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The thumb is the first digit of the hand, next to the index finger. When a person is standing in the medical anatomical position (where the palm is facing to the front), the thumb is the outermost digit. The Medical Latin English noun for thumb is (compare hallux for big toe), and the corresponding adjective for thumb is .
Linguistically, it appears that the original sense was the first 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.
The thumb shares the following with each of the other four fingers:
The thumb contrasts with each of the other four fingers by being the only one that:
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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 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). Arguably, this definition was chosen to underline what is unique to the human thumb.
The spider monkey compensates for being virtually thumbless by using the hairless part of its long, prehensile tail for grabbing objects. In 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.
Darwinius, an Eocene primate transitional fossil between prosimian and simian, had hands and feet with highly flexible digits featuring opposable thumbs and halluces.
Additionally, in many polydactyl cats, both the innermost toe and outermost toe (Little finger) may become opposable, allowing the cat to perform more complex tasks.
In addition to these, some other dinosaurs may have had partially or completely opposed toes in order to manipulate food and/or grasp prey.
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.
Three dorsal forearm muscles act on the thumb:
The 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. 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 (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 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).
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. 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).
The abductor pollicis brevis (APB) originates on the Scaphoid bone 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 (C8-T1).
The 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, Trapezoid bone, and Capitate bone. 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 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.
Other muscles involved are:
The adductor pollicis also has two heads. The transversal head originates along the entire third metacarpal bone, while the oblique head originates on the carpal bones proximal to the third metacarpal. The muscle is inserted onto the ulnar sesamoid bone of the metacarpophalangeal joint. It adducts the thumb, and assists in opposition and flexion. It is innervated by the deep branch of the ulnar nerve (C8-T1).
The first dorsal 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.
It has been suggested that the variation is an autosomal recessive trait, called a hitchhiker's thumb, with homozygous carriers having an angle close to 90°. However this theory has been disputed, since the variation in thumb angle is known to fall on a continuum and shows little evidence of the bi-modality seen in other recessive genetic traits.
Other variations of the thumb include brachydactyly type D (which is a thumb with a congenitally short distal phalanx), a triphalangeal thumb (which is a thumb which has 3 phalanges instead of the usual two), and polysyndactyly (which is a combination of radial polydactyly and syndactyly).
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 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 phalanges 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 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.
Opposable thumbs are shared by some , including most Catarrhini. The climbing and suspensory behaviour in orthograde apes, such as chimpanzees, has resulted in elongated hands while the thumb has remained short. As a result, these primates are unable to perform the pad-to-pad grip associated with opposability. However, in pronograde monkeys such as , an adaptation to a terrestrial lifestyle has led to reduced finger length and thus hand proportions similar to those of humans. Consequently, these primates have dexterous hands and are able to grasp objects using a pad-to-pad grip. It can thus be difficult to identify hand adaptations to manipulation-related tasks based solely on thumb proportions.
The evolution of the fully opposable thumb is usually associated with Homo habilis, a forerunner of Homo sapiens.: "In the pollex is well developed and fully opposable and the hand is capable not only of a power grip but of, at least, a simple and usually well developed precision grip." This, however, is the suggested result of evolution from Homo erectus (around 1 mya) via a series of intermediate simian stages, and is therefore a much more complicated link.
Modern humans are unique in the musculature of their forearm and hand. Yet, they remain autapomorphic, meaning each muscle is found in one or more non-human primates. The extensor pollicis brevis and flexor pollicis longus allow modern humans to have great manipulative skills and strong flexion in the thumb.
However, a more likely scenario may be that the specialized precision gripping hand (equipped with opposable thumb) of Homo habilis preceded walking, with the specialized adaptation of the spine, pelvis, and lower extremities preceding a more advanced hand. And, it is logical that a conservative, highly functional adaptation be followed by a series of more complex ones that complement it. With Homo habilis, an advanced grasping-capable hand was accompanied by facultative bipedalism, possibly implying, assuming a co-opted evolutionary relationship exists, that the latter resulted from the former as obligate bipedalism was yet to follow. Walking may have been a by-product of busy hands and not vice versa.
HACNS1 (also known as Human Accelerated 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". 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.
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