I Have 5 Fingers But Not Alive? 126 Most Correct Answers

Manipulation and sensation organ in the hands of humans and other primates

A finger is a limb and a type of finger, an actuation and sensory organ found in the hands of most tetrapods, including humans and other primates. Most terrestrial vertebrates have five fingers (pentadactyly).[1][2]

Fingers of land vertebrates

The five-rayed forelimbs of terrestrial vertebrates can be derived phylogenetically from the pectoral fins of fish. Within the taxa of terrestrial vertebrates, the basic pentadactyl body plan, and thus the fingers and phalanges, is subject to many variations.[3] Morphologically, the different fingers of terrestrial vertebrates are homologous. The wings of birds and bats are not homologous, they are analogous flight organs. However, the phalanges in them are homologous.[4]

Chimpanzees have lower limbs specialized for manipulation and (arguably) also have fingers on their lower limbs. In primates in general, the digits of the hand are commonly referred to as “fingers”.[5][6] Primate fingers have both fingernails and fingerprints.[7]

Studies of embryonic development in domestic chickens have shown that interdigital tissue forms between the tissues that will become the toes, which subsequently regresses through apoptosis. If apoptosis does not occur, the interdigital skin remains intact. Many animals have developed webs or skin between their fingers from this, such as Wallace’s flying frog.

human fingers

Humans normally have five fingers on each hand[11], the bones of which are called phalanges,[2] although some people have more or fewer than five fingers due to congenital disorders such as polydactyly or oligodactyly, or accidental or intentional amputation. The first digit is the thumb, followed by the index finger, middle finger, ring finger, and pinky or little finger. Depending on the definition, the thumb may or may not be called a finger.

English dictionaries describe fingers as either one of the five digits including the thumb, or one of the four digits excluding the thumb (in which case they are numbered 1 through 4, starting with the index finger closest to the thumb). 2][12]

anatomy

skeleton

Illustration depicting the bones of the human hand

The thumb (connected to the trapeze) is on one of the sides, parallel to the arm.

The palm has five bones known as the metacarpals, one for each of the five fingers. Human hands contain fourteen digital bones, also called phalanges or phalanx bones: two in the thumb (the thumb has no middle phalanx) and three in each of the four fingers. These are the end link that supports the nail, the middle link, and the base link. Where two or more of these bones meet, joints form. Each of the fingers has three joints:

Metacarpophalangeal Joint (MCP) – the joint at the base of the finger

Proximal Interphalangeal Joint (PIP) – the joint in the middle of the finger

Distal Interphalangeal Joint (DIP) – the joint closest to the tip of the finger.

Sesamoid bones are small ossified knots embedded in the tendons to provide extra leverage and relieve pressure on underlying tissues. Many exist around the palm at the bases of the digits; the exact number varies between different people.

The articulations are: interphalangeal articulations between phalangeal bones and metacarpophalangeal joints connecting the phalanges to the metacarpal bones.

muscles

[13] The precision of finger movements in space and time is highlighted in this motion-tracing of the fingers of two pianists playing the same piece (slow motion, no sound).

Each finger can flex and extend, abduct and adduct, and encircle as well. Flexion is by far the strongest movement. In humans, there are two major muscles that create the flexion of each finger and additional muscles that increase the movement. The muscle mass that moves each finger may be partially intermingled, and the tendons may be attached to one another by a network of fibrous tissue, preventing total free movement. Although each finger appears to move independently, moving one finger moves the other fingers slightly as well, referred to as finger interdependence or finger enslavement.[14][15][16]

Fingers contain no muscles (except arrector pili). The muscles that move the finger joints are in the palm and forearm. One can observe the long tendons that provide movement from the forearm muscles moving under the skin on the wrist and back of the hand.

Finger muscles can be divided into extrinsic and intrinsic muscles. The outer muscles are the long flexors and extensors. They are called extrinsic because the muscle belly is on the forearm.

The fingers have two long flexors located on the underside of the forearm. They attach to the phalanges of the fingers through tendons. The deep flexor inserts at the distal phalanx and the superficial flexor at the middle phalanx. The flexors allow the actual flexing of the fingers. The thumb has a long flexor and a short flexor in the thenar muscle group. The human thumb also has other muscles in the thenar group (opponens and abductor brevis) that move the thumb in opposition and allow gripping.

The extensors are located on the back of the forearm and are more complex than the flexors associated with the back of the fingers. The tendons unite with the interosseous and lumbrical muscles to form the extensor mechanism. The primary function of the extensors is to straighten the digits. The thumb has two extensors in the forearm; the tendons of these form the anatomical snuff-box. In addition, the index finger and the little finger have an additional extensor, which is used, for example, for pointing. The extensors are located in six separate compartments. The first compartment contains abductor pollicis longus and extensor pollicis brevis. The second compartment contains the extensors carpi radialis longus and brevis. The third compartment contains the extensor pollicis longus muscle. The extensor digitorum indicis and the extensor digitorum communis are in the fourth compartment. Extensor digiti minimi is in the fifth and extensor carpi ulnaris in the sixth.

The intrinsic muscle groups are the thenar and hypothenar muscles (thenar refers to the thumb, hypothenar refers to the little finger), the dorsal and palmar interosseous muscles (between the metacarpals), and the psoas. The lumbricals arise from the deep flexor (and are special because they have no bony origin) and attach to the mechanism of the dorsal extensor hood.

skin

Aside from the genitals, the fingertips possess the highest concentration of tactile and thermoreceptor receptors of any area of ​​human skin, making them extremely sensitive to temperature, pressure, vibration, texture, and moisture. Recent studies [when?] suggest that fingers can sense nanoscale creases on an apparently smooth surface, a level of sensitivity not previously measured.[17] This makes fingers commonly used sensory probes to determine characteristics of objects encountered in the world, making them vulnerable to injury.

The pulp of a finger is the fleshy mass on the palm of the finger end.[18]

Fingertip creases in the water

Although a widespread phenomenon, the underlying functions and mechanisms of fingertip wrinkling after water immersion are relatively unexplored. It was originally thought [by whom?] that the folds were simply the result of the skin swelling in the water, [citation needed], but it is now clear that the ridges are caused by the blood vessels constricting in response to signals from the sympathetic nervous system in response to water exposure.[19][20] One hypothesis for why this occurs, the “rain profile” hypothesis, posits that the folds may help fingers grip things when wet, possibly an adaptation from a time when humans lived in forested primate habitats with rain and dew had to do.[19] A 2013 study supporting this hypothesis found that the wrinkled fingertips allowed for better handling of wet items, but offered no benefit for handling dry items.[21] However, a 2014 study that attempted to reproduce these results failed to show any improvement in handling wet objects with wrinkled fingertips.[20]

Fingertip regrowth

Fingertips have been observed to regrow in less than 8 weeks after children have been torn off.[22] However, these fingertips do not look the same, although they look more appealing than a skin graft or a stitched fingertip. If the crack occurs below the nail, healing will not take place. This works because the distal phalanges are regenerative in youth and stem cells in the nails form new tissue that ends up as the fingertip.[23]

representation of the brain

Each finger has an ordered somatotopic representation on the cerebral cortex in somatosensory cortex area 3b,[24] part of area 1[25] and a distributed, overlapping representation in the supplemental motor area and the primary motor area.[26]

The somatosensory cortex representation of the hand is a dynamic reflection of the fingers on the outer hand: in syndactyly, people have a club hand made up of interconnected, shortened fingers. However, not only are the fingers of their hands fused, but the cortical maps of their individual fingers also form a club hand. The fingers can be surgically split to make a more useful hand. Surgeons at the Institute of Reconstructive Plastic Surgery in New York did this on a 32-year-old man with the initials OG. They touched OG’s fingers before and after surgery while using MRI brain scans. Before the surgery, the fingers mapped on his brain were tightly fused together; after that, the cards on his individual fingers actually separated and took on the layout that corresponded to a normal hand.[27]

Clinical Significance

anomalies, injuries and diseases

X-ray of type 1 syndactyly

A rare anatomical variation affects 1 in 500 people in which the individual has more than the usual number of fingers; This is called polydactyly. A human can also be born without one or more fingers or an underdevelopment of some fingers such as symbrachydactyly. Additional fingers may be functional. One person with seven fingers not only used them, but claimed that they gave him “some advantage in playing the piano.”[28]

Phalanges are often broken. A damaged tendon can lead to a significant loss of fine motor function, e.g. B. a mallet finger. You can be damaged by cold, including frostbite and non-freezing cold injuries (NFCI); and heat, including burns.

Fingers are commonly affected by diseases such as rheumatoid arthritis and gout. Diabetics often use their fingers to collect blood samples for regular glucose testing. Raynaud’s phenomenon and paroxysmal hand hematoma are neurovascular disorders affecting the fingers.

Research has linked index to ring finger length ratio to higher testosterone levels and various physical and behavioral traits such as penis length[29] and risk of developing alcohol dependence[30] or video game addiction[31] ]

etymology

The English word finger comes from the Old English finger, ultimately from the Proto-Germanic *fingraz (‘finger’). It is related to Gothic figgren, Old Norse fingers, or Old High German fingers. Linguists generally assume that *fingraz is a Ro stem derived from an earlier form *fimfe, ultimately from Proto-Indo-European *pénkʷe (‘five’).

The name Pinkie derives from the Dutch pinkje, whose origin is uncertain. In English, only the digits on the hand are called fingers. However, in some languages, the translated version of fingers can mean either the digits on the hand or the feet. In English, a digit on a foot has the unique name toe.

See also

Remarks

references

Aye-aye lemurs look a bit like gremlins, with prominent, clawed middle fingers, and these primates’ hands have fascinated scientists for years. New research shows that the tiny lemurs, which are ugly enough to inspire superstition in their native range of Madagascar, have an extra tiny fingerprinted thumb, giving these animals six fingers.

“It has the most amazing anatomy of any primate,” says Adam Hartstone-Rose, associate professor of biological sciences at North Carolina State University and lead author of a study published today in the American Journal of Physical Anthropology. “There’s nothing better on earth than an aye-aye.”

Aye-ayes are slightly larger than the average house cat, with massive ears that look like a cross between a bat and a cow. Ears are equipped for echolocation, and aye-ays are the world’s largest nocturnal primates, says Hartstone-Rose. They have huge incisors that never stop growing and are covered in wiry hair, and they have the largest brains of any living lemur species. Even their babies look like a version of the chupacabra – a blood-sucking animal in Latin American folklore.

But for all of the aye-aye’s bizarre features, her hands are perhaps her strangest attribute. The four fingers, primarily thumbs, are long and spindly. “It looks like a cat walking on spiders,” says Hartstone-Rose.

Hartstone-Rose has dissected hundreds of primates, often focusing on the anatomy of the forearm, where most of the muscles that control the fingers are located. He studies how primate muscles have adapted to different behaviors. He had long wanted to take a look at an aye-aye’s anatomy, but even dead ones weren’t easy to come by. “Aye-ayes are super rare animals. There are only about 30 of them in captivity in the United States,” he says.

Colleagues at Duke University’s Lemur Center eventually found that Hartstone’s rose was an aye-aye to study, prompting a spate of research into the relatively unknown species. The dissection revealed that fingerhens have an extra thumb, which researchers call a pseudo-thumb. The small structure of bone and cartilage can move in three different directions, similar to how human thumbs move.

Katharine Thompson, a graduate student in anthropology at Stonybrook University who studies lemurs, says the animals are very difficult to observe in the wild because of their cryptic nature. “So much of their behavior is best understood in terms of their morphology.”

According to Hartstone-Rose, pseudo-thumbs are known from a few different animals. All bears used to have these digits, but most living species have lost them while stomping on the ground. The giant panda is the only bear that still has a pseudo thumb, which it uses to grasp the bamboo it feeds on. Some rodents also evolved pseudo-thumbs to grip twigs and grass for similar reasons.

Some species of extinct aquatic reptiles also had pseudo-thumbs so they could widen their fins and improve their swimming efficiency. Some moles also have a pseudo thumb to help them dig.

But aye-ayes developed this digit for completely different reasons. Hartstone-Rose says it probably happened because their fingers and actual thumb are so specialized in finding food. Aye-ayes have very peculiar food and nutritional habits. They tap their fingers on rotting wood and use their massive ears to find cavities that indicate the tunnels built by wood-boring beetles.

“They create a mental map with their enormous brains,” says Hartstone-Rose. Once they find an intersection of these tunnels, they gnaw into the wood with large incisors. At this point, they use their long, slender middle finger with a large claw.

“They stick this horrible middle finger in the hole to fish out larvae from all sorts of directions,” says Hartstone-Rose, adding that “one of the scariest things they do is bite a hole in the top of an egg.” and using this finger to scoop it up and slurp it up.”

But all this feeding specialization means the tree-dwelling fingerhens would have trouble grasping them with their four fingers and normal thumb, which is also long and scrawny. The development of the pseudo-thumb adds a great deal of grip and exerts a force equivalent to half the lemur’s body weight.

Nancy Stevens, a professor of biomedical sciences at Ohio University who was not involved in the recent study, says Hartstone-Rose and colleagues’ research “underscores not only the profound uniqueness of the aye-aye, but also the fact that still so much remains to be done to learn more about primate adaptations to the natural world.”

Stevens, who has studied fossil lemurs on mainland Africa, says that aye digits and some of their extinct relatives are so unique that they may have colonized Madagascar independently of other lemurs. “It is important to emphasize that habitat conservation remains key to expanding knowledge and ensuring the long-term survival of the aye-aye, among numerous other animals, plants and other fascinating aspects of Madagascar’s incredible biodiversity,” she says.

Aye-ayes are listed as Vulnerable by the International Union for Conservation of Nature, mainly due to deforestation and habitat loss, although its creepy appearance also makes it a target for kills in some parts of Madagascar. Some local beliefs consider the creature taboo, says Hartstone-Rose.

But since Aye-Ays are the only living member of their family, he says their extinction would be “a terrible loss to science.”

It’s the kind of question that usually pops up in kindergarten classes or 2 a.m. college dormitories after metabolizing popular tetrahydrocannabinol derivatives:

Why do we even have 10 fingers and 10 toes?

As Lissette Padilla explains in this DNews story, the truth is we don’t really know. But scientists have some pretty good guesses.

When in doubt, consult a theoretical neurobiologist, we always say. In 2001, author and researcher Mark Changizi proposed a mathematical hypothesis called Limb Law. His equation tries to figure out the ideal number of limbs an animal needs depending on its environment and body size.

Applying the limb law to 190 animal species from seven different phyla, Changizi found his math held up — and the researcher surmises the same equation might explain why we have 10 fingers. In this case, our hands are the “animal” and our fingers are the “limbs.” Crunch the number and you get 4.71 fingers per hand. Mother Nature, it seems, rounds up.

RELATED: ‘Yeti Finger’ mystery solved

However, ask any evolutionary biologist and you’ll likely get a much simpler answer: We have 10 fingers and 10 toes because somewhere in our species’ past Darwinian migrations those numbers gave us an evolutionary advantage. If events had turned out differently, we might have eight fingers and twelve toes.

An interesting side note: Harvard geneticist Cliff Tabin, who specializes in the development of vertebrate limbs, suggests that if we ever evolved a sixth finger, it would likely grow out of our wrists like a panda.

Speaking of genetics, there’s another way to approach the question. In 2014, a research team led by James Sharpe of the Center for Genomic Regulation in Barcelona, ​​Spain, found that fingers and toes are patterned by three specific embryonic molecules – Bmp, Wnt and Sox9.

Incidentally, the basis for this discovery was a theory first put forward in 1952 by science superhero Alan Turing. This guy has thought of everything.

Continue reading:

Science Daily: A mathematical theory proposed by Alan Turing in 1952 can explain the formation of fingers

National Geographic: How Did You Get Five Fingers?

Discovery News: Human hands more primitive than chimpanzee hands

The thumb is the shortest and thickest finger on the human hand. The anatomy and function of the thumb is slightly different from the other fingers, so some people may not think of it as a finger. However, as one of the five terminal joints of the hand, most medical guides refer to the thumb as a finger. Also known as the pollex or digitus primus manus, the thumb is anatomically distinct from the other four fingers of the hand. These differences allow the thumb to move and function differently than the rest of the fingers. Humans and other primates have the distinct ability to use their hands with greater awareness and dexterity. Because the thumb is opposable, people can position it against other fingers of the hand to manipulate or hold an object. Some evidence suggests that this ability accounts for about 40% of hand function in the thumb. This article describes the function and anatomy of the thumb and common conditions that can affect this digit.

Is the thumb a finger? Share on Pinterest mrs/Getty Images The thumb and the rest of the fingers are appendages or fingers of the hand. While the thumb is distinctly different from the other four fingers, many medical professionals consider it a finger due to differences in size, bone, joints, and function. This reflects the Latin name digitus primus manus, which translates to “first finger of the hand”. However, some people prefer to say they have 10 fingers or eight fingers and two thumbs rather than 10 fingers.

Thumb Anatomy Like the rest of the hand, the thumb is made up of many different structures that allow for movement and dexterity. Each finger has three bones, or phalanges, except for the thumb, which has two phalanges instead. The first bone – the terminal phalanx – extends from the tip of the thumb to the knuckle. The second bone – proximal phalanx – then extends from the knuckle to the base of the thumb. The thumb then connects to the hand bone or thumb metacarpal. Because the thumb has only two phalanxes, it has only one joint, while the other fingers have two joints. The interphalangeal (IP) joint is located at the tip of the thumb, just before the fingernail begins, and is similar to the distal interphalangeal joint of the other four fingers. This joint allows a person to flex the tip of their thumb. There are also two other joints where the thumb connects to the hand, which aids in movement. The metacarpophalangeal (MP) joint is where the metacarpal meets the phalanges. This joint allows a person to flex and extend the thumb. The carpometacarpal (CMC) joint is where the metacarpal meets the carpal or wrist bones. This joint allows people to move the thumb away from and towards the hand. Nine muscles contribute to the movement of the thumb. Experts can divide these into two groups: the extrinsic muscles in the forearm and the intrinsic muscles in the hand.

The four external muscles include: Abductor pollicis longus

extensor pollicis brevis

extensor pollicis longus

Flexor pollicis longus The five intrinsic muscles, also known as the thenar muscle group, consist of: Abductor pollicis brevis

flexor pollicis brevis

opponens pollicis

first dorsal interosseous

Adductor pollicis

Thumb Function and Purpose The thumb’s primary function is to work either with or against the other fingers to manipulate objects and perform actions such as pinching or grasping. Along with the other fingers of the hand, the thumb plays a crucial role in performing coordinated hand movements for precise tool use. Its unique position and proportional length allows the thumb to firmly touch the other fingers and other objects. The opposing nature of the thumb and thenar muscles provide the dexterity and strength to perform these actions. There are two main types of grips: power and precision grips. Power grips require the use of the whole hand for greater stability and power on larger objects. The thumb’s ability to position itself opposite the fingers allows people to hold and move heavy objects in a controlled manner, such as when lifting weights at the gym. A precision grip requires only the fingertip to apply force between the fingertips on smaller objects. The thumb works opposite one or more fingertips to allow the fingers to grasp objects. This type of grip is important for moving small and delicate objects, e.g. B. when writing with a pen.

Common Thumb Disorders Many people use their thumbs every day to perform a variety of functions. When injured, people can experience thumb pain, which can make it difficult to perform everyday activities. Common conditions that can affect the thumb include: Arthritis Thumb arthritis, also known as basal thumb arthritis, is the second most common arthritis of the hand. It affects the CMC joint, the thumb’s unique saddle joint. The cause is often age-related cartilage wear. However, diseases that cause joint degeneration, such as lupus and rheumatoid arthritis, can also cause arthritis in the CMC joint. DeQuervain’s tenosynovitis (DQT) De Quervain’s tenosynovitis is a painful condition often caused by overuse. A person with DQT usually feels pain on the side of the wrist near the base of the thumb. The pain can worsen if a person raises their thumb while bending their wrist, such as when bending their wrist. B. when opening a glass, writing a text message or wringing out a towel. DQT is a common hand and wrist injury. It often occurs during and after pregnancy. For this reason, the condition is also commonly known as mother’s thumb. Learn more about exercises for DQT here. Carpal Tunnel Syndrome Carpal tunnel syndrome occurs when pressure pinches the median nerve in the carpal tunnel. This nerve gives sensation to the palmar side of the thumb to allow the thumb to flex and resist. In milder cases, a person may feel tingling and mild pain in the wrist and thumb. In severe cases, the thumb muscles can weaken and shrink. Skier’s Thumb Skier’s thumb results from a sprain or tear in the ulnar collateral ligament (UCL) of the thumb. A person often suffers this injury when falling from an outstretched hand, such as falling on a ski slope with one hand strapped to a ski pole. Improper healing can cause pain and weakness during pinching activities. Trigger finger Trigger finger is a condition where a person’s finger locks up when they try to straighten or bend it. While it can affect any finger, most people experience the condition on the ring finger, pinky, or thumb. If a person needs treatment, it may include avoiding activities and taking medication. In more severe cases, it may be necessary for a person to have surgery.

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Limb at the end of an arm or foreleg

A hand is a tangible, multi-fingered appendage found at the end of the forearm or forelimb of primates such as humans, chimpanzees, monkeys, and lemurs. Some other vertebrates, such as the koala (which has two opposing thumbs on each “hand” and fingerprints very similar to human fingerprints) are often described as having “hands” instead of paws on their front legs. The raccoon is usually described as having “hands”, although opposable thumbs are absent.[1]

Some evolutionary anatomists use the term hand to refer more generally to the finger appendage on the forelimb—for example, in the context of whether the three fingers of the bird hand involved the same homologous loss of two fingers as in the dinosaur hand. 2]

The human hand typically has five fingers: four fingers plus a thumb;[3][4] but these are often referred to collectively as five fingers, counting the thumb as one of the fingers.[3][5][] 6] It has 27 bones, excluding the sesamoid bone, the number of which varies from person to person,[7] 14 of which are the phalanges (proximal, intermediate and distal) of the fingers and thumb. The metacarpal bones connect the fingers and carpal bones of the wrist. Each human hand has five metacarpal bones[8] and eight carpal bones.

Fingers contain some of the densest arrays of nerve endings in the body and are the richest source of tactile feedback. They also have the greatest body positioning ability; Therefore, the sense of touch is closely linked to the hands. Like other paired organs (eyes, feet, legs), each hand is dominantly controlled by the opposite brain hemisphere, so handedness—the preferred hand choice for one-handed activities like writing with a pencil—reflects individual brain function.

In humans, hands play an important role in body language and sign language. Likewise, the ten digits of two hands and the twelve phalanxes of four fingers (touchable with the thumb) have produced number systems and arithmetic techniques.

structure

Many mammals and other animals have prehensile appendages that are similar in shape to a hand, such as paws, talons, and talons, but these are not scientifically considered prehensile hands. The scientific use of the term hand in this sense to distinguish the ends of the front paws from the hind paws is an example of anthropomorphism. The only true prehensile hands occur in the primate mammalian order. Hands must also have opposable thumbs, as described later in the text.

The hand is at the distal end of each arm. Monkeys and monkeys are sometimes described as having four hands because the toes are long and the hallux is opposable and looks more like a thumb, allowing the feet to be used as hands.

The word “hand” is sometimes used by evolutionary anatomists to refer to the appendages of digits on the forelimb, for example when exploring the homology between the three digits of the bird’s hand and the dinosaur’s hand.[2]

An adult human hand weighs about a pound.[9]

areas

Human hand parts

Areas of the human hand include:

The palm (volar), which is the central area of ​​the front part of the hand, is superficial to the metacarpal bone. The skin in this area contains dermal papillae to increase friction, like those found on fingers and used for fingerprinting.

(Volar), this is the central region of the front part of the hand that is superficial to the metacarpal bone. The skin in this area contains dermal papillae to increase friction, like those found on fingers and used for fingerprinting. The opisthenar area (dorsal) is the corresponding area at the back of the hand.

Area (dorsal) is the corresponding area at the back of the hand. The heel of the hand is the area in front of the bases of the metacarpal bones, located in the proximal part of the palm. It is the area that takes the most pressure when the palm is used for support, e.g. B. in handstand.

Five fingers are attached to the hand, specifically with a nail attached to the end instead of the normal claw. The four fingers can be folded over the palm, allowing for grasping objects. Each finger, starting with the one closest to the thumb, has a slang name to distinguish it from the others:

Index finger, index finger, index finger or 2nd digit

Middle finger or long finger or 3rd digit

Ring finger or 4th digit

pinky, pinky, pinky, baby finger, or 5th digit

The thumb (connected to the first metacarpal and the trapezium) is on one of the sides, parallel to the arm. A reliable way to identify human hands is the presence of opposable thumbs. Opposable thumbs are identified by the ability to be opposed to the fingers, a muscular action known as opposition.

Bone

bones of the human hand

Hand bone animation (metacarpal motion is exaggerated except for the thumb)

The skeleton of the human hand consists of 27 bones:[10] The eight short carpal bones of the wrist are organized into a proximal row (navicular, lunate, triquetral, and pisiform) that articulates with the bones of the forearm, and a distal row (trapezium, trapezium, capitate, and hamate) articulating with the bases of the five metacarpal bones of the hand. The heads of the metacarpals, in turn, articulate with the bases of the proximal phalanx of the fingers and thumb. These articulations with the fingers are the metacarpophalangeal joints, known as the knuckles. At the palm of the first metacarpophalangeal joints are small, almost spherical bones called sesamoid bones. The fourteen phalanxes make up the fingers and thumb and are numbered I-V (thumb to little finger) when the hand is viewed from an anatomical position (palm up). The four fingers each consist of three phalanx bones: proximal, middle and distal. The thumb consists only of a base and end phalanx.[11] Together with the phalanges of the fingers and thumb, these metacarpal bones form five rays, or multi-linked chains.

Because of the addition of supination and pronation (rotation about the axis of the forearm) to the wrist’s two axes of motion, the ulna and radius are sometimes considered part of the hand skeleton.

There are numerous sesame bones in the hand, small ossified knots embedded in tendons; The exact number varies from person to person:[7] While a pair of sesamoids can be found at virtually all basal joints of the thumb, sesamoids are also common at the interphalangeal joint of the thumb (72.9%) and at the basal joints of the pinky finger (82.5%). ) and index finger (48%). In rare cases, sesame bones have been found in all metacarpophalangeal joints and all distal interphalangeal joints except that of the long finger.

The articulations are:

Sheets

Red: one of the oblique arches

Brown: one of the longitudinal arcs of the numerals

Dark green: transverse carpal arch

Light green: transverse medial hand arcs Red: one of the oblique arcs Brown: one of the longitudinal arcs of the numerals Dark green: transverse medial carpal arc Light green: transverse median arc

The fixed and mobile parts of the hand adapt to various everyday tasks by forming bony arches: longitudinal arches (the rays formed by the phalanges and associated metacarpals), transverse arches (formed by the carpal bones and the distal ends of the metacarpals). ) and oblique arcs (between thumb and four fingers):

Of the longitudinal arches or rays of the hand, that of the thumb is the most mobile (and least elongated). While the ray formed by the little finger and associated metacarpal bone still offers some mobility, the remaining rays are rigid. However, the phalangeal joints of the index finger offer some independence to its finger due to the arrangement of its flexor and extensor tendons.[12]

The carpal bones form two transverse rows, each forming a concave arch on the palmar side. Since the proximal arch must simultaneously conform to the articular surface of the radius and to the distal carpal row, it is inherently flexible. In contrast, the capitate, the “keystone” of the distal arch, moves with the metacarpals and the distal arch is therefore rigid. The stability of these arches depends more on the ligaments and capsules of the wrist than on the interlocking shapes of the carpal bones, and the wrist is therefore more stable in flexion than in extension.[12] The distal carpal arch affects the function of the CMC joints and hands, but not the function of the wrist or the proximal carpal arch. The ligaments that support the distal carpal arches are the transverse carpal ligament and the intercarpal ligaments (also transversely oriented). These ligaments also form the carpal tunnel and contribute to the deep and superficial palmar arches. Several muscle tendons that attach to the TCL and distal carpal bones also help maintain the carpal arch.[13]

Compared to the carpal arches, the arch formed by the distal ends of the metacarpals is flexible due to the mobility of the peripheral metacarpals (thumb and little finger). As these two metacarpal bones approach each other, the palmar groove deepens. The most central metacarpal (middle finger) is the most rigid. It and its two neighbors are connected to the carpus by the interlocking shapes of the metacarpal bones. The thumb metacarpal articulates only with the trapezium and is therefore fully independent, while the fifth metacarpal (pinky) is semi-independent from the fourth metacarpal (ring finger), which forms a transitional element to the fifth metacarpal.[12]

Together with the thumb, the four fingers form four oblique arcs, of which the arc of the index finger is functionally the most important, particularly for precise grip, while the arc of the little finger contributes an important locking mechanism for power grip. The thumb is undoubtedly the “master digit” of the hand, giving value to all the other fingers. Together with the index and middle fingers, it forms the dynamic tridactyl configuration responsible for most holds that do not require strength. The ring and pinky fingers are more static, a reserve ready to interact with the palm when great force is required.[12]

muscles

Muscles and other structures of the wrist and palm

The muscles acting on the hand can be divided into two groups: the extrinsic and the intrinsic muscle group. The outer muscle groups are the long flexors and extensors. They are called extrinsic because the muscle belly is on the forearm.

intrinsic

The intrinsic muscle groups are the thenar (thumb) and hypothenar (pinky) muscles; the interosseous muscles (four dorsal and three volar) arising between the metacarpals; and the lumbar muscles, which arise from the deep flexor (and are special because they are not of bony origin) to be deployed at the dorsal extensor hood mechanism.[14]

extrinsic

Wrist extensors (back of the hand)

The fingers have two long flexors located on the underside of the forearm. They attach to the phalanges of the fingers through tendons. The deep flexor inserts at the distal phalanx and the superficial flexor at the middle phalanx. The flexors allow the actual flexing of the fingers. The thumb has a long flexor and a short flexor in the thenar muscle group. The human thumb also has other muscles in the thenar group (opponens and abductor brevis) that move the thumb in opposition and allow gripping.

The extensors are located on the back of the forearm and are more complex than the flexors associated with the back of the fingers. The tendons unite with the interosseous and lumbrical muscles to form the extensor mechanism. The primary function of the extensors is to straighten the digits. The thumb has two extensors in the forearm; the tendons of these form the anatomical snuff-box. In addition, the index finger and the little finger have an additional extensor, which is used, for example, for pointing. The extensors are located in 6 separate compartments.

The first four compartments are located in the grooves on the back of the inferior side of the radius, while the 5th compartment lies between the radius and the ulna. The 6th compartment is located in the groove on the back of the inferior side of the ulna.

nerve supply

Cutaneous innervation of the upper extremity

The hand is innervated by the radial, median, and ulnar nerves.

engine

The radial nerve supplies the finger extensors and thumb abductor, which are the muscles that extend from the wrist and the metatarsophalangeal joints (knuckles); and that hijacks and lengthens the thumb. The median nerve supplies the flexors of the wrist and digits, the abductors and opponens of the thumb, the first and second lumbrical nerves. The ulnar nerve supplies the rest of the intrinsic muscles of the hand.[15]

All muscles of the hand are innervated by the brachial plexus (C5–T1) and can be classified by innervation:[16]

Sensory

The radial nerve supplies the skin on the back of the hand from the thumb to the ring finger and the dorsal aspects of the index, middle, and semiring fingers to the proximal interphalangeal joints. The median nerve supplies the palm of the thumb, index, middle, and semiring fingers. Dorsal branches innervate the distal phalanges of the index, middle, and semiring fingers. The ulnar nerve supplies the ulnar third of the hand, both on the palm and on the back of the hand, as well as the little and half ring fingers.[15]

There is considerable variation in this general pattern, except for the little finger and the volar surface of the index finger. For example, in some individuals the ulnar nerve supplies the entire ring finger and the ulnar aspect of the middle finger, while in others the median nerve supplies the entire ring finger.[15]

blood supply

hand arteries

The hand is supplied with blood from two arteries, the ulnar artery and the radial artery. These arteries form three arches over the dorsal and palmar aspects of the hand, the dorsal carpal arch (over the back of the hand), the deep palmar arch, and the superficial palmar arch. Together, these three arches and their anastomoses supply oxygenated blood to the palm, fingers, and thumb.

The hand is emptied through the dorsal venous network of the hand, with deoxygenated blood leaving the hand via the cephalic and basilic veins.

skin

The glabrous (hairless) skin on the front of the hand, the palm, is relatively thick and can be flexed along the flex lines of the hand, where the skin is firmly attached to the underlying tissue and bones. Compared to the rest of the skin on the body, the palms of the hands (like the soles of the feet) are usually lighter – and in black people, much lighter than the other side of the hand. In fact, genes specifically expressed in the dermis of palmoplantar skin inhibit melanin production and hence the ability to tan and promote thickening of the stratum lucidum and stratum corneum layers of the epidermis. All areas of skin involved in grasping are covered with papillae ridges (fingerprints) that act as friction pads. In contrast, the hairy skin on the back is thin, soft, and pliable, allowing the skin to spring back when fingers are stretched. On the dorsal side, the skin can be moved up to 3 cm over the hand; an important input are the cutaneous mechanoreceptors.[17]

The web of the hand is a “fold of skin connecting the fingers”.[18] These webs, located between each set of digits, are called skin folds (interdigital folds or plica interdigitalis). They are defined as “any of the skin folds or vestigial webs between the fingers and toes.”[19]

variation

The ratio of the length of the index finger to the length of the ring finger in adults is influenced by the level of exposure to male sex hormones in the embryo in utero. This digit ratio is less than 1 for both sexes, but is lower for men than for women on average.

Clinical Significance

X-ray of the left hand of a ten-year-old boy with polydactyly

A number of genetic disorders affect the hand. Polydactyly is the presence of more than the usual number of fingers. One of the disorders that can cause this is Catel-Manzke Syndrome. The fingers may be fused in a disorder known as syndactyly. Or one or more middle fingers are missing — a condition known as ectrodactyly. Also, some people are born without one or both hands (Amelia). Hereditary multiple exostoses of the forearm – also known as hereditary multiple osteochondromas – are another cause of hand and forearm deformities in children and adults.[20]

There are several skin conditions that can affect the hand, including the nails.

The autoimmune disease rheumatoid arthritis can affect the hand, particularly the finger joints.

Some conditions can be treated with hand surgery. These include carpal tunnel syndrome, a painful condition of the hand and fingers caused by compression of the median nerve, and Dupuytren’s contracture, a condition in which the fingers bend toward the palm and cannot straighten. Similarly, injury to the ulnar nerve can result in a condition where some of the fingers cannot flex.

A common fracture of the hand is a scaphoid fracture – a fracture of the scaphoid, one of the carpal bones. This is the most common carpal bone fracture and can be slow to heal due to limited blood flow to the bone. There are different types of fractures at the base of the thumb; These are known as Rolando’s fracture, Bennet’s fracture, and Gamekeeper’s thumb. Another common fracture, known as a Boxer’s fracture, affects the neck of a metacarpal. You can also have a broken finger.

evolution

The prehensile hands and feet of primates evolved from the agile hands of half-tree shrews, which lived about . This evolution has been accompanied by important changes in the brain and the relocation of the eyes to the front of the face, which together allow for the muscle control and stereoscopic vision required for controlled grasping. Also known as a power grip, this grip is complemented by the precise grip between the thumb and distal fingertips made possible by the opposable thumbs. For example, Hominidae (great apes including humans) acquired an upright bipedal posture that freed the hands from the task of locomotion and paved the way for the precision and range of motion of the human hand.[21] Functional analyzes of features unique to the modern human hand have shown that they are consistent with the stresses and demands associated with the effective use of Paleolithic stone tools.[22] It is possible that the refinement of the biped posture in the earliest hominids evolved to facilitate the use of the torso as a lever to accelerate the hand.[23]

While the human hand exhibits unique anatomical features, including a longer thumb and fingers that can be more individually controlled, the hands of other primates are anatomically similar and the dexterity of the human hand cannot be explained by anatomical factors alone. The neural machinery underlying hand movements is an important factor; Primates have evolved direct connections between neurons in cortical motor areas and spinal motor neurons, giving the cerebral cortex monosynaptic control over the motor neurons of the hand muscles; Placing the hands “closer” to the brain.[24] The recent development of the human hand is thus a direct result of the development of the central nervous system, and the hand is therefore a direct tool of our consciousness – the main source of differentiated tactile sensations – and a precise working organ enabling gestures – the expression of our personality.[25 ]

A gorilla, a large primate with small thumbs, and the hand skeleton of Ardipithecus ramidus, a Pliocene large primate with relatively human-like thumbs

However, there are some primitive features in the human hand, including pentadactyly (having five fingers), the hairless skin of the palm and fingers, and the central bone found in human embryos, prosimians, and great apes. In addition, the precursors to the intrinsic hand muscles are present in the earliest fish, suggesting that the hand evolved from the pectoral fin and is thus evolutionarily much older than the arm.[21]

The proportions of the human hand are plesiomorphic (shared between ancestral and extant primate species); The elongated thumbs and short hands resemble the hand proportions of Miocene monkeys more than those of modern primates.[26] Humans did not evolve from ankle-walking apes,[27] and chimpanzees and gorillas independently acquired elongated metacarpal bones as part of their adaptation to their mode of locomotion.[28] Several primitive hand features are still present in the hands of Australopithecus, Paranthropus, and Homo floresiensis, most likely present in the last common ancestor of chimpanzee-man (CHLCA) and absent in modern humans. This suggests that the inferred changes in modern humans and Neanderthals did not evolve before or after the appearance of the earliest Acheulian stone tools, and that these changes are associated with tool-related tasks beyond those observed in other hominins.[29] The thumbs of Ardipithecus ramidus, an early hominin, are almost as robust as humans, so this may be a primitive feature, while the palms of other extant higher primates are so elongated that some of the thumb’s original function has been lost (before especially in very arboreal primates like the spider monkey). Thus, in humans, the big toe is more derived than the thumb.[28]

There is a hypothesis that the shape of the modern human hand is particularly conducive to the formation of a compact fist, presumably for combat purposes. The fist is compact, making it effective as a weapon. It also provides protection for the fingers.[30][31][32] However, this is not generally considered to be one of the primary selective pressures acting on hand morphology throughout human evolution, with tool use and production thought to be far more influential.[22]

Additional Pictures

Illustration of hand and wrist bones.

bones of the left hand. volar surface.

bones of the left hand. back surface.

Static physical characteristics of the adult human hand.

X-ray shows joints

Anatomy of the hand bones

See also

This article uses anatomical terminology

It’s the kind of question that usually pops up in kindergarten classes or 2 a.m. college dormitories after metabolizing popular tetrahydrocannabinol derivatives:

Why do we even have 10 fingers and 10 toes?

As Lissette Padilla explains in this DNews story, the truth is we don’t really know. But scientists have some pretty good guesses.

When in doubt, consult a theoretical neurobiologist, we always say. In 2001, author and researcher Mark Changizi proposed a mathematical hypothesis called Limb Law. His equation tries to figure out the ideal number of limbs an animal needs depending on its environment and body size.

Applying the limb law to 190 animal species from seven different phyla, Changizi found his math held up — and the researcher surmises the same equation might explain why we have 10 fingers. In this case, our hands are the “animal” and our fingers are the “limbs.” Crunch the number and you get 4.71 fingers per hand. Mother Nature, it seems, rounds up.

RELATED: ‘Yeti Finger’ mystery solved

However, ask any evolutionary biologist and you’ll likely get a much simpler answer: We have 10 fingers and 10 toes because somewhere in our species’ past Darwinian migrations those numbers gave us an evolutionary advantage. If events had turned out differently, we might have eight fingers and twelve toes.

An interesting side note: Harvard geneticist Cliff Tabin, who specializes in the development of vertebrate limbs, suggests that if we ever evolved a sixth finger, it would likely grow out of our wrists like a panda.

Speaking of genetics, there’s another way to approach the question. In 2014, a research team led by James Sharpe of the Center for Genomic Regulation in Barcelona, ​​Spain, found that fingers and toes are patterned by three specific embryonic molecules – Bmp, Wnt and Sox9.

Incidentally, the basis for this discovery was a theory first put forward in 1952 by science superhero Alan Turing. This guy has thought of everything.

Continue reading:

Science Daily: A mathematical theory proposed by Alan Turing in 1952 can explain the formation of fingers

National Geographic: How Did You Get Five Fingers?

Discovery News: Human hands more primitive than chimpanzee hands

Have you ever wondered why you have five fingers? We’ve been using our hands for more than twenty years and you may have the same question I have about why we have 5 fingers.

To better understand why, let’s look at a special case of “polydactyly.” Briefly, polydactyly refers to the situation where one hand (or foot) has more than five fingers. There are two types of polydactyly related to type A and type B. In Type A, the extra finger can function properly like other fingers and have full bone structure. In Type B, the extra finger is non-functional and appears to be “floating”.

To answer our question at the beginning, we need to know the cause of Type A. The doubling of the entire finger is due to an abnormal function of a gene called the “sonic hedgehog” gene or “SHH” gene, which plays an important role in the growth of digits on the limbs and the organization of the brain. During the fifth week of embryonic development, the SHH gene will synthesize a signaling protein called sonic hedgehog protein, which acts on the ulnar side of the “hand” (the black dot in the picture) and fingers begin to form.

How can a “thumbs up” sign help us remember five processes that influence evolution?

There is a five-finger trick from TEDEd to understand and remember the five processes – small population, non-random mating, mutations, gene flow, adaptation – that affect evolution (i.e. the changes in a population’s gene pool from generation to generation ). Narrated by Paul Andersen and animated by Alan Foreman, this video is truly excellent.

Related links: What is microevolution?

Then watch more videos about evolution including Why dogs have floppy ears, an animated story, Evidence of Evolution that you can find on your body, and Evolution 101 & how natural selection works.