Finger

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Finger
Human fingers both sides 2.jpg
The fingers of a left hand seen from both sides. The hand past the knuckles is displayed in grayscale for de-emphasis.
Details
Identifiers
Latin Digiti manus
MeSH D005385
TA98 A01.1.00.030
TA2 150
FMA 9666
Anatomical terminology

A finger is a limb of the human body and a type of digit, an organ of manipulation and sensation found in the hands of humans and other primates. [1] [2] Normally humans have five digits, [3] the bones of which are termed phalanges, [2] on each hand, although some people have more or fewer than five due to congenital disorders such as polydactyly or oligodactyly, or accidental or intentional amputations. The first digit is the thumb, followed by index finger, middle finger, ring finger, and little finger or pinkie. According to different definitions, the thumb can be called a finger, or not.

Contents

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). [1] [2] [4]

Anatomy

Skeleton

Illustration depicting the bones of the human hand Scheme human hand bones-en.svg
Illustration depicting the bones of the human hand

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.

Sesamoid bones 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

The precision of finger movements in space and time is highlighted in this motion tracking of two pianists' fingers playing the same piece (slow motion, no sound). [5]

Each finger may flex and extend, 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. Each finger may move independently of the others, though 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.

Fingers do not contain muscles (other than arrector pili). The muscles 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. Also, the index finger and the little finger have an extra extensor, used for instance for pointing. The extensors are situated within 6 separate compartments. The 1st compartment contains abductor pollicis longus and extensor pollicis brevis. The 2nd compartment contains extensors carpi radialis longus and brevis. The 3rd compartment contains extensor pollicis longus. The extensor digitorum indicis and extensor digitorum communis are within the 4th compartment. Extensor digiti minimi is in the fifth, and extensor carpi ulnaris is in the 6th.

The intrinsic muscle groups are the thenar 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 thermoreceptors among all areas of the human skin,[ citation needed ] 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. [6] This makes the fingers commonly used sensory probes to ascertain properties of objects encountered in the world, making them prone to injury.

The pulp of a finger is the fleshy mass on the palmar aspect of the extremity of the finger. [7]

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[ by whom? ] that the wrinkles were simply the result of the skin swelling in water,[ citation needed ] but it is now understood that the furrows are caused by the blood vessels constricting due to signalling by the sympathetic nervous system in response to water exposure. [8] [9] 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. [8] 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. [10] However, a 2014 study attempting to reproduce these results was unable to demonstrate any improvement of handling wet objects with wrinkled fingertips. [9]

Brain representation

Each finger has an orderly somatotopic representation on the cerebral cortex in the somatosensory cortex area 3b, [11] part of area 1 [12] and a distributed, overlapping representations in the supplementary motor area and primary motor area. [13]

The somatosensory cortex representation of the hand is a dynamic reflection of the fingers on the external hand: in syndactyly people have a clubhand 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. [14]

Other animals

Chimpanzees have lower limbs that are specialized for manipulation, and (arguably) have fingers on their lower limbs as well. The term 'finger' is not applied to the digits of most other animals, such as canines, felines, or ungulates, none of which can engage in fine manipulation with their forelimbs as a primate can.

Clinical significance

Anomalies, injuries and diseases

Radiograph of Type 1 Syndactyly Syndactyly type1 hands.jpg
Radiograph of Type 1 Syndactyly

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. 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". [15]

Phalanges are commonly fractured. A damaged tendon can cause significant loss of function in fine motor control, such as with a mallet finger. 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 affects 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 [16] and risk for development of alcohol dependence [17] or video game addiction. [18]

Etymology

The English word finger stems from Old English finger, ultimately from Proto-Germanic *fingraz ('finger'). It is cognate with Gothic figgrs, Old Norse fingr, or Old High German fingar. Linguists generally assume that *fingraz is a ro-stem deriving from a previous form *fimfe, ultimately from Proto-Indo-European *pénkʷe ('five'). [19]

The name pinkie derives from Dutch pinkje, 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

Notes

  1. 1 2 Chambers 1998 p. 603
  2. 1 2 3 Oxford Illustrated pp. 311, 380
  3. Tracy L. Kivell; Pierre Lemelin; Brian G. Richmond; Daniel Schmitt (2016). The Evolution of the Primate Hand: Anatomical, Developmental, Functional, and Paleontological Evidence. Springer. pp. 7–. ISBN   978-1-4939-3646-5.
  4. Oxford Advanced p. 326
  5. Goebl, W.; Palmer, C. (2013). Balasubramaniam, Ramesh (ed.). "Temporal Control and Hand Movement Efficiency in Skilled Music Performance". PLOS ONE. 8 (1): e50901. Bibcode:2013PLoSO...850901G. doi:10.1371/journal.pone.0050901. PMC   3536780 . PMID   23300946.
  6. "Feeling small: Fingers can detect nano-scale wrinkles even on a seemingly smooth surface". Science Daily . September 16, 2013.
  7. medilexicon.com > Medical Dictionary - 'Pulp Of Finger' Citing: Stedman's Medical Dictionary. 2006
  8. 1 2 Changizi, M.; Weber, R.; Kotecha, R.; Palazzo, J. (2011). "Are Wet-Induced Wrinkled Fingers Primate Rain Treads?". Brain, Behavior and Evolution. 77 (4): 286–90. doi: 10.1159/000328223 . PMID   21701145.
  9. 1 2 Haseleu, Julia; Omerbašić, Damir; Frenzel, Henning; Gross, Manfred; Lewin, Gary R. (2014). Goldreich, Daniel (ed.). "Water-Induced Finger Wrinkles Do Not Affect Touch Acuity or Dexterity in Handling Wet Objects". PLOS ONE. 9 (1): e84949. Bibcode:2014PLoSO...984949H. doi:10.1371/journal.pone.0084949. PMC   3885627 . PMID   24416318.
  10. Kareklas, K.; Nettle, D.; Smulders, T. V. (2013). "Water-induced finger wrinkles improve handling of wet objects". Biology Letters. 9 (2): 20120999. doi:10.1098/rsbl.2012.0999. PMC   3639753 . PMID   23302867.
  11. Van Westen, D; Fransson, P; Olsrud, J; Rosén, B; Lundborg, G; Larsson, EM (2004). "Fingersomatotopy in area 3b: an fMRI-study". BMC Neurosci. 5: 28. doi:10.1186/1471-2202-5-28. PMC   517711 . PMID   15320953.
  12. Nelson, AJ; Chen, R (2008). "Digit somatotopy within cortical areas of the postcentral gyrus in humans". Cereb Cortex. 18 (10): 2341–51. doi: 10.1093/cercor/bhm257 . PMID   18245039.
  13. Kleinschmidt, A; Nitschke, MF; Frahm, J (1997). "Somatotopy in the human motor cortex hand area. A high-resolution functional MRI study". Eur J Neurosci. 9 (10): 2178–86. doi:10.1111/j.1460-9568.1997.tb01384.x. PMID   9421177. S2CID   21042040.
  14. Mogilner, A; Grossman, JA; Ribary, U; Joliot, M; Volkmann, J; Rapaport, D; Beasley, RW; Llinás, RR (1993). "Somatosensory cortical plasticity in adult humans revealed by magnetoencephalography". Proc Natl Acad Sci USA. 90 (8): 3593–97. Bibcode:1993PNAS...90.3593M. doi:10.1073/pnas.90.8.3593. PMC   46347 . PMID   8386377.
  15. Dwight, T (1892). "Fusion of hands". Memoirs of the Boston Society of Natural History. 4: 473–86.
  16. Researchers Find Association Between Penile Length and Ratio of Length of Men's Fingers
  17. Kornhuber, J; Erhard, G; Lenz, B; Kraus, T; Sperling, W; Bayerlein, K; Biermann, T; Stoessel, C (2011). "Low digit ratio 2D:4D in alcohol dependent patients". PLOS ONE. 6 (4): e19332. Bibcode:2011PLoSO...619332K. doi:10.1371/journal.pone.0019332. PMC   3081847 . PMID   21547078.
  18. Kornhuber, J.; Zenses, EM; Lenz, B; Stoessel, C; Bouna-Pyrrou, P; Rehbein, F; Kliem, S; Mößle, T (2013). "Low digit ratio 2D:4D associated with video game addiction". PLOS ONE. 8 (11): e79539. Bibcode:2013PLoSO...879539K. doi:10.1371/journal.pone.0079539. PMC   3827365 . PMID   24236143.
  19. Kroonen, Guus (2013). Etymological Dictionary of Proto-Germanic. Brill. p. 141. ISBN   978-90-04-18340-7.

Related Research Articles

Thumb First digit of the hand

The thumb is the first finger of the hand, next to the index finger. When a person is standing in the medical anatomical position, the thumb is the outermost finger. The Medical Latin English noun for thumb is pollex, and the corresponding adjective for thumb is pollical.

Median nerve Nerve of the upper limb

The median nerve is a nerve in humans and other animals in the upper limb. It is one of the five main nerves originating from the brachial plexus.

The flexor digitorum profundus is a muscle in the forearm of humans that flexes the fingers. It is considered an extrinsic hand muscle because it acts on the hand while its muscle belly is located in the forearm.

Upper limb

The upper limbs or upper extremities are the forelimbs of an upright-postured tetrapod vertebrate, extending from the scapulae and clavicles down to and including the digits, including all the musculatures and ligaments involved with the shoulder, elbow, wrist and knuckle joints. In humans, each upper limb is divided into the arm, forearm and hand, and is primarily used for climbing, lifting and manipulating objects.

Phalanx bone Digital bone in the hands and feet of most vertebrates

The phalanges 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.

Extensor digitorum muscle

The extensor digitorum muscle is a muscle of the posterior forearm present in humans and other animals. It extends the medial four digits of the hand. Extensor digitorum is innervated by the posterior interosseous nerve, which is a branch of the radial nerve.

In human anatomy, the extensor pollicis longus muscle (EPL) is a skeletal muscle located dorsally on the forearm. It is much larger than the extensor pollicis brevis, the origin of which it partly covers and acts to stretch the thumb together with this muscle.

The flexor pollicis longus is a muscle in the forearm and hand that flexes the thumb. It lies in the same plane as the flexor digitorum profundus.

In human anatomy, the abductor pollicis longus (APL) is one of the extrinsic muscles of the hand. As the name implies, its major function is to abduct the thumb at the wrist. Its tendon forms the anterior border of the anatomical snuffbox.

Adductor pollicis muscle

In human anatomy, the adductor pollicis muscle is a muscle in the hand that functions to adduct the thumb. It has two heads: transverse and oblique.

Palmar interossei muscles

In human anatomy, the palmar or volar interossei are three small, unipennate muscles in the hand that lie between the metacarpal bones and are attached to the index, ring, and little fingers. They are smaller than the dorsal interossei of the hand.

Dorsal interossei of the hand

In human anatomy, the dorsal interossei (DI) are four muscles in the back of the hand that act to abduct (spread) the index, middle, and ring fingers away from hand's midline and assist in flexion at the metacarpophalangeal joints and extension at the interphalangeal joints of the index, middle and ring fingers.

The posterior compartment of the forearm contains twelve muscles which are chiefly responsible for extension of the wrist and digits, and supination of the forearm. It is separated from the anterior compartment by the interosseous membrane between the radius and ulna.

Muscles of the hand

The muscles of the hand are the skeletal muscles responsible for the movement of the hand and fingers. The muscles of the hand can be subdivided into two groups: the extrinsic and intrinsic muscle groups. The extrinsic muscle groups are the long flexors and extensors. They are called extrinsic because the muscle belly is located on the forearm. The intrinsic group are the smaller muscles located within the hand itself. The muscles of the hand are innervated by the radial, median, and ulnar nerves from the brachial plexus.

Hand Extremity at the end of an arm or forelimb

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 are often described as having "hands" instead of paws on their front limbs. The raccoon is usually described as having "hands" though opposable thumbs are lacking.

Median nerve palsy Medical condition

Injuries to the arm, forearm or wrist area can lead to various nerve disorders. One such disorder is median nerve palsy. The median nerve controls the majority of the muscles in the forearm. It controls abduction of the thumb, flexion of hand at wrist, flexion of digital phalanx of the fingers, is the sensory nerve for the first three fingers, etc. Because of this major role of the median nerve, it is also called the eye of the hand. If the median nerve is damaged, the ability to abduct and oppose the thumb may be lost due to paralysis of the thenar muscles. Various other symptoms can occur which may be repaired through surgery and tendon transfers. Tendon transfers have been very successful in restoring motor function and improving functional outcomes in patients with median nerve palsy.

Extrinsic extensor muscles of the hand

The extrinsic extensor muscles of the hand are located in the back of the forearm and have long tendons connecting them to bones in the hand, where they exert their action. Extrinsic denotes their location outside the hand. Extensor denotes their action which is to extend, or open flat, joints in the hand. They include the extensor carpi radialis longus (ECRL), extensor carpi radialis brevis (ECRB), extensor digitorum (ED), extensor digiti minimi (EDM), extensor carpi ulnaris (ECU), abductor pollicis longus (APL), extensor pollicis brevis (EPB), extensor pollicis longus (EPL), and extensor indicis (EI).

Muscles of the thumb

The muscles of the thumb are nine skeletal muscles located in the hand and forearm. The muscles allow for flexion, extension, adduction, abduction and opposition of the thumb. 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.

Extensor pollicis et indicis communis muscle

In human anatomy, the extensor pollicis et indicis communis is an aberrant muscle in the posterior compartment of forearm. It was first described in 1863. The muscle has a prevalence from 0.5% to 4%.

References