Finger (other)
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A finger is a prominent
digit Digit may refer to: Mathematics and science * Numerical digit, as used in mathematics or computer science ** Hindu-Arabic numerals, the most common modern representation of numerical digits * Digit (anatomy), the most distal part of a limb, such ...
on the
forelimb A forelimb or front limb is one of the paired articulated appendages (limbs) attached on the cranial ( anterior) end of a terrestrial tetrapod vertebrate's torso. With reference to quadrupeds, the term foreleg or front leg is often used instead. ...
s of most tetrapod vertebrate animals, especially those with
prehensile Prehensility is the quality of an appendage or organ (anatomy), organ that has Adaptation (biology), adapted for grasping or holding. The word is derived from the Latin term ''prehendere'', meaning "to grasp". The ability to grasp is likely der ...
extremities (i.e.
hand A hand is a prehensile, multi-fingered appendage located at the end of the forearm or forelimb of primates such as humans, chimpanzees, monkeys, and lemurs. A few other vertebrates such as the koala (which has two opposable thumbs on each "h ...
s) such as humans and primates. Most tetrapods have five digits (
pentadactyly In biology, dactyly is the arrangement of digits (fingers and toes) on the hands, feet, or sometimes wings of a tetrapod animal. It comes from the Greek word δακτυλος (''dáktylos'') = "finger". Sometimes the ending "-dactylia" is used ...
), Chambers 1998 p. 603 Oxford Illustrated pp. 311, 380 and short digits (i.e. significantly shorter than the metacarpal/ metatarsals) are typically referred to as toes, while those that are notably elongated are called fingers. In humans, the fingers are flexibly articulated and opposable, serving as an important organ of tactile sensation and fine movements, which are crucial to the dexterity of the hands and the ability to
grasp A grasp is an act of taking, holding or seizing firmly with (or as if with) the hand. An example of a grasp is the handshake, wherein two people grasp one of each other's like hands. In zoology particularly, prehensility is the quality of an ap ...
and manipulate objects.


Land vertebrate fingers

As terrestrial vertebrates were evolved from lobe-finned fish, their forelimbs are
phylogenetically In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...
equivalent to the pectoral fins of fish. Within the taxa of the terrestrial vertebrates, the basic pentadactyl plan, and thus also the metacarpals and
phalanges The phalanges (singular: ''phalanx'' ) are digital bones in the hands and feet of most vertebrates. In primates, the thumbs and big toes have two phalanges while the other digits have three phalanges. The phalanges are classed as long bones. ...
, undergo many variations. Morphologically the different fingers of terrestrial vertebrates are homolog. The wings of birds and those of
bats Bats are mammals of the order Chiroptera.''cheir'', "hand" and πτερόν''pteron'', "wing". With their forelimbs adapted as wings, they are the only mammals capable of true and sustained flight. Bats are more agile in flight than most bi ...
are not homologous, they are analogue flight organs. However, the
phalanges The phalanges (singular: ''phalanx'' ) are digital bones in the hands and feet of most vertebrates. In primates, the thumbs and big toes have two phalanges while the other digits have three phalanges. The phalanges are classed as long bones. ...
within them are homologous.
Chimpanzee The chimpanzee (''Pan troglodytes''), also known as simply the chimp, is a species of great ape native to the forest and savannah of tropical Africa. It has four confirmed subspecies and a fifth proposed subspecies. When its close relative th ...
s have lower limbs that are specialized for manipulation, and (arguably) have fingers (instead of toes) on their lower limbs as well. In the case of
primates Primates are a diverse order of mammals. They are divided into the strepsirrhines, which include the lemurs, galagos, and lorisids, and the haplorhines, which include the tarsiers and the simians (monkeys and apes, the latter including huma ...
in general, the digits of the hand are overwhelmingly referred to as "fingers". Primate fingers have both fingernails and
fingerprints A fingerprint is an impression left by the friction ridges of a human finger. The recovery of partial fingerprints from a crime scene is an important method of forensic science. Moisture and grease on a finger result in fingerprints on surf ...
. Research has been carried out on the
embryonic development An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sperm ...
of domestic chickens showing that an
interdigital webbing Interdigital webbing is the presence of membranes of skin between the digits. Normally in mammals, webbing is present in the embryo but resorbed later in development, but in various mammal species it occasionally persists in adulthood. In humans, ...
forms between the tissues that become the toes, which subsequently regresses by
apoptosis Apoptosis (from grc, ἀπόπτωσις, apóptōsis, 'falling off') is a form of programmed cell death that occurs in multicellular organisms. Biochemical events lead to characteristic cell changes (morphology) and death. These changes incl ...
. If apoptosis fails to occur, the interdigital skin remains intact. Many animals have developed
webbed feet The webbed foot is a specialized limb with interdigital membranes (webbings) that aids in aquatic locomotion, present in a variety of tetrapod vertebrates. This adaptation is primarily found in semiaquatic species, and has convergently evolved m ...
or skin between the fingers from this like the
Wallace's flying frog Wallace's flying frog (''Rhacophorus nigropalmatus''), also known as the gliding frog or the Abah River flying frog, is a moss frog found at least from the Malay Peninsula into western Indonesia, and is present in Borneo and Sumatra. It is named ...
.


Human fingers

Usually humans have five digits, the bones of which are termed phalanges, on each hand, although some people have more or fewer than five due to congenital disorders such as polydactyly or
oligodactyly Oligodactyly (from Ancient Greek ''oligos'' 'few' and δάκτυλος ''daktylos'' 'finger') is the presence of fewer than five fingers or toes on a hand or foot.
, or accidental or intentional
amputation Amputation is the removal of a limb by trauma, medical illness, or surgery. As a surgical measure, it is used to control pain or a disease process in the affected limb, such as malignancy or gangrene. In some cases, it is carried out on indi ...
s. The first digit is the
thumb 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 ...
, followed by the index finger, middle finger, ring finger, and little finger or pinkie. According to different definitions, the thumb can be called a finger, or not. English dictionaries describe finger as meaning either one of the five digits including the thumb, or one of the four excluding the thumb (in which case they are numbered from 1 to 4 starting with the index finger closest to the thumb).


Anatomy


Skeleton

The thumb (connected to the trapezium) is located on one of the sides, parallel to the arm. The palm has five bones known as metacarpal bones, one to each of the five digits. 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 distal phalanx, carrying the nail, the middle phalanx, and the proximal phalanx. Joints are formed wherever two or more of these bones meet. 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 fingertip.
Sesamoid bone In anatomy, a sesamoid bone () is a bone embedded within a tendon or a muscle. Its name is derived from the Arabic word for ' sesame seed', indicating the small size of most sesamoids. Often, these bones form in response to strain, or can be pres ...
s are small ossified nodes embedded in the tendons to provide extra leverage and reduce pressure on the underlying tissue. 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

Each finger may
flex Flex or FLEX may refer to: Computing * Flex (language), developed by Alan Kay * FLEX (operating system), a single-tasking operating system for the Motorola 6800 * FlexOS, an operating system developed by Digital Research * FLEX (protocol), a comm ...
and
extend Extension, extend or extended may refer to: Mathematics Logic or set theory * Axiom of extensionality * Extensible cardinal * Extension (model theory) * Extension (predicate logic), the set of tuples of values that satisfy the predicate * Ex ...
, abduct and adduct, and so also circumduct. Flexion is by far the strongest movement. In humans, there are two large muscles that produce flexion of each finger, and additional muscles that augment the movement. The muscle bulks that move each finger may be partly blended, and the tendons may be attached to each other by a net of fibrous tissue, preventing completely free movement. Although each finger seems to move independently, moving one finger also moves the other fingers slightly which is called finger interdependence or finger enslaving. Fingers do not contain muscles (other than
arrector pili The arrector pili muscles, also known as hair erector muscles, are small muscles attached to hair follicles in mammals. Contraction of these muscles causes the hairs to stand on end, known colloquially as goose bumps (piloerection). Structure E ...
). The
muscle Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of muscl ...
s that move the finger joints are in the palm and forearm. The long tendons that deliver motion from the forearm muscles may be observed to move under the skin at the wrist and on the back of the hand. Muscles of the fingers can be subdivided into extrinsic and intrinsic muscles. The extrinsic muscles are the long flexors and extensors. They are called extrinsic because the muscle belly is located on the forearm. The fingers have two long flexors, located on the underside of the forearm. They insert by tendons to the phalanges of the fingers. The deep flexor attaches to the distal phalanx, and the superficial flexor attaches to the middle phalanx. The flexors allow for the actual bending of the fingers. The thumb has one 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 muscle), moving the thumb in opposition, making grasping possible. The extensors are located on the back of the forearm and are connected in a more complex way than the flexors to the dorsum of the fingers. The tendons unite with the interosseous and lumbrical muscles to form the extensorhood mechanism. The primary function of the extensors is to straighten out the digits. The thumb has two extensors in the forearm; the tendons of these form the
anatomical snuff box The anatomical snuff box or snuffbox or foveola radialis is a triangular deepening on the radial, dorsal aspect of the hand—at the level of the carpal bones, specifically, the scaphoid and trapezium bones forming the floor. The name originate ...
. Also, the index finger and the little finger have an extra extensor, used for instance for pointing. The extensors are situated within six separate compartments. The first compartment contains abductor pollicis longus and extensor pollicis brevis. The second compartment contains extensors carpi radialis longus and brevis. The third compartment contains extensor pollicis longus. The extensor digitorum indicis and extensor digitorum communis are within the fourth compartment. Extensor digiti minimi is in the fifth, and extensor carpi ulnaris is in the sixth. The intrinsic muscle groups are the
thenar The thenar eminence is the mound formed at the base of the thumb on the palm of the hand by the intrinsic group of muscles of the thumb. The skin overlying this region is the area stimulated when trying to elicit a palmomental reflex. The word ...
and hypothenar muscles (thenar referring to the thumb, hypothenar to the small finger), the dorsal and palmar interossei muscles (between the metacarpal bones) and the lumbrical muscles. The lumbricals arise from the deep flexor (and are special because they have no bony origin) and insert on the dorsal extensor hood mechanism.


Skin

Aside from the genitals, the fingertips possess the highest concentration of touch receptors and
thermoreceptor A thermoreceptor is a non-specialised sense receptor, or more accurately the receptive portion of a sensory neuron, that codes absolute and relative changes in temperature, primarily within the innocuous range. In the mammalian peripheral nervous s ...
s among all areas of the human skin, making them extremely sensitive to temperature, pressure, vibration, texture and moisture. Recent studies suggest fingers can feel nano-scale wrinkles on a seemingly smooth surface, a level of sensitivity not previously recorded. This makes the fingers commonly used sensory probes to ascertain properties of objects encountered in the world, making them prone to
injury An injury is any physiological damage to living tissue caused by immediate physical stress. An injury can occur intentionally or unintentionally and may be caused by blunt trauma, penetrating trauma, burning, toxic exposure, asphyxiation, o ...
. The '' of a finger'' is the fleshy mass on the palmar aspect of the extremity of the finger.


Fingertip wrinkling in water

Although a common phenomenon, the underlying functions and mechanism of fingertip wrinkling following immersion in water are relatively unexplored. Originally it was assumed that the wrinkles were simply the result of the skin swelling in water, but it is now understood that the furrows are caused by the blood vessels constricting due to signalling by the
sympathetic nervous system The sympathetic nervous system (SNS) is one of the three divisions of the autonomic nervous system, the others being the parasympathetic nervous system and the enteric nervous system. The enteric nervous system is sometimes considered part of th ...
in response to water exposure. One hypothesis for why this occurs, the "rain tread" hypothesis, posits that the wrinkles may help the fingers grip things when wet, possibly being an adaption from a time when humans dealt with rain and dew in forested primate habitats. A 2013 study supporting this hypothesis found that the wrinkled fingertips provided better handling of wet objects but gave no advantage for handling dry objects. However, a 2014 study attempting to reproduce these results was unable to demonstrate any improvement of handling wet objects with wrinkled fingertips.


Regrowth of the fingertips

Fingertips, after having been torn off children, have been observed to regrow in less than 8 weeks. However, these fingertips do not look the same, although they do look more appealing than a skin graft or a sewn fingertip. No healing occurs if the tear happens below the nail. This works because the distal phalanges are regenerative in youth, and stem cells in the nails create new tissue that ends up as the fingertip.


Brain representation

Each finger has an orderly somatotopic representation on the cerebral cortex in the somatosensory cortex area 3b, part of area 1 and a distributed, overlapping representations in the supplementary motor area and primary motor area. The somatosensory cortex representation of the hand is a dynamic reflection of the fingers on the external hand: in syndactyly people have a
clubhand Radial dysplasia, also known as radial club hand or radial longitudinal deficiency, is a congenital difference occurring in a longitudinal direction resulting in radial deviation of the wrist and shortening of the forearm. It can occur in different ...
of webbed, 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 divided to make a more useful hand. Surgeons did this at the Institute of Reconstructive Plastic Surgery in New York to a 32-year-old man with the initials O. G.. They touched O. G.'s fingers before and after surgery while using MRI brain scans. Before the surgery, the fingers mapped onto his brain were fused close together; afterward, the maps of his individual fingers did indeed separate and take the layout corresponding to a normal hand.


Clinical significance


Anomalies, injuries and diseases

A rare anatomical variation affects 1 in 500 humans, in which the individual has more than the usual number of digits; this is known as polydactyly. A human may also be born without one or more fingers or underdevelopment of some fingers such as
symbrachydactyly Symbrachydactyly is a congenital abnormality, characterized by limb anomalies consisting of brachydactyly, cutaneous syndactyly and global hypoplasia of the hand or foot. In many cases, bones will be missing from the fingers and some fingers or toes ...
. Extra fingers can be functional. One individual with seven fingers not only used them but claimed that they "gave him some advantages in playing the piano".
Phalanges The phalanges (singular: ''phalanx'' ) are digital bones in the hands and feet of most vertebrates. In primates, the thumbs and big toes have two phalanges while the other digits have three phalanges. The phalanges are classed as long bones. ...
are commonly fractured. A damaged tendon can cause significant loss of function in fine motor control, such as with a
mallet finger A mallet finger, also known as hammer finger or PLF finger or Hannan finger, is an extensor tendon injury at the farthest away finger joint. This results in the inability to extend the finger tip without pushing it. There is generally pain and ...
. They can be damaged by cold, including frostbite and non-freezing cold injury (NFCI); and heat, including burns. The fingers are commonly affected by diseases such as rheumatoid arthritis and gout. Diabetics often use the fingers to obtain blood samples for regular blood sugar testing. Raynaud's phenomenon and Paroxysmal hand hematoma are neurovascular disorders that affect the fingers. Research has linked the ratio of lengths between the index and ring fingers to higher levels of testosterone, and to various physical and behavioral traits such as penis length and risk for development of
alcohol dependence Alcohol dependence is a previous (DSM-IV and ICD-10) psychiatric diagnosis in which an individual is physically or psychologically dependent upon alcohol (also chemically known as ethanol). In 2013, it was reclassified as alcohol use disorde ...
or video game addiction.


Etymology

The English word ''finger'' stems from
Old English Old English (, ), or Anglo-Saxon, is the earliest recorded form of the English language, spoken in England and southern and eastern Scotland in the early Middle Ages. It was brought to Great Britain by Anglo-Saxon settlement of Britain, Anglo ...
''finger'', ultimately from Proto-Germanic ' ('finger'). It is
cognate In historical linguistics, cognates or lexical cognates are sets of words in different languages that have been inherited in direct descent from an etymology, etymological ancestor in a proto-language, common parent language. Because language c ...
with
Gothic Gothic or Gothics may refer to: People and languages *Goths or Gothic people, the ethnonym of a group of East Germanic tribes **Gothic language, an extinct East Germanic language spoken by the Goths **Crimean Gothic, the Gothic language spoken b ...
', Old Norse ', or Old High German '. Linguists generally assume that ' is a ''ro''-stem deriving from a previous form ', ultimately from Proto-Indo-European ' ('five'). The name pinkie derives from Dutch , of uncertain origin. In English only the digits on the hand are known as fingers. However, in some languages the translated version of fingers can mean either the digits on the hand or feet. In English a digit on a foot has the distinct name of toe.


See also

* Finger snapping * Fingerprint * Nail (anatomy) * Paroxysmal hand hematoma * Toe


Notes


References

* * *


External links

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