Finger

Finger
Details
Identifiers
Latin	Digiti manus
MeSH	A01.378.800.667.430
TA	A01.1.00.030
FMA	75592
Anatomical terminology [edit on Wikidata]

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, 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 medical amputations. The first digit is the thumb, followed by index finger, middle finger, ring finger, and little finger or pinky. According to different definitions, the thumb can be called a finger, or not.

Anatomy

Skeleton

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 5 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).^[3]

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 digititorum 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, 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.^[4] Thus fingers are commonly used as sensory probes to ascertain properties of objects encountered in the world, and so they are prone to injury.

The pulp of a finger is the fleshy mass on the palmar aspect of the extremity of the finger.^[5]

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 in response to water exposure.^[6][7] 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.^[6] 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.^[8] However, a 2014 study attempting to reproduce these results was unable to demonstrate any improvement of handling wet objects with wrinkled fingertips.^[7]

Brain representation

Each finger has an orderly somatotopic representation on the cerebral cortex in the somatosensory cortex area 3b,^[9] part of area 1^[10] and a distributed, overlapping representations in the supplementary motor area and primary motor area.^[11]

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.^[12]

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

A rare anatomical variation affects 1 in 500 humans,^[13] 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".^[14]

Phalanges are commonly fractured. A damaged tendon can cause significant loss of function in fine motor control, such as with a mallet finger.

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 is a neurovascular disorder 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^[15] and risk for development of alcohol dependence^[16] or video game addiction.^[17]

History

Etymology

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][18] Linguistically, it appears that the original sense was to include the thumb as a finger: the word is derived from *penkwe-ros (also rendered as *penqrós) which was, in the Proto-Indo-European language, a suffixed form of *penkwe (or *penqe), "five".^[19] 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)

Notes

1. Chambers 1998 page 603
2. Oxford Illustrated pages 311,380
3. Goebl, W.; Palmer, C. (2013). Balasubramaniam, Ramesh, ed. "Temporal Control and Hand Movement Efficiency in Skilled Music Performance". PLoS ONE 8 (1): e50901. doi:10.1371/journal.pone.0050901. PMC 3536780. PMID 23300946.
4. "Feeling small: Fingers can detect nano-scale wrinkles even on a seemingly smooth surface". Science Daily. September 16, 2013.
5. medilexicon.com > Medical Dictionary - 'Pulp Of Finger' Citing: Stedman's Medical Dictionary. 2006
6. 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.
7. 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: e84949. doi:10.1371/journal.pone.0084949.
8. 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.
9. 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.
10. 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.
11. 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.
12. 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 U S A 90 (8): 3593–7. doi:10.1073/pnas.90.8.3593. PMC 46347. PMID 8386377.
13. Greene, Alan (May 19, 1997). "Polydactylism". drgreene.com. Retrieved 2008-06-26.
14. Dwight, T (1892). "Fusion of hands". Memoirs of the Boston Society of Natural History 4: 473–486.
15. Researchers Find Association Between Penile Length and Ratio of Length of Men's Fingers
16. 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. doi:10.1371/journal.pone.0019332. PMC 3081847. PMID 21547078.
17. 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. doi:10.1371/journal.pone.0079539.
18. Oxford Advanced page 326
19. http://www.etymonline.com/index.php?term=finger&allowed_in_frame=0

References

• The Chambers Dictionary. Edinburgh: Chambers Harrap Publishers Ltd. 2000 [1998]. ISBN 0-550-14000-X.
• The Oxford Illustrated Dictionary. Great Britain: Oxford University Press. 1976 [1975].
• Oxford Advanced Learner's Dictionary of Current English. London: Oxford University Press. 1974 [1974]. ISBN 0-19-431102-3.