Thumb

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This article is about the digit of the hand. Thumb_sentence_0

For other uses, see Thumb (disambiguation). Thumb_sentence_1

Thumb_table_infobox_0

ThumbThumb_header_cell_0_0_0
DetailsThumb_header_cell_0_1_0
ArteryThumb_header_cell_0_2_0 Princeps pollicis arteryThumb_cell_0_2_1
VeinThumb_header_cell_0_3_0 Dorsal venous network of handThumb_cell_0_3_1
NerveThumb_header_cell_0_4_0 Dorsal digital nerves of radial nerve, proper palmar digital nerves of median nerveThumb_cell_0_4_1
LymphThumb_header_cell_0_5_0 Infraclavicular lymph nodesThumb_cell_0_5_1
IdentifiersThumb_header_cell_0_6_0
LatinThumb_header_cell_0_7_0 pollex

digitus I manus digitus primus manusThumb_cell_0_7_1

MeSHThumb_header_cell_0_8_0 Thumb_cell_0_8_1
TA98Thumb_header_cell_0_9_0 Thumb_cell_0_9_1
TA2Thumb_header_cell_0_10_0 Thumb_cell_0_10_1
FMAThumb_header_cell_0_11_0 Thumb_cell_0_11_1

The thumb is the first digit of the hand. Thumb_sentence_2

When a person is standing in the medical anatomical position (where the palm is facing to the front), the thumb is the outermost digit. Thumb_sentence_3

The Medical Latin English noun for thumb is (compare hallux for big toe), and the corresponding adjective for thumb is . Thumb_sentence_4

Definition Thumb_section_0

Thumb and fingers Thumb_section_1

The English word "finger" has two senses, even in the context of appendages of a single typical human hand: Thumb_sentence_5

Thumb_ordered_list_0

  1. Any of the four terminal members of the hand, specifically those other than the thumb.Thumb_item_0_0
  2. Any of the five digits of the hand.Thumb_item_0_1

Linguistically, it appears that the original sense was the second 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-family words (tens of them defined in English dictionaries) that involve, or stem from, concepts of fiveness. Thumb_sentence_6

The thumb shares the following with each of the other four fingers: Thumb_sentence_7

Thumb_unordered_list_1

  • Having a skeleton of phalanges, joined by hinge-like joints that provide flexion toward the palm of the handThumb_item_1_2
  • Having a dorsal surface that features hair and a nail, and a hairless palmar aspect with fingerprint ridgesThumb_item_1_3

The thumb contrasts with each of the other four by being the only digit that: Thumb_sentence_8

Thumb_unordered_list_2

  • Is opposable to the other four fingersThumb_item_2_4
  • Has two phalanges rather than threeThumb_item_2_5
  • Has greater breadth in the distal phalanx than in the proximal phalanxThumb_item_2_6
  • Is attached to such a mobile metacarpus (which produces most of the opposability)Thumb_item_2_7

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 fingers. Thumb_sentence_9

Opposition and apposition Thumb_section_2

Humans Thumb_section_3

Anatomists and other researchers focused on human anatomy have hundreds of definitions of opposition. Thumb_sentence_10

Some anatomists 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. Thumb_sentence_11

To anatomists, this makes sense as two intrinsic hand muscles are named for this specific movement (the opponens pollicis and opponens digiti minimi respectively). Thumb_sentence_12

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. Thumb_sentence_13

Moving a limb back to its neutral position is called reposition and a rotary movement is referred to as circumduction. Thumb_sentence_14

Primatologists and hand research pioneers John and Prudence Napier defined opposition as: "A movement by which the pulp surface of the thumb is placed squarely in contact with – or diametrically opposite to – the terminal pads of one or all of the remaining digits." Thumb_sentence_15

For this true, pulp-to-pulp opposition to be possible, the thumb must rotate about its long axis (at the carpometacarpal joint). Thumb_sentence_16

Arguably, this definition was chosen to underline what is unique to the human thumb. Thumb_sentence_17

Other primates Thumb_section_4

Thumb_unordered_list_3

The spider monkey compensates for being virtually thumbless by using the hairless part of its long, prehensile tail for grabbing objects. Thumb_sentence_18

In apes and Old World monkeys, the thumb can be rotated around its axis, but the extensive area of contact between the pulps of the thumb and index finger is a human characteristic. Thumb_sentence_19

Darwinius masillae, an Eocene primate transitional fossil between prosimian and simian, had hands and feet with highly flexible digits featuring opposable thumbs and halluces. Thumb_sentence_20

Other placental mammals Thumb_section_5

Thumb_unordered_list_4

  • Giant pandas — five clawed fingers plus an extra-long sesamoid bone beside the true first digit that, though not a true digit, works like an opposable thumb.Thumb_item_4_15
  • In some Muridae the hallux is clawless and fully opposable, including arboreal species such as Hapalomys, Chiropodomys, Vandeleuria, and Chiromyscus; and saltatorial, bipedal species such as Notomys and possibly some Gerbillinae.Thumb_item_4_16
  • The East African maned rat (Lophiomys imhausi), an arboreal, porcupine-like rodent, has four digits on its hands and feet and a partially opposable thumb.Thumb_item_4_17
  • Most rodents have a partly opposable toe on each front paw, letting them grasp.Thumb_item_4_18

Additionally, in many polydactyl cats, both the innermost toe and outermost toe (pinky) may become opposable, allowing the cat to perform more complex tasks. Thumb_sentence_21

Marsupials Thumb_section_6

Thumb_unordered_list_5

  • In most phalangerid marsupials (a family of possums) 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 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.Thumb_item_5_19
  • Similar to phalangerids though in a different order, koalas 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.Thumb_item_5_20
  • Opossums 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).Thumb_item_5_21
  • The mouse-like microbiotheres were a group of South American marsupials most closely related to Australian marsupials. The only extant member, Dromiciops gliroides, is not closely related to opossums but has paws similar to these animals, each having opposable toes adapted for gripping.Thumb_item_5_22

Reptiles Thumb_section_7

Thumb_unordered_list_6

  • The front feet of chameleons are organized into a medial bundle of digits 1, 2 and 3, and a lateral bundle of digits 4 and 5, and the hind feet are organized into a medial bundle of digits 1 and 2, and a lateral bundle of digits 3, 4 and 5.Thumb_item_6_23

Dinosaurs Thumb_section_8

Thumb_unordered_list_7

  • The bird-like dinosaur Troodon had a partially opposable finger. It is possible that this adaptation was used to better manipulate ground objects or moving undergrowth branches when searching for prey.Thumb_item_7_24
  • The small predatory dinosaur Bambiraptor may have had mutually opposable first and third fingers and a forelimb maneouverability 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.Thumb_item_7_25
  • Nqwebasaurus — a coelurosaur with a long, three-fingered hand which included a partially opposable thumb (a "killer claw").Thumb_item_7_26

In addition to these, some other dinosaurs may have had partially or completely opposed digits in order to manipulate food and/or grasp prey. Thumb_sentence_22

Birds Thumb_section_9

See also: Dactyly § Anisodactyly Thumb_sentence_23

Thumb_unordered_list_8

  • Most birds have at least one opposable digit on the foot, in various configurations, though these are seldom called "thumbs". They are more often known simply as halluxes.Thumb_item_8_27

Amphibians Thumb_section_10

Thumb_unordered_list_9

  • Phyllomedusa, a genus of frogs native to South America.Thumb_item_9_28

Human anatomy Thumb_section_11

Skeleton Thumb_section_12

The skeleton of the thumb consists of the first metacarpal bone which articulates proximally with the carpus at the carpometacarpal joint and distally with the proximal phalanx at the metacarpophalangeal joint. Thumb_sentence_24

This latter bone articulates with the distal phalanx at the interphalangeal joint. Thumb_sentence_25

Additionally, there are two sesamoid bones at the metacarpophalangeal joint. Thumb_sentence_26

Muscles Thumb_section_13

Main article: Muscles of the thumb Thumb_sentence_27

The muscles of the thumb can be compared to guy-wires supporting a flagpole; tension from these muscular guy-wires must be provided in all directions to maintain stability in the articulated column formed by the bones of the thumb. Thumb_sentence_28

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. Thumb_sentence_29

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. Thumb_sentence_30

Extrinsic Thumb_section_14

A ventral forearm muscle, the flexor pollicis longus (FPL) originates on the anterior side of the radius distal to the radial tuberosity and from the interosseous membrane. Thumb_sentence_31

It passes through the carpal tunnel in a separate tendon sheath, after which it lies between the heads of the flexor pollicis brevis. Thumb_sentence_32

It finally attaches onto the base of the distal phalanx of the thumb. Thumb_sentence_33

It is innervated by the anterior interosseus branch of the median nerve (C7-C8) It is a persistence of one of the former contrahentes muscles that pulled the fingers or toes together. Thumb_sentence_34

Three dorsal forearm muscles act on the thumb: Thumb_sentence_35

The abductor pollicis longus (APL) originates on the dorsal sides of both the ulna and the radius, and from the interosseous membrane. Thumb_sentence_36

Passing through the first tendon compartment, it inserts to the base of the first metacarpal bone. Thumb_sentence_37

A part of the tendon reaches the trapezium, while another fuses with the tendons of the extensor pollicis brevis and the abductor pollicis brevis. Thumb_sentence_38

Except for abducting the hand, it flexes the hand towards the palm and abducts it radially. Thumb_sentence_39

It is innervated by the deep branch of the radial nerve (C7-C8). Thumb_sentence_40

The extensor pollicis longus (EPL) originates on the dorsal side of the ulna and the interosseous membrane. Thumb_sentence_41

Passing through the third tendon compartment, it is inserted onto the base of the distal phalanx of the thumb. Thumb_sentence_42

It uses the dorsal tubercle on the lower extremity of the radius as a fulcrum to extend the thumb and also dorsiflexes and abducts the hand at the wrist. Thumb_sentence_43

It is innervated by the deep branch of the radial nerve (C7-C8). Thumb_sentence_44

The 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. Thumb_sentence_45

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. Thumb_sentence_46

It extends the thumb and, because of its close relationship to the long abductor, also abducts the thumb. Thumb_sentence_47

It is innervated by the deep branch of the radial nerve (C7-T1). Thumb_sentence_48

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). Thumb_sentence_49

Intrinsic Thumb_section_15

There are three thenar muscles: Thumb_sentence_50

The abductor pollicis brevis (APB) originates on the scaphoid tubercle and the flexor retinaculum. Thumb_sentence_51

It inserts to the radial sesamoid bone and the proximal phalanx of the thumb. Thumb_sentence_52

It is innervated by the median nerve (C8-T1). Thumb_sentence_53

The flexor pollicis brevis (FPB) has two heads. Thumb_sentence_54

The superficial head arises on the flexor retinaculum, while the deep head originates on three carpal bones: the trapezium, trapezoid, and capitate. Thumb_sentence_55

The muscle is inserted onto the radial sesamoid bone of the metacarpophalangeal joint. Thumb_sentence_56

It acts to flex, adduct, and abduct the thumb, and is therefore also able to oppose the thumb. Thumb_sentence_57

The superficial head is innervated by the median nerve, while the deep head is innervated by the ulnar nerve (C8-T1). Thumb_sentence_58

The opponens pollicis originates on the tubercle of the trapezium and the flexor retinaculum. Thumb_sentence_59

It is inserted onto the radial side of the first metacarpal. Thumb_sentence_60

It opposes the thumb and assists in adduction. Thumb_sentence_61

It is innervated by the median nerve. Thumb_sentence_62

Other muscles involved are: Thumb_sentence_63

The adductor pollicis also has two heads. Thumb_sentence_64

The transversal head originates along the entire third metacarpal bone, while the oblique head originates on the carpal bones proximal to the third metacarpal. Thumb_sentence_65

The muscle is inserted onto the ulnar sesamoid bone of the metacarpophalangeal joint. Thumb_sentence_66

It adducts the thumb, and assists in opposition and flexion. Thumb_sentence_67

It is innervated by the deep branch of the ulnar nerve (C8-T1). Thumb_sentence_68

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. Thumb_sentence_69

Variations Thumb_section_16

There is a variation of the human thumb where the angle between the first and second phalanges varies between 0° and almost 90° when the thumb is in a thumbs-up gesture. Thumb_sentence_70

It has been suggested that the variation is an autosomal recessive trait, called a "Hitchhiker's thumb", with homozygous carriers having an angle close to 90°. Thumb_sentence_71

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. Thumb_sentence_72

Other formations of the thumb include a triphalangeal thumb and polydactyly. Thumb_sentence_73

Grips Thumb_section_17

One of the earlier significant contributors to the study of hand grips was orthopedic primatologist and paleoanthropologist 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. Thumb_sentence_74

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. Thumb_sentence_75

Napier proposed two primary prehensile grips: the precision grip and the power grip. Thumb_sentence_76

The precision and power grip are defined by the position of the thumb and fingers where: Thumb_sentence_77

Thumb_unordered_list_10

  • 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.Thumb_item_10_29
  • 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).Thumb_item_10_30

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). Thumb_sentence_78

Nevertheless, precision grips are usually only found in higher apes, and only in degrees significantly more restricted than in humans. Thumb_sentence_79

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 of the index finger. Thumb_sentence_80

Most non-human primates have to flex their long fingers in order for the small thumb to reach them. Thumb_sentence_81

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. Thumb_sentence_82

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. Thumb_sentence_83

In robotics, almost all robotic hands have a long and strong opposable thumb. Thumb_sentence_84

Like human hands, the thumb of a robotic hand also plays a key role in gripping an object. Thumb_sentence_85

One inspiring approach of robotic grip planning is to mimic human thumb placement. Thumb_sentence_86

In a sense, human thumb placement indicates which surface or part of the object is good for grip. Thumb_sentence_87

Then the robot places its thumb to the same location and plans the other fingers based on the thumb placement. Thumb_sentence_88

The function of the thumb declines physiologically with aging. Thumb_sentence_89

This can be demonstrated by assessing the motor sequencing of the thumb. Thumb_sentence_90

Human evolution Thumb_section_18

A primitive autonomization of the first carpometacarpal joint (CMC) may have occurred in dinosaurs. Thumb_sentence_91

A real differentiation appeared perhaps 70 mya in early primates, while the shape of the human thumb CMC finally appears about 5 mya. Thumb_sentence_92

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. Thumb_sentence_93

Opposable thumbs are shared by some primates, including most catarrhines. Thumb_sentence_94

The climbing and suspensory behaviour in orthograde apes, such as chimpanzees, has resulted in elongated hands while the thumb has remained short. Thumb_sentence_95

As a result, these primates are unable to perform the pad-to-pad grip associated with opposability. Thumb_sentence_96

However, in pronograde monkeys such as baboons, an adaptation to a terrestrial lifestyle has led to reduced digit length and thus hand proportions similar to those of humans. Thumb_sentence_97

Consequently, these primates have dexterous hands and are able to grasp objects using a pad-to-pad grip. Thumb_sentence_98

It can thus be difficult to identify hand adaptations to manipulation-related tasks based solely on thumb proportions. Thumb_sentence_99

The evolution of the fully opposable thumb is usually associated with Homo habilis, a forerunner of Homo sapiens. Thumb_sentence_100

This, however, is the suggested result of evolution from Homo erectus (around 1 mya) via a series of intermediate anthropoid stages, and is therefore a much more complicated link. Thumb_sentence_101

Modern humans are unique in the musculature of their forearm and hand. Thumb_sentence_102

Yet, they remain autapomorphic, meaning each muscle is found in one or more non-human primates. Thumb_sentence_103

The extensor pollicis brevis and flexor pollicis longus allow modern humans to have great manipulative skills and strong flexion in the thumb. Thumb_sentence_104

However, a more likely scenario may be that the specialized precision gripping hand (equipped with opposable thumb) of Homo habilis preceded walking, with the specialized adaptation of the spine, pelvis, and lower extremities preceding a more advanced hand. Thumb_sentence_105

And, it is logical that a conservative, highly functional adaptation be followed by a series of more complex ones that complement it. Thumb_sentence_106

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. Thumb_sentence_107

Walking may have been a by-product of busy hands and not vice versa. Thumb_sentence_108

HACNS1 (also known as Human Accelerated Region 2) is a 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". Thumb_sentence_109

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. Thumb_sentence_110

See also Thumb_section_19

Thumb_unordered_list_11


Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Thumb.