Okay, it is true that a thumb has two joints and other fingers have two, it's just.. it's strange to point out such a thing, maybe you watched enough of the wiggles to prove me wrong, but when reffering to a hand nobody points out four finger and a thumb, it is perfectly normal to say a hand has five fingers. If we are going to point out each finger why not call it; Thumb, index, forefinger, middle finger, ring finger and pinky, or why not just use the medical terms; Pollex, digitus secundus manus, digitus tertius, digitus medius, digitus annularis, diginus minimus manus. Even in medical terms the count starts with thumb(Pollex) and the next is the index finger which goes by (digitus (secundus meaning second) manus.
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-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 distal phalanx than in the 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 fingers.
Opposition and apposition
A bonobo "fishing" for termites, an example of incomplete/"untrue" opposition.[2]
In humans, opposition and apposition are two movements unique to the thumb but these words are not synonyms:
Primatologists and hand research pioneers J. Napier and P. 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." For this true, pulp-to-pulp opposition to be possible, the thumb must rotate about its long axis (at the carpometacarpal joint).[3] 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.[4] Some anatomists[5] 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 and opponens digiti minimi respectively).
Other researchers use another definition,[4] 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.
Human anatomy
Skeleton
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. This latter bone articulates with the distal phalanx at the interphalangeal joint. Additionally, there are two sesamoid bones at the metacarpophalangeal joint.
Muscles
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. 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.[6]
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.[7]
Extrinsic
Flexor pollicis longus (left) and deep muscles of dorsal forearm (right)
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. 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 interosseus branch of the median nerve (C7-C8)[8]
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 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).[9]
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 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).[9]
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).[9]
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
Thenar (left) and dorsal interossei (right) muscles
There are three thenar muscles:
The abductor pollicis brevis (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 (C8-T1).[10]
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, and 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).[10]
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.[10]
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).[10]
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
If you are reffering to joints in our fingers, sure, it's a thumb, but a palm has five fingers. Flexible thumbs are what differs us from animals, that's why people don't reffer to them as a seperate body part.
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