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
John is a common English name and surname:
* John (given name)
* John (surname)
John may also refer to:
New Testament
Works
* Gospel of John, a title often shortened to John
* First Epistle of John, often shortened to 1 John
* Secon ...
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 joints in the wrist that articulate the distal row of carpal bones and the proximal bases of the five metacarpal bones.
The CMC joint of the thumb or the first CMC joint, also known as the trapeziometaca ...
).
Arguably, this definition was chosen to underline what is unique to the human thumb.
Other primates
![BonoboFishing02 cropped](https://upload.wikimedia.org/wikipedia/commons/0/00/BonoboFishing02_cropped.jpeg)
*
Primate
Primates are a diverse order of mammals. They are divided into the strepsirrhines, which include the lemurs, galagos, and lorisids, and the haplorhines, which include the tarsiers and the simians (monkeys and apes, the latter including huma ...
s fall into one of six groups:
** Thumbless:
spider monkey
Spider monkeys are New World monkeys belonging to the genus ''Ateles'', part of the subfamily Atelinae, family Atelidae. Like other atelines, they are found in tropical forests of Central and South America, from southern Mexico to Brazil. The ...
and
colobus
Black-and-white colobuses (or colobi) are Old World monkeys of the genus ''Colobus'', native to Africa. They are closely related to the red colobus monkeys of genus '' Piliocolobus''. There are five species of this monkey, and at least eight subs ...
** Nonopposable thumbs:
tarsiers (which are found in the islands of Southeast Asia),
marmoset
The marmosets (), also known as zaris or sagoin, are 22 New World monkey species of the genera '' Callithrix'', '' Cebuella'', '' Callibella'', and ''Mico''. All four genera are part of the biological family Callitrichidae. The term "marmoset" ...
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 and South America: Callitrichidae, Cebidae, Aotidae, Pitheciidae, and Atelidae. The five families are ranked together as the Ceboidea ...
)
** Pseudo-opposable thumbs: all
strepsirrhine
Strepsirrhini or Strepsirhini (; ) is a suborder of primates that includes the lemuriform primates, which consist of the lemurs of Madagascar, galagos ("bushbabies") and pottos from Africa, and the lorises from India and southeast Asia. Colle ...
s (lemurs, pottos and lorises) and
Cebidae (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 and South America: Callitrichidae, Cebidae, Aotidae, Pitheciidae, and Atelidae. The five families are ranked together as the Ceboidea ...
)
** Opposable thumbs:
Old World monkey
Old World monkey is the common English name for a family of primates known taxonomically as the Cercopithecidae (). Twenty-four genera and 138 species are recognized, making it the largest primate family. Old World monkey genera include baboons ...
s (Circopithecidae) except colobus, and all
great apes
The Hominidae (), whose members are known as the great apes or hominids (), are a taxonomic family of primates that includes eight extant species in four genera: '' Pongo'' (the Bornean, Sumatran and Tapanuli orangutan); ''Gorilla'' (the ...
** Opposable with comparatively long thumbs:
gibbon
Gibbons () are apes in the family Hylobatidae (). The family historically contained one genus, but now is split into four extant genera and 20 species. Gibbons live in subtropical and tropical rainforest from eastern Bangladesh to Northeast India ...
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 and South America: Callitrichidae, Cebidae, Aotidae, Pitheciidae, and Atelidae. The five families are ranked together as the Ceboidea ...
(
tamarin
The tamarins are squirrel-sized New World monkeys from the family Callitrichidae in the genus ''Saguinus''. They are the first offshoot in the Callitrichidae tree, and therefore are the sister group of a clade formed by the lion tamarins, Goel ...
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 (collectively Hominoidea ) are a clade of Old World simians native to sub-Saharan Africa and Southeast Asia (though they were more widespread in Africa, most of Asia, and as well as Europe in prehistory), which together with its sister g ...
s and
Old World monkeys
Old World monkey is the common English name for a family of primates known taxonomically as the Cercopithecidae (). Twenty-four genera and 138 species are recognized, making it the largest primate family. Old World monkey genera include baboon ...
, 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
The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene' ...
primate
transitional fossil
A transitional fossil is any fossilized remains of a life form that exhibits traits common to both an ancestral group and its derived descendant group. This is especially important where the descendant group is sharply differentiated by gross a ...
between
prosimian
Prosimians are a group of primates that includes all living and extinct Strepsirrhini, strepsirrhines (lemurs, Lorisoidea, lorisoids, and Adapiformes, adapiforms), as well as the Haplorhini, haplorhine tarsiers and their extinct relatives, the O ...
and
simian
The simians, anthropoids, or higher primates are an infraorder (Simiiformes ) of primates containing all animals traditionally called monkeys and apes. More precisely, they consist of the parvorders New World monkeys (Platyrrhini) and Catarrhi ...
, 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 species endemic to China. It is characterised by its bold black-and-white coat and rotund body. The name "giant panda" is sometimes us ...
s — five clawed fingers plus an extra-long
sesamoid bone
In anatomy, a sesamoid bone () is a bone embedded within a tendon or a muscle. Its name is derived from the Arabic word for ' sesame seed', indicating the small size of most sesamoids. Often, these bones form in response to strain, or can be pres ...
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 and of mammals, containing approximately 1,383 species, including many species of mice, rats, and gerbils found naturally throughout Eurasia, Africa, and Australia.
The name Muridae come ...
'' the hallux is clawless and fully opposable, including
arboreal
Arboreal locomotion is the Animal locomotion, locomotion of animals in trees. In habitats in which trees are present, animals have evolved to move in them. Some animals may scale trees only occasionally, but others are exclusively arboreal. Th ...
species such as ''
Hapalomys
''Hapalomys'' is a genus of rodent in the family Muridae endemic to Southeast Asia. It is the only member of the tribe
The term tribe is used in many different contexts to refer to a category of human social group. The predominant worldw ...
'', ''
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 bee ...
'', ''
Vandeleuria
''Vandeleuria'' is a small genus of rodent from Asia with only three species. It is the only member of the tribe
The term tribe is used in many different contexts to refer to a category of human social group. The predominant worldwide us ...
'', and ''
Chiromyscus
Fea's tree rat or Indochinese chiromyscus (''Chiromyscus chiropus'') is a species of rodent in the family Muridae. It is found in Yunnan (China), eastern Myanmar, northern Thailand, Laos, and Vietnam
Vietnam or Viet Nam ( vi, Việt Nam, ...
''; and
saltatorial
This glossary of entomology describes terms used in the formal study of insect species by entomologists.
A–C
A synthetic chlorinated hydrocarbon insecticide, toxic to vertebrates. Though i ...
, bipedal species such as ''
Notomys
A hopping mouse is any of about ten different Australian native mice in the genus ''Notomys''. They are rodents, not marsupials, and their ancestors are thought to have arrived from Asia about 5 million years ago.
All are brown or fawn, fading ...
'' and possibly some
Gerbillinae
Gerbillinae is one of the subfamilies of the rodent family Muridae and includes the gerbils, jirds, and sand rats. Once known as desert rats, the subfamily includes about 110 species of African, Indian, and Asian rodents, including sand rats a ...
.
* The East African
maned rat
The maned rat or (African) crested rat (''Lophiomys imhausi'') is a nocturnal, long-haired and bushy-tailed East African rodent that superficially resembles a porcupine. The world's only poisonous rodent, the maned rat borrows toxins from plants ...
(''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 , 'to gnaw') are mammals of the order Rodentia (), which are characterized by a single pair of continuously growing incisors in each of the upper and lower jaws. About 40% of all mammal species are rodents. They are nat ...
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
* In most
phalangerid marsupial
Marsupials are any members of the mammalian infraclass Marsupialia. All extant marsupials are endemic to Australasia, Wallacea and the Americas. A distinctive characteristic common to most of these species is that the young are carried in a po ...
s (a family of
possums
Possum may refer to:
Animals
* Phalangeriformes, or possums, any of a number of arboreal marsupial species native to Australia, New Guinea, and Sulawesi
** Common brushtail possum (''Trichosurus vulpecula''), a common possum in Australian urban a ...
) except species ''
Trichosurus
The brushtail possums are the members of the genus ''Trichosurus'' in the Phalangeridae, a family of marsupials. They are native to Australia (including Tasmania) and some small nearby islands. Unique among marsupials, they have shifted the hyp ...
'' and ''
Wyulda
''Wyulda'' is a genus of phalanger. The scaly-tailed possum is the only extant species, but a single extinct species ('' Wyulda asherjoeli'') from the Miocene
The Miocene ( ) is the first geological epoch of the Neogene Period and extends f ...
'' 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 though in a different family,
koala
The koala or, inaccurately, koala bear (''Phascolarctos cinereus''), is an arboreal herbivorous marsupial native to Australia. It is the only extant representative of the family Phascolarctidae and its closest living relatives are the womb ...
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 Didelphimorphia () endemic to the Americas. The largest order of marsupials in the Western Hemisphere, it comprises 93 species in 18 genera. Opossums originated in South America and entered North ...
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 of the family Didelphidae.* It is the only living member of its genus, ''Chironectes''. This semiaquatic creature is found in and near freshwater ...
, 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
The monito del monte or colocolo opossum, ''Dromiciops gliroides'', also called ''chumaihuén'' in Mapudungun, is a diminutive marsupial native only to southwestern South America (Argentina and Chile). It is the only extant species in the ancien ...
'', 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 Chamaeleonidae) are a distinctive and highly specialized clade of Old World lizards with 202 species described as of June 2015. The members of this family are best known for their distinct range of colors, bein ...
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 belong to the family of bird-like dinosaur
Troodontidae
Troodontidae is a clade of bird-like theropod dinosaurs. During most of the 20th century, troodontid fossils were few and incomplete and they have therefore been allied, at various times, with many dinosaurian lineages. More recent fossil disco ...
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
''Nqwebasaurus'' (; anglicized as or ) is a basal coelurosaur and is the basal-most member of the coelurosaurian clade Ornithomimosauria from the Early Cretaceous of South Africa. The name ''Nqwebasaurus'' is derived from the Xhosa word "Nqweb ...
'' — a
coelurosaur
Coelurosauria (; from Greek, meaning "hollow tailed lizards") is the clade containing all theropod dinosaurs more closely related to birds than to carnosaurs.
Coelurosauria is a subgroup of theropod dinosaurs that includes compsognathids, tyran ...
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 vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweigh ...
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 digits (fingers) of the foot of a tetrapod. Animal species such as cats that walk on their toes are described as being '' digitigrade''. Humans, and other animals that walk on the soles of their feet, are described as being '' pl ...
es.
Pterosaurs
* The
wukongopterid
Wukongopteridae is a group of basal pterosaurs, found in China and the UK. It contains eight species in five genera, all dated to the Middle to Late Jurassic period,
The Wukongopteridae were first named by Wang ''et al.'' in 2009, not yet givi ...
pterosaur ''
Kunpengopterus
''Kunpengopterus'' is a genus of wukongopterid pterosaur from the middle-late Jurassic Tiaojishan Formation of northeastern China. The genus contains two species, the type species ''K. sinensis'' and ''K. antipollicatus''.
History of discovery
...
'' 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 subfamily Phyllomedusinae found in tropical and subtropical South America (south to northern Argentina) and Panama. It has 16 recognised species.
Taxonomy
The following species are recognised i ...
'', 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 of the thumb is the first bone proximal to the thumb. It is connected to the trapezium of the carpus at the first carpometacarpal joint and to the proximal thumb phalanx at the first metacarpophal ...
which articulates
proximal
Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...
ly with the
carpus
In human anatomy, the wrist is variously defined as (1) the carpus or carpal bones, the complex of eight bones forming the proximal skeletal segment of the hand; "The wrist contains eight bones, roughly aligned in two rows, known as the carpal ...
at the
carpometacarpal joint
The carpometacarpal (CMC) joints are five joints in the wrist that articulate the distal row of carpal bones and the proximal bases of the five metacarpal bones.
The CMC joint of the thumb or the first CMC joint, also known as the trapeziometaca ...
and
distal
Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...
ly with the
proximal phalanx
The phalanges (singular: ''phalanx'' ) are digital bones in the hands and feet of most vertebrates. In primates, the thumbs and big toes have two phalanges while the other digits have three phalanges. The phalanges are classed as long bones. ...
at the
metacarpophalangeal joint
The metacarpophalangeal joints (MCP) are situated between the metacarpal bones and the proximal phalanges of the fingers. These joints are of the condyloid kind, formed by the reception of the rounded heads of the metacarpal bones into shallow cav ...
. This latter bone articulates with the
distal phalanx
The phalanges (singular: ''phalanx'' ) are digital bones in the hands and feet of most vertebrates. In primates, the thumbs and big toes have two phalanges while the other digits have three phalanges. The phalanges are classed as long bones. ...
at the
interphalangeal joint. Additionally, there are two
sesamoid bone
In anatomy, a sesamoid bone () is a bone embedded within a tendon or a muscle. Its name is derived from the Arabic word for ' sesame seed', indicating the small size of most sesamoids. Often, these bones form in response to strain, or can be pres ...
s at the metacarpophalangeal joint.
Muscles
The muscles of the thumb can be compared to
guy-wire
A guy-wire, guy-line, guy-rope, or stay, also called simply a guy, is a tensioned cable designed to add stability to a free-standing structure. They are used commonly for ship masts, radio masts, wind turbines, utility poles, and tents. A thi ...
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 in the forearm and hand that flexes the thumb. It lies in the same plane as the flexor digitorum profundus. This muscle is un ...
(FPL) originates on the anterior side of the
radius
In classical geometry, a radius ( : radii) of a circle or sphere is any of the line segments from its center to its perimeter, and in more modern usage, it is also their length. The name comes from the latin ''radius'', meaning ray but also the ...
distal to the
radial tuberosity
Beneath the neck of the radius, on the medial side, is an eminence, the radial tuberosity; its surface is divided into:
* a ''posterior, rough portion'', for the insertion of the tendon of the biceps brachii.
* an ''anterior, smooth portion'', on w ...
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
* Interosseou ...
. It passes through the
carpal tunnel
In the human body, the carpal tunnel or carpal canal is the passageway on the palmar side of the wrist that connects the forearm to the hand.
The tunnel is bounded by the bones of the wrist and flexor retinaculum from connective tissue. Norma ...
in a separate
tendon sheath
A tendon sheath is a layer of synovial membrane around a tendon. It permits the tendon to stretch and not adhere to the surrounding fascia.
It has two layers:
* synovial sheath
A synovial sheath is one of the two membranes of a tendon sheath wh ...
, 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 in humans and other animals in the upper limb. It is one of the five main nerves originating from the brachial plexus.
The median nerve originates from the lateral and medial cords of the brachial plexus, and has contr ...
(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. Its major function is to abduct the thumb at the wrist. Its tendon forms the anterior border of the anatomical snuffbox.
Structure
The abductor p ...
(APL) originates on the dorsal sides of both the
ulna
The ulna (''pl''. ulnae or ulnas) is a long bone found in the forearm that stretches from the elbow to the smallest finger, and when in anatomical position, is found on the medial side of the forearm. That is, the ulna is on the same side of t ...
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 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 bones ar ...
. 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 winds to the back of the forearm around the lateral side of the radius between the two planes of fibers of the ...
(C7-C8).
The
extensor pollicis longus
In human anatomy, the extensor pollicis longus muscle (EPL) is a skeletal muscle located dorsally on the forearm. It is much larger than the extensor pollicis brevis, the origin of which it partly covers and acts to stretch the thumb together w ...
(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 pivots.
Fulcrum may also refer to:
Companies and organizations
* Fulcrum (Anglican think tank), a Church of England think tank
* Fulcrum Press, a British publisher of poetry
* Fulcrum Wheels, a bicy ...
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 winds to the back of the forearm around the lateral side of the radius between the two planes of fibers of the ...
(C7-C8).
The
extensor pollicis brevis
In human anatomy, the extensor pollicis brevis is a skeletal muscle on the dorsal side of the forearm. It lies on the medial side of, and is closely connected with, the abductor pollicis longus. The extensor pollicis brevis (EPB) belongs to the ...
(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 winds to the back of the forearm around the lateral side of the radius between the two planes of fibers of the ...
(C7-T1).
The tendons of the extensor pollicis longus and extensor pollicis brevis form what is known as the
anatomical snuff box
The anatomical snuff box or snuffbox or foveola radialis is a triangular deepening on the radial, dorsal aspect of the hand—at the level of the carpal bones, specifically, the scaphoid and trapezium bones forming the floor. The name originates ...
(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 intrinsic group of muscles of the thumb. The skin overlying this region is the area stimulated when trying to elicit a palmomental reflex. The word ...
:
The
abductor pollicis brevis
The abductor pollicis brevis is a muscle in the hand that functions as an 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 therefore contributes to the ...
(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 in humans and other animals in the upper limb. It is one of the five main nerves originating from the brachial plexus.
The median nerve originates from the lateral and medial cords of the brachial plexus, and has contr ...
(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 fl ...
(FPB) has two heads. The superficial head arises on the flexor retinaculum, while the deep head originates on three carpal bones: the
trapezium,
trapezoid
A quadrilateral with at least one pair of parallel sides is called a trapezoid () in American and Canadian English. In British and other forms of English, it is called a trapezium ().
A trapezoid is necessarily a Convex polygon, convex quadri ...
, and
capitate
The capitate bone is a bone in the human wrist found in the center of the carpal bone region, located at the distal end of the radius and ulna bones. It articulates with the third metacarpal bone (the middle finger) and forms the third carpomet ...
. 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 in humans and other animals in the upper limb. It is one of the five main nerves originating from the brachial plexus.
The median nerve originates from the lateral and medial cords of the brachial plexus, and has contr ...
, while the deep head is innervated by the
ulnar nerve
In human anatomy, the ulnar nerve is a nerve that runs near the ulna bone. The ulnar collateral ligament of elbow joint is in relation with the ulnar nerve. The nerve is the largest in the human body unprotected by muscle or bone, so injury is ...
(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. It lies deep to the abductor pollicis brevis and lateral to the flexor pollicis brevis.
Structure
The opp ...
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 in humans and other animals in the upper limb. It is one of the five main nerves originating from the brachial plexus.
The median nerve originates from the lateral and medial cords of the brachial plexus, and has contr ...
.
Other muscles involved are:
The
adductor pollicis
In human anatomy, the adductor pollicis muscle is a muscle in the hand that functions to adduct the thumb. It has two heads: transverse and oblique.
It is a fleshy, flat, triangular, and fan-shaped muscle deep in the thenar compartment beneath ...
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
In human anatomy, the ulnar nerve is a nerve that runs near the ulna bone. The ulnar collateral ligament of elbow joint is in relation with the ulnar nerve. The nerve is the largest in the human body unprotected by muscle or bone, so injury is ...
(C8-T1).
The first
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
![Hitchhikers thumb](https://upload.wikimedia.org/wikipedia/commons/4/42/Hitchhikers_thumb.jpg)
There is a variation of the human thumb where the angle between the first and second (proximal and distal)
phalanges
The phalanges (singular: ''phalanx'' ) are digital bones in the hands and feet of most vertebrates. In primates, the thumbs and big toes have two phalanges while the other digits have three phalanges. The phalanges are classed as long bones.
...
varies between 0° and almost 90° when the thumb is in a
thumbs-up
A thumb signal, usually described as a thumbs-up or thumbs-down, is a common hand gesture achieved by a closed fist held with the thumb extended upward or downward in approval or disapproval, respectively. These gestures have become metaphors i ...
gesture.
It has been suggested that the variation is an autosomal
recessive trait
In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The first variant is termed dominant and t ...
, called a
hitchhiker's thumb
Hitchhiker's thumb, also known as ''distal hyperextensibility of the thumb'' is the condition of having a thumb that has a distal phalange that bends backwards in an angle of 90°, it is thought to be inherited in an autosomal recessive manner, h ...
, with
homozygous
Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism.
Mo ...
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
bi-modality seen in other recessive genetic traits.
Other variations of the thumb include
brachydactyly type D
Brachydactyly type D, also known as short thumb or stub thumb and inaccurately referred to as clubbed thumb, is a condition clinically recognised by a thumb being relatively short and round with an accompanying wider nail bed. The distal phalanx ...
(which is a thumb with a congenitally short distal phalanx), a
triphalangeal thumb
Triphalangeal thumb (TPT) is a congenital malformation where the thumb has three phalanges instead of two. The extra phalangeal bone can vary in size from that of a small pebble to a size comparable to the phalanges in non-thumb digits. The true ...
(which is a thumb which has 3 phalanges instead of the usual two), and
polysyndactyly (which is a combination of radial polydactyly and syndactyly).
Grips
One of the earlier significant contributors to the study of hand grips was orthopedic primatologist and paleoanthropologist
John Napier
John Napier of Merchiston (; 1 February 1550 – 4 April 1617), nicknamed Marvellous Merchiston, was a Scottish landowner known as a mathematician, physicist, and astronomer. He was the 8th Laird of Merchiston. His Latinized name was Ioann ...
, 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
Prehensility is the quality of an appendage or organ (anatomy), organ that has Adaptation (biology), adapted for grasping or holding. The word is derived from the Latin term ''prehendere'', meaning "to grasp". The ability to grasp is likely der ...
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).
![Distal-phalanges-thumb-index Journal](https://upload.wikimedia.org/wikipedia/commons/7/71/Distal-phalanges-thumb-index_Journal.pone.0011727.g001.png)
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 (
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 bones in the hands and feet of most vertebrates. In primates, the thumbs and big toes have two phalanges while the other digits have three phalanges. The phalanges are classed as long bones. ...
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 hand
A robotic arm is a type of mechanical arm, usually programmable, with similar functions to a human arm; the arm may be the sum total of the mechanism or may be part of a more complex robot. The links of such a manipulator are connected by joints ...
s 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 joints in the wrist that articulate the distal row of carpal bones and the proximal bases of the five metacarpal bones.
The CMC joint of the thumb or the first CMC joint, also known as the trapeziometaca ...
(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
primate
Primates are a diverse order of mammals. They are divided into the strepsirrhines, which include the lemurs, galagos, and lorisids, and the haplorhines, which include the tarsiers and the simians (monkeys and apes, the latter including huma ...
s, including most
catarrhines
The parvorder Catarrhini , catarrhine monkeys, Old World anthropoids, or Old World monkeys, consisting of the Cercopithecoidea and apes (Hominoidea). In 1812, Geoffroy grouped those two groups together and established the name Catarrhini, "Old W ...
. The climbing and suspensory behaviour in
orthograde apes, such as
chimpanzee
The chimpanzee (''Pan troglodytes''), also known as simply the chimp, is a species of great ape native to the forest and savannah of tropical Africa. It has four confirmed subspecies and a fifth proposed subspecies. When its close relative th ...
s, 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
Quadrupedalism is a form of locomotion where four limbs are used to bear weight and move around. An animal or machine that usually maintains a four-legged posture and moves using all four limbs is said to be a quadruped (from Latin ''quattuor ...
monkeys such as
baboon
Baboons are primates comprising the genus ''Papio'', one of the 23 genera of Old World monkeys. There are six species of baboon: the hamadryas baboon, the Guinea baboon, the olive baboon, the yellow baboon, the Kinda baboon and the chacma ba ...
s, 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
''Homo habilis'' ("handy man") is an extinct species of archaic human from the Early Pleistocene of East and South Africa about 2.31 million years ago to 1.65 million years ago (mya). Upon species description in 1964, ''H. habilis'' was highly ...
'', a forerunner of ''
Homo sapiens
Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...
''. This, however, is the suggested result of evolution from ''
Homo erectus
''Homo erectus'' (; meaning "upright man") is an extinct species of archaic human from the Pleistocene, with its earliest occurrence about 2 million years ago. Several human species, such as '' H. heidelbergensis'' and '' H. antecessor' ...
'' (around 1
mya) via a series of intermediate
anthropoid
Anthropoid means 'ape/human feature' and may refer to:
* Simian, monkeys and apes (anthropoids, or suborder Anthropoidea, in earlier classifications)
*Anthropoid apes - apes that are closely related to humans (e.g., former family Pongidae and some ...
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
''Homo habilis'' ("handy man") is an extinct species of archaic human from the Early Pleistocene of East and South Africa about 2.31 million years ago to 1.65 million years ago (mya). Upon species description in 1964, ''H. habilis'' was highly ...
'' 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
''Homo habilis'' ("handy man") is an extinct species of archaic human from the Early Pleistocene of East and South Africa about 2.31 million years ago to 1.65 million years ago (mya). Upon species description in 1964, ''H. habilis'' was highly ...
'', an advanced grasping-capable hand was accompanied by facultative
bipedalism
Bipedalism is a form of terrestrial locomotion where an organism moves by means of its two rear limbs or legs. An animal or machine that usually moves in a bipedal manner is known as a biped , meaning 'two feet' (from Latin ''bis'' 'double' a ...
, 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
Arf-GAP with GTPase, ANK repeat and PH domain-containing protein 1 is an enzyme that in humans is encoded by the ''AGAP1'' gene.
Function
CENTG2 belongs to an ADP-ribosylation factor GTPase-activating (ARF-GAP) protein family involved in membra ...
(also known as
Human Accelerated Region Human accelerated regions (HARs), first described in August 2006, are a set of 49 segments of the human genome that are conserved throughout vertebrate evolution but are strikingly different in humans. They are named according to their degree of dif ...
2) is a
gene enhancer
In genetics, an enhancer is a short (50–1500 bp) region of DNA that can be bound by proteins ( activators) to increase the likelihood that transcription of a particular gene will occur. These proteins are usually referred to as transcriptio ...
"that may have contributed to the evolution of the uniquely opposable human thumb, and possibly also modifications in the
ankle
The ankle, or the talocrural region, or the jumping bone (informal) is the area where the foot and the leg meet. The ankle includes three joints: the ankle joint proper or talocrural joint, the subtalar joint, and the inferior tibiofibular join ...
or
foot
The foot ( : feet) is an anatomical structure found in many vertebrates. It is the terminal portion of a limb which bears weight and allows locomotion. In many animals with feet, the foot is a separate organ at the terminal part of the leg made ...
that allow humans to
walk
Walking (also known as ambulation) is one of the main gaits of terrestrial locomotion among legged animals. Walking is typically slower than running and other gaits. Walking is defined by an ' inverted pendulum' gait in which the body vaults ...
on two legs". Evidence to date shows that of the 110,000 gene enhancer sequences identified in the human
genome
In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding ge ...
, HACNS1 has undergone the most change during the
human evolution
Human evolution is the evolutionary process within the history of primates that led to the emergence of ''Homo sapiens'' as a distinct species of the hominid family, which includes the great apes. This process involved the gradual development of ...
since the
chimpanzee-human last common ancestor.
See also
*
Prehensility
*
Thumb signal
A thumb signal, usually described as a thumbs-up or thumbs-down, is a common hand gesture achieved by a closed fist held with the thumb extended upward or downward in approval or disapproval, respectively. These gestures have become metapho ...
*
Thumb twiddling
Thumb twiddling is an activity that is done with the hands of an individual whereby the fingers are interlocked and the thumbs circle around a common point, usually in the middle of the distance between the two thumbs.
While it is an expression ...
*
Thumb war
A thumb war (also called thumb wrestling, pea-knuckle or pea-knuckle war in New Zealand) is a game played by two players in which the thumbs are used to simulate fighting. The objective of the game is to pin the opponent's thumb, often to a count ...
*
Pollicization
Notes
References
*
*
*
* (
NCBI
The National Center for Biotechnology Information (NCBI) is part of the United States National Library of Medicine (NLM), a branch of the National Institutes of Health (NIH). It is approved and funded by the government of the United States. The ...
)
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
External links
*
*
{{Authority control
Fingers