The thumb is the first finger|digit of the hand . When a person is standing in the medical anatomical position (where the palm is facing anteriorly), the thumb is the Human anatomical terms#Anatomical directions|lateral -most digit. The Medical Latin English noun for thumb is wikt:pollex|pollex (compare hallux for big toe), and the corresponding adjective for thumb is wikt:pollical|pollical .
Definition
As one of five digits, and as companion to four other fingers
The English word "finger" has two senses, even in the context of appendages of a single typical human hand: # Any of the five digits. # Any of the five terminal members of the hand, especially those other than the thumb.
Linguistically, it appears that the original sense was the broader of these two: penkwe-ros (also rendered as penqrós ) was, in the inferred Proto-Indo-European language , a suffixed form of penkwe (or penqe ), which has given rise to many Indo-European languages|Indo-European -family words (tens of them defined in English dictionaries) that involve or flow from concepts of fiveness.
The thumb shares the following with each of the other four fingers:
Having a skeleton of phalanges , joined by hinge-like joints that provide flexion toward the palm of the hand
Having a "back" surface that features hair and a nail, and a hairless palm-of-the-hand side with fingerprint ridges instead
The thumb contrasts with each of the other four by being the only digit that:
Is opposable to the other four fingers
Has two phalanges rather than three
Has greater breadth in the Anatomical terms of location#Proximal_and_distal|distal phalanx than in the Anatomical terms of location#Proximal_and_distal|proximal phalanx
Is attached to such a mobile metacarpus (which produces most of the opposability)
and hence the etymology of the word: "tum" is Proto-Indo-European for " swelling " (cf "tumour" and "thigh") since the thumb is the stoutest of the digits.
Opposition and apposition
| title = The Thumb is the Hero | publisher = The New York Times | quote = The "fishing rod" a chimp strips of leaves and pokes into a termite nest to bring up a snack is as far as he'll ever get toward orbiting the planets. | date = January 11, 1981 | accessdate = November 2010 | url = http://query.nytimes.com/gst/fullpage.html? res=9F04E3D8173BF932A25752C0A967948260
In humans, opposition and apposition are two movements unique to the thumb but these words are not synonyms:
Primatologists and hand research pioneers John Napier (primatologist)|J. Napier and Prudence Hero Napier|P. Napier defined opposition as: "A movement by which the pulp surface of the thumb is placed squarely in contact with& nbsp;- or diametrically opposite to& nbsp;- the terminal pads of one or all of the remaining digits." For this true , pulp-to-pulp opposition to be possible, the thumb must rotate about its long axis (at the carpometacarpal joint ).cite web | title = Primates FAQ: Do any primates have opposable thumbs? | publisher = Wisconsin Regional Primate Research Center | url = http://www.primates.com/faq/ | accessdate = November 2010 Arguably, this definition was chosen to underline what is unique to the human thumb.
Anatomists and other researchers focused exclusively on human anatomy, on the other hand, tend to elaborate this definition in various ways and, consequently, there are hundreds of definitions.harvnb|van Nierop|van der Helm|Overbeeke|Djajadiningrat|2008| p=34 Some anatomistsharvnb|Brown|Freeman|Cuccurullo|Freeman|2004 restrict opposition to when the thumb is approximated to the fifth digit (little finger) and refer to other approximations between the thumb and other digits as apposition . To anatomists, this makes sense as two intrinsic hand muscles are named for this specific movement (the Opponens pollicis muscle|opponens pollicis and Opponens digiti minimi muscle|opponens digiti minimi respectively).
Other researchers use another definition, referring to opposition-apposition as the transition between flexion-abduction and extension-adduction; the side of the distal thumb phalanx thus approximated to the palm or the hand's radial side (side of index finger) during apposition and the pulp or "palmar" side of the distal thumb phalanx approximated to either the palm or other digits during opposition .
Moving a limb back to its neutral position is called reposition and a rotary movement is referred to as Circumduction (anatomy)|circumduction .
Human anatomy
Skeleton
The skeleton of the thumb consists of the first metacarpal bone which articulates proximal ly with the carpus at the Carpometacarpal joint#Thumb|carpometacarpal joint and distal ly with the proximal phalanges|proximal phalanx at the metacarpophalangeal joint . This latter bone articulates with the Distal phalanges|distal phalanx at the interphalangeal articulations of hand|interphalangeal joint . Additionally, there are two sesamoid bone s at the metacarpophalangeal joint.
Muscles
The muscles of the thumb can be compared to guy-wire s supporting a flagpole; tension from these musclular 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.harvnb|Austin|2005| p=339
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 muscles bellies located in the hand proper.cite web | title = Muscles of the thumb | publisher = Eaton hand | url = http://www.eatonhand.com/mus/mus131.htm | accessdate = April 2010
Extrinsic
double image|right|FPL.png|85|Gray419.png|100|Flexor pollicis longus (left) and deep muscles of dorsal forearm (right)A ventral forearm muscle, the Flexor pollicis longus muscle|flexor pollicis longus (FPL) originates on the anterior side of the Radius (bone)|radius distal to the radial tuberosity and from the Interosseous membrane of forearm|interosseous membrane . It passes through the carpal tunnel in a separate tendon sheath , after which it lies between the heads of the flexor pollicis brevis. It finally attaches onto the base of the distal phalanx of the thumb. It is innervated by the Anterior interosseous nerve|anterior interosseus branch of the median nerve (C7-C8)harvnb|Platzer|2004| p=162
Three dorsal forearm muscles act on the thumb:
The Abductor pollicis longus muscle|abductor pollicis longus (APL) originates on the dorsal sides of both the ulna and the radius, and from the interosseous membrane. Passing through the first tendon compartment, it inserts to the base of the first Metacarpus|metacarpal bone . A part of the tendon reaches the trapezium, while another fuses with the tendons of the extensor pollicis brevis and the abductor pollicis brevis. Except for abducting the hand, it flexes the hand towards the palm and abducts it radially. It is innervated by the deep branch of the radial nerve (C7-C8).
The Extensor pollicis longus muscle|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 Lever|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 (C7-C8).harvnb|Platzer|2004| p=168
The Extensor pollicis brevis muscle|extensor pollicis brevis (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 (C7-T1).
The tendons of the extensor pollicis longus and extensor pollicis brevis form what is known as the anatomical snuff box (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
double image|right|Gray426.png|160|Gray428.png|100|Thenar (left) and dorsal interossei (right) musclesThere are four Thenar eminence|thenar muscles :
The Abductor pollicis brevis muscle|abductor pollicis brevis (APB) originates on the Scaphoid bone|scaphoid tubercle and the Flexor retinaculum of the hand|flexor retinaculum . It inserts to the radial sesamoid bone and the proximal phalanx of the thumb. It is innervated by the median nerve (C8-T1).harvnb|Platzer|2004| p=176
The Flexor pollicis brevis muscle|flexor pollicis brevis (FPB) has two heads. The superficial head arises on the flexor retinaculum, while the deep head originates on three carpal bones: the Trapezium (bone)|trapezium , Trapezoid bone|trapezoid , and Capitate bone|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 , while the deep head is innervated by the ulnar nerve (C8-T1).
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 Opponens pollicis muscle|opponens pollicis 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 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.harvnb|Platzer|2004| p=174
Variation
"Hitchhiker's thumb" is an autosomal recessive trait more formally known as "distal hyperextensibility of the thumb".cite web|url= http://www.ncbi.nlm.nih.gov/entrez/dispomim.cgi? id=274200|title=Thumb, Distal Hyperextensibility of |work=OMIM|publisher=NCBI|accessdate=5 February 2010 Homozygous carriers can extend the top of the thumb backwards nearly 90° when the thumb is extended in a " Thumbs Up|thumbs-up ". Cases of Hitchhiker's thumb are subject to a variation in range of motion. Some who have this condition are able to hyperextend thumbs backwards 90° while still able to bend thumbs forward partially or with an added normal forward range of motion. Those with a more traditional case of Hitchhiker's thumbs can hyperextend backwards 90° while unable to bend past a straight position of the thumb.harvnb|Hsu|2008| p=S148 Malformations include a triphalangeal thumb and polydactyly .
Grips
double image|right|Gripclosed.jpg|130|Iverson grip.jpg|120|Left: In a power grip the object is in contact with the palm. Right: Cricketer Jack Iverson|Jack Iverson's "bent finger grip", an unusual pad-to-side precision grip designed to confuse batsmen.One of the earlier significant contributors to the study of hand grips was orthopedic primatologist and paleoathropologist 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.harvnb|Slocum|Pratt|1946, harvnb|McBride|1942| p=631 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 .harvnb|Napier|1956| pp=902–913 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 makes 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).harvnb|Costello|Fragaszy|1988| pp=235–245 Nevertheless, precision grips are usually only found in higher apes, and only in degrees significantly more restricted than in humans.harvnb|Young|2003| pp=165–174, harvnb|Christel|Kitzel|Niemitz|2004| pp=165–194, harvnb|Byrne|Byrne|1993| p=241
Evolution
The opposable thumb has helped the human species develop more accurate fine motor skill s. It is also thought to have directly led to the development of tools, not just in humans or their evolutionary ancestors, but other primates as well. The opposable thumb ensured that important human functions such as writing were possible.cite web | url = http://www.nationalgeographic.com/xpeditions/lessons/01/g68/lonsdorf.html | title = Lesson Plans& nbsp;— Chimps, Humans, Thumbs, and Tools | publisher = National Geographic | year = 2006 | accessdate = April 26, 2007cite web | last = Damonte | first = Kathleen | url = http://www.nsta.org/main/news/stories/science_and_children.php? news_story_ID=49036 | title = Thumbs Are Handy Digits | publisher = National Science Teachers Association : Science & Children: The Elementary Science Classroom | date = February 2004 | accessdate = April 26, 2007 The thumb, in conjunction with the other fingers, makes human hands and those of other species with similar hands some of the most dexterous in the world.cite web | last = Chaisson | first = Eric J. | url = http://www.tufts.edu/as/wright_center/cosmic_evolution/docs/text/text_bio_6.html | title = Cosmic Evolution& nbsp;— Epoch 6 - Biological Evolution | publisher = Tufts University | year = 2007 | accessdate = April 26, 2007
A primitive autonomization of the first carpometacarpal joint (CMC) may have occurred in dinosaurs. A real differentiation appeared perhaps 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.harvnb|Brunelli|1999| p=167
Opposable thumbs are shared by many primate s, including most simian s, and some prosimian s. The climbing and suspensory behaviour in Orthograde posture|orthograde apes, such as chimpanzee 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 monkeys such as baboon s, an adaptation to a terrestrial lifestyle has led to reduced digit 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.harvnb|Moyà-Solà|Köhler|Rook|1999 | loc= pp. 315–6
The evolution of the fully opposable thumb is usually associated with Homo habilis , the forerunner of Homo sapiens .cite web | url = http://www.esalenctr.org/display/confpage.cfm? confid=10& pageid=103& pgtype=1 | title = The Evolution of the Human Species (from Evolutionary Theory Conference Summary) | publisher = Esalen Center for Theory & Research | first = Christopher | last = Wills cite web | url = http://www.fortunecity.com/tatooine/acegarp/898/hominids.htm | title = Hominids | publisher = The NEXUS: Technology Timeline | accessdate = December 2009 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.
It is possible, though, that 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.harvnb|Harcourt-Smith|Aiello|2004 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 .cite web | url = http://www.sciencecodex.com/gene_enhancer_in_evolution_of_human_opposable_thumb | title = HACNS1: Gene enhancer in evolution of human opposable thumb | publisher = Science Codex | date = September 4, 2008 | accessdate = December 2009
Other animals with opposable digits
Refimprove section|date=February 2010Prose|section|date=May 2010Many animals also have some kind of opposable thumb or toe. An animal species is said to have opposable thumbs if the thumb is capable of bending in such a way that it can touch all the other digits on the hand. Most species do not have opposable thumbs. Opposable thumbs are a signature feature of the primate family, and played a large role in the ancient humans' invention and use of tool s.
; Primates
Primate s fall into one of four groups:harvnb|Ankel-Simons|2007| p=345
* Nonopposable thumbs: tarsier s and marmoset s
* Pseudo-opposable thumbs: all strepsirrhine s and Cebidae
* Opposable thumbs: Old World monkey s and all Hominidae|great ape s
* Opposable with comparatively long thumbs: gibbon s (or lesser apes)
Darwinius|Darwinius masillae , an Eocene primate fossil often described as a missing link between prosimian and simian , had hands and feet with highly flexible digits featuring opposable thumbs and halluces.harvnb|Franzen|Gingerich|Habersetzer|Hurum|2009| pp=15–18Dubious|date=February 2012Undue-inline|date=February 2012 ; Other placental mammals
Giant Panda s& nbsp;— five clawed fingers plus an extra-long Sesamoid bone#Panda anatomy|sesamoid bone beside the true first digit that, though not a true digit, works like an opposable thumb.cite web
| title = The Panda's Thumb | publisher = Athro | url = http://www.athro.com/evo/pthumb.html | year = 2000 | accessdate = November 2010
Lophiomys & nbsp;— a genus of African cricetid (rodent), with only one extant species (Lophiomys imhausi), but several extinct species, with opposable hallux|halluces .cite book
| title = An Introduction to the Study of Mammals Living and Extinct | publisher = A. and C. Black | year = 1891 | url = http://books.google.com/books? id=jeTjaFiNZtwC& pg=PA460 | author = Flower, William Henry; Lydekker, Richard | isbn = 978-1-4400-5891-2
Additionally, in many polydactyl cats , both the innermost and outermost (" Little finger|pinky ") toes may become opposable, allowing the cat to perform more complex tasks.
; Marsupials Double image|right|Ailurops ursinus Naemundung 5 North Sulawesi cropped.JPG|105|Oppossumthumb.jpg|180|Left: Opposable digits of Sulawesi Bear Cuscus forelimb Right: Opposable thumb on rear foot of an opossum
In most phalangerid marsupial s (a family of possum s) except species Trichosurus and Wyulda the first and second digits 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 Dactyly|syndactylous , united by skin at the top joint while the two separate nails serve as hair combs. The fourth and fifth digits are the largest of the hind foot.harvnb|Nowak|1999| p=89
Similar to phalangerids though in a different order, koala s have five digits on their fore and hind feet with sharp curved claws except for the first digit of the hind foot. The first and second digits 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 digits of the hind foot are fused but have separate claws.harvnb|McDade|2003| loc=vol 13, p. 44
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 webbed feet of which restrict opposability).harvnb|McDade|2003| loc=vol 12, p. 250
The mouse-like Microbiotheria|microbiotheres were a group of South American marsupials most closely related to Australian marsupials. The only extant member, Monito del Monte|Dromiciops gliroides , is not closely related to opossums but has paws similar to these animals, each having opposable toes adapted for gripping.harvnb|McDade|2003| loc=vol 12, p. 274
; Reptiles
Troodon & nbsp;— a birdlike dinosaur& nbsp;— partially opposable thumbs.
Bambiraptor & nbsp;— a small, predatory dinosaur& nbsp;— it could touch the outer two of its three digits together in an opposable grip.
Nqwebasaurus & nbsp;— a coelurosaur with a long, three-fingered hand which included a partially opposable thumb (a "killer claw").harvnb|de Klerk|Forster|Sampson|Chinsamy|2000| p=327. The left manus shows that the ?exed digit I had the potential to partially oppose digits II and III.
Most bird s have at least one opposable digit on the foot, in Dactyly#In_birds|various configurations , but these are seldom called "thumbs".
; Amphibians
Phyllomedusa , a genus of frogs native to South America.
See also
Prehensility
Thumbcuffs
Notes
Reflist|colwidth=30em
References
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Commons category-inline|Thumbswiktionary-inline|thumb human anatomical features Category:Fingers Category:Article Feedback 5
