Thumb

This article is about the digit of the hand. For other uses, seeThumb (disambiguation).

Thumb
Human left thumb
Bones of the thumb, visible at far left
Details
Artery	Princeps pollicis artery
Vein	Dorsal venous network of hand
Nerve	Dorsal digital nerves of radial nerve,proper palmar digital nerves of median nerve
Lymph	Infraclavicular lymph nodes
Identifiers
Latin	pollex digitus I manus digitus primus manus
MeSH	D013933
TA98	A01.1.00.053
TA2	151
FMA	24938
Anatomical terminology

Thethumbis the first digit of thehand,next to theindex finger.When a person is standing in the medicalanatomical position(where the palm is facing to the front), the thumb is the outermost digit. The Medical Latin English noun for thumb ispollex(comparehalluxfor big toe), and the corresponding adjective for thumb ispollical.

Definition

Thumb and fingers

The English wordfingerhas twosenses,even in the context of appendages of a single typical human hand: 1) Any of the five terminal members of the hand. 2) Any of the four terminal members of the hand, other than the thumb

Linguistically, it appears that the original sense was the first of these two:*penkwe-ros(also rendered as*penqrós) was, in the inferredProto-Indo-European language,a suffixed form of*penkwe(or*penqe), which has given rise to manyIndo-European-family words (tens of them defined in English dictionaries) that involve, or stem from, concepts of fiveness.

The thumb shares the following with each of the other four fingers:

• Having a skeleton ofphalanges,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 withfingerprintridges

The thumb contrasts with each of the other four fingers by being the only one that:

• Is opposable to the other four fingers
• Has twophalangesrather than three. However, recently there have been reports that the thumb, like other fingers, has three phalanges, but lacks a metacarpal bone.
• Has greater breadth in thedistalphalanx than in theproximalphalanx
• Is attached to such a mobilemetacarpus(which produces most of the opposability)
• Curls horizontally instead of vertically

and hence the etymology of the word:*tumis Proto-Indo-European for 'swelling' (cf 'tumor' and 'thigh') since the thumb is the stoutest of the fingers.

Opposition and apposition

Humans

Anatomists and other researchers focused on human anatomy have hundreds of definitions ofopposition.Some anatomistsrestrictoppositionto when the thumb is approximated to the fifth finger (little finger) and refer to other approximations between the thumb and other fingers asapposition.To anatomists, this makes sense as two intrinsic hand muscles are named for this specific movement (theopponens pollicisandopponens digiti minimirespectively).

Other researchers use another definition,referring to opposition-apposition as the transition between flexion-abduction and extension-adduction; thesideof the distal thumb phalanx thus approximated to the palm or the hand's radial side (side of index finger) duringappositionand thepulpor "palmar" side of the distal thumb phalanx approximated to either the palm or other fingers duringopposition.

Moving a limb back to its neutral position is called reposition and a rotary movement is referred to ascircumduction.

Primatologists and hand research pioneersJohnandPrudence Napierdefined 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 thistrue,pulp-to-pulp opposition to be possible, the thumb must rotate about its long axis (at thecarpometacarpal joint).Arguably, this definition was chosen to underline what is unique to the human thumb.

Other primates

Abonobo"fishing" for termites, an example of incomplete/ "untrue" opposition

• Primatesfall into one of six groups:
  • Thumbless:spider monkeyandcolobus
  • Nonopposable thumbs:tarsiers(which are found in the islands of Southeast Asia),marmosets(which areNew World monkeys)
  • Pseudo-opposable thumbs: allstrepsirrhines(lemurs, pottos and lorises) andCebidae(capuchin and squirrel monkeys, which areNew World monkeys)
  • Opposable thumbs:Old World monkeys(Circopithecidae) except colobus, and allgreat apes
  • Opposable with comparatively long thumbs:gibbons(or lesser apes)
  • Yet to be classified: otherNew World monkeys(tamarins,Aotidae: night or owl monkeys, Pitheciidae: titis, sakis and uakaris, Atelidae: howler and woolly monkeys)

The spider monkey compensates for being virtually thumbless by using the hairless part of its long, prehensile tail for grabbing objects. InapesandOld World monkeys,the thumb can be rotated around its axis, but the extensive area of contact between the pulps of the thumb and index finger is a human characteristic.

Darwinius masillae,anEoceneprimatetransitional fossilbetweenprosimianandsimian,had hands and feet with highly flexible digits featuring opposable thumbs and halluces.

Other placental mammals

• Giant pandas— five clawed fingers plus an extra-longsesamoid bonebeside the true first finger that, though not a true finger, works like an opposable thumb.
• Mostrodentshave a partly opposable toe on each front paw, letting them grasp.
  • In somemice,the hallux ( "big toe" ) is clawless and fully opposable, includingarborealspecies such asHapalomys,Chiropodomys,Vandeleuria,andChiromyscus;andsaltatorial,bipedal species such asNotomysand possibly someGerbillinae.
  • The East Africanmaned rat(Lophiomys imhausi), an arboreal, porcupine-like rodent, has four fingers on its hands and feet and a partially opposable thumb.

Additionally, in manypolydactyl cats,both theinnermost toeandoutermost toe(pinky) may become opposable, allowing the cat to perform more complex tasks.

Marsupials

Left: Opposable toes of theSulawesi bear cuscusforelimb
Right: Opposable thumb on rear foot of an opossum

• In mostphalangeridmarsupials(a family ofpossums) except speciesTrichosurusandWyulda,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 partlysyndactylous,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.
• Koalashave 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.
• Opossumsare New World marsupials with opposable thumbs in the hind feet giving these animals their characteristic grasping capability (with the exception of thewater opossum,the webbed feet of which restrict opposability).
• The mouse-likemicrobiothereswere 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 ofchameleonsare organized into a medial bundle of toes 1, 2 and 3, and a lateral bundle of toes 4 and 5, and the hind feet are organized into a medial bundle of toes 1 and 2, and a lateral bundle of toes 3, 4 and 5.

Dinosaurs

• Dinosaurs belonging to the family of bird-like dinosaurTroodontidaehad 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 dinosaurBambiraptormay 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— acoelurosaurwith a long, three-fingered hand which included a partially opposable thumb (a "killer claw" ).

In addition to these, some other dinosaurs may have had partially or completely opposed toes in order to manipulate food and/or grasp prey.

Birds

Four types of bird feet
(right foot diagrams)

See also:Dactyly § Anisodactyly

• Mostbirdshave at least one opposable toe on the foot, invarious configurations,though these are seldom called "thumbs". They are more often known simply ashalluxes.

Pterosaurs

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

Amphibians

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

Human anatomy

Skeleton

The skeleton of the thumb consists of thefirst metacarpal bonewhich articulatesproximallywith thecarpusat thecarpometacarpal jointanddistallywith theproximal phalanxat themetacarpophalangeal joint.This latter bone articulates with thedistal phalanxat theinterphalangeal joint.Additionally, there are twosesamoid bonesat the metacarpophalangeal joint.

Muscles

Main article:Muscles of the thumb

The muscles of the thumb can be compared toguy-wiressupporting 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

Flexor pollicis longus (left) and deep muscles of dorsal forearm (right)

A ventral forearm muscle, theflexor pollicis longus(FPL) originates on the anterior side of theradiusdistal to theradial tuberosityand from theinterosseous membrane.It passes through thecarpal tunnelin a separatetendon 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 theanterior interosseus branchof themedian nerve(C7-C8)It is a persistence of one of the formercontrahentesmuscles that pulled the fingers or toes together.

Three dorsal forearm muscles act on the thumb:

Theabductor pollicis longus(APL) originates on the dorsal sides of both theulnaand the radius, and from the interosseous membrane. Passing through the first tendon compartment, it inserts to the base of the firstmetacarpal 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 thedeep branch of the radial nerve(C7-C8).

Theextensor 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 afulcrumto extend the thumb and also dorsiflexes and abducts the hand at the wrist. It is innervated by thedeep branch of the radial nerve(C7-C8).

Theextensor 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 thedeep branch of the radial nerve(C7-T1).

The tendons of the extensor pollicis longus and extensor pollicis brevis form what is known as theanatomical 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

Thenar (left) and dorsal interossei (right) muscles

There are threethenar muscles:

Theabductor pollicis brevis(APB) originates on thescaphoid tubercleand theflexor retinaculum.It inserts to the radial sesamoid bone and the proximal phalanx of the thumb. It is innervated by themedian nerve(C8-T1).

Theflexor pollicis brevis(FPB) has two heads. The superficial head arises on the flexor retinaculum, while the deep head originates on three carpal bones: thetrapezium,trapezoid,andcapitate.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 themedian nerve,while the deep head is innervated by theulnar nerve(C8-T1).

Theopponens pollicisoriginates 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 themedian nerve.

Other muscles involved are:

Theadductor pollicisalso 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 theulnar nerve(C8-T1).

The firstdorsal 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

Further information:Hitchhiker's thumb,brachydactyly type D,triphalangeal thumb,andpolydactyly

Hitchhiker's thumb

There is a variation of the human thumb where the angle between the first and second (proximal and distal)phalangesvaries between 0° and almost 90° when the thumb is in athumbs-upgesture.

It has been suggested that the variation is an autosomalrecessive trait,called ahitchhiker's thumb,withhomozygouscarriers 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 thebi-modalityseen in other recessive genetic traits.

Other variations of the thumb includebrachydactyly type D(which is a thumb with a congenitally short distal phalanx), atriphalangeal thumb(which is a thumb which has 3 phalanges instead of the usual two), andpolysyndactyly(which is a combination of radial polydactyly and syndactyly).

Grips

Left: In a power grip the object is in contact with the palm.
Right: CricketerJack 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 paleoanthropologistJohn 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 primaryprehensilegrips: theprecision gripand thepower grip.The precision and power grip are defined by the position of the thumb and fingers where:

• Thepower gripis 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 usingboth your palm and fingers,and during pullups.
• Theprecision gripis 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 jarwith the fingertips alone,and gripping a ball (only if the ball is not tight against the palm).

Thumb and index finger during pad-to-pad precision grasping

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 Capuchinsfor 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 thedistal phalanxof 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 allrobotic handshave 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 firstcarpometacarpal joint(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 someprimates,including mostcatarrhines.The climbing and suspensory behaviour inorthogradeapes, such aschimpanzees,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, inpronogrademonkeys such asbaboons,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 withHomo habilis,a forerunner ofHomo sapiens.This, however, is the suggested result of evolution fromHomo erectus(around 1mya) via a series of intermediateanthropoidstages, 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) ofHomo habilispreceded 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. WithHomo habilis,an advanced grasping-capable hand was accompanied by facultativebipedalism,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 asHuman Accelerated Region2) is agene enhancer"that may have contributed to the evolution of the uniquely opposable human thumb, and possibly also modifications in theankleorfootthat allow humans towalkon two legs ". Evidence to date shows that of the 110,000 gene enhancer sequences identified in the humangenome,HACNS1 has undergone the most change during thehuman evolutionsince thechimpanzee–human last common ancestor.

See also

• Pollicization
• Prehensility
• Thumb signal
• Thumb twiddling
• Thumb war

References

• Almécija, S.; Moyà-Solà, S.; Alba, D. M. (2010)."Early Origin for Human-Like Precision Grasping: A Comparative Study of Pollical Distal Phalanges in Fossil Hominins".PLOS ONE.5(7): e11727.Bibcode:2010PLoSO...511727A.doi:10.1371/journal.pone.0011727.PMC2908684.PMID20661444.
• Ankel-Simons, Friderun (2007). "Chapter 8: Postcranial Skeleton".Primate Anatomy(3rd ed.). Academic Press. p. 345.ISBN978-0-12-372576-9.
• Austin, Noelle M. (2005). "Chapter 9: The Wrist and Hand Complex". In Levangie, Pamela K.; Norkin, Cynthia C. (eds.).Joint Structure and Function: A Comprehensive Analysis(4th ed.). Philadelphia: F. A. Davis Company.ISBN978-0-8036-1191-7.
• Brown, David P.; Freeman, Eric D.; Cuccurullo, Sara; Freeman, Ted L. (2004)."Upper Extremities—Hand Region: Range of Motion of the Digits".In Cuccurullo, Sara (ed.).Physical Medicine and Rehabilitation Board Review.Demos Medical Publishing.ISBN978-1-888799-45-3.(NCBI)
• Brunelli, Giovanni R. (1999)."Stability in the first carpometacarpal joint".In Brüser, Peter; Gilbert, Alain (eds.).Finger bone and joint injuries.Taylor & Francis.ISBN978-1-85317-690-6.
• Byrne, R.W.; Byrne, J.M.E. (1993)."Complex Leaf-Gathering Skills of Mountain Gorillas (Gorilla g. beringei): Variability and Standardization"(PDF).American Journal of Primatology.31(4): 241–261.doi:10.1002/ajp.1350310402.ISSN0275-2565.PMID31936992.S2CID84429453.Archived fromthe original(PDF)on September 20, 2009.
• Christel, Marianne I.; Kitzel, Stefanie;Niemitz, Carsten(30 November 2004). "How Precisely do Bonobos (Pan paniscus) Grasp Small Objects?".International Journal of Primatology.19(1): 165–194.doi:10.1023/A:1020319313219.S2CID23567551.
• Costello, Michael B.; Fragaszy, Dorothy M. (March 1988)."Prehension in Cebus and Saimiri: I. Grip type and hand preference".American Journal of Primatology.15(3): 235–245.doi:10.1002/ajp.1350150306.PMID31968893.S2CID86556774.
• de Klerk, W.J.; Forster, C.A.; Sampson, S.D.; Chinsamy, A.; Ross, C.F. (2000)."A new coelurosaurian dinosaur from the Early Cretaceous of South Africa"(PDF).Journal of Vertebrate Paleontology.20(2): 324–332.doi:10.1671/0272-4634(2000)020[0324:ancdft]2.0.co;2.S2CID128622530.Archived fromthe original(PDF)on 2012-01-30.
• Diogo, R; Richmond, BG; Wood, B (2012). "Evolution and homologies of primate and modern human hand and forearm muscles, with notes on thumb movements and tool use".Journal of Human Evolution.63(1): 64–78.doi:10.1016/j.jhevol.2012.04.001.PMID22640954.
• Ellerman, John Reeves (1941).The families and genera of living rodents. Vol. II. Family Muridae.London: British Museum (Natural History).
• Franzen, JL; Gingerich, PD; Habersetzer, J; Hurum, JH; von Koenigswald, W; et al. (2009). Hawks, John (ed.)."Complete Primate Skeleton from the Middle Eocene of Messel in Germany: Morphology and Paleobiology".PLOS ONE.4(5): e5723.Bibcode:2009PLoSO...4.5723F.doi:10.1371/journal.pone.0005723.PMC2683573.PMID19492084.
• Harcourt-Smith, W E H; Aiello, L C (May 2004)."Fossils, feet and the evolution of human bipedal locomotion".Journal of Anatomy.204(5): 403–16.doi:10.1111/j.0021-8782.2004.00296.x.PMC1571304.PMID15198703.
• Hsu, Ar-Tyan; Meng-Tsu Hu; Fong Ching Su (July 2008)."Effect of Gender, Flexibility and Thumb Type on Thumb Tip Generation".Journal of Biomechanics.41(Supplement 1): S148.doi:10.1016/S0021-9290(08)70148-9.
• Jones, Lynette A.;Lederman, Susan J.(2006).Human hand function.Oxford University Press US.ISBN9780195173154.
• Leakey, LSB;Tobias, PV;Napier, JR(April 1964)."A New Species of Genus Homo from Olduvai Gorge"(PDF).Nature.202(4927): 7–9.Bibcode:1964Natur.202....7L.doi:10.1038/202007a0.PMID14166722.S2CID12836722.
• McBride, Earl Duwain (1942).Disability evaluation: principles of treatment of compensable injuries.Lippincott. p. 631.
• McDade, Melissa C. (2003). "Koalas (Phascolartidae) ". In Hutchins, Michael; Kleiman, Devra G.; Geist, Valerius; et al. (eds.).Grzimek's animal life encyclopedia: Volumes 12–16, Mammals I–V(2nd ed.). Farmington Hills, MI: Gale Group.
• Moyà-Solà, Salvador; Köhler, Meike; Rook, Lorenzo (January 5, 1999)."Evidence of hominid-like precision grip capability in the hand of the Miocene ape Oreopithecus"(PDF).PNAS.96(1): 313–317.Bibcode:1999PNAS...96..313M.doi:10.1073/pnas.96.1.313.PMC15136.PMID9874815.
• Napier, John Russell (November 1956)."The prehensile movements of the human hand".J Bone Joint Surg Br.38(4): 902–913.doi:10.1302/0301-620X.38B4.902.PMID13376678.
• Nowak, Ronald M. (1999).Walker's mammals of the world, Volume 2(6th ed.). JHU Press.ISBN978-0-8018-5789-8.
• Platzer, Werner (2004).Color Atlas of Human Anatomy, Vol. 1: Locomotor System(5th ed.). Thieme.ISBN3-13-533305-1.
• Senter, Phil (2006). "Comparison of forelimb function betweenDeinonychusandBambiraptor(Theropoda: Dromaeosauridae) ".Journal of Vertebrate Paleontology.26(4): 897–906.doi:10.1671/0272-4634(2006)26[897:COFFBD]2.0.CO;2.S2CID85919882.
• Slocum, D.B.; Pratt, D.R. (1946)."Disability Evaluation for the Hand"(PDF).Journal of Bone and Joint Surgery.28(3): 491–5.PMID20992193.
• van Nierop, Onno A.; van der Helm, Aadjan; Overbeeke, Kees J.; Djajadiningrat, Tom J.P. (2008)."A natural human hand model"(PDF).Visual Comput.24(1): 31–44.doi:10.1007/s00371-007-0176-x.
• Young, Richard W. (January 2003)."Evolution of the human hand: the role of throwing and clubbing".Journal of Anatomy.202(1): 165–174.doi:10.1046/j.1469-7580.2003.00144.x.PMC1571064.PMID12587931.

External links

• Media related toThumbsat Wikimedia Commons
• The dictionary definition ofthumbat Wiktionary