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Nail (anatomy)

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For the nail on a bird's beak, see Beak § Nail.
Human nails
Human fingers and nails
Fingernails
Human toes and nails
Toenails

A nail is a horn-like envelope covering the dorsal aspect of the terminal phalanges of fingers and toes in humans, most non-human primates, and a few other mammals. Nails are similar to claws in other animals. Fingernails and toenails are made of a tough protein called keratin, as are animals' hooves and horns. The mammalian nail, claw, and hoof are all examples of unguis [plural ungues].

Human anatomy

The nail consists of the nail plate, the nail matrix and the nail bed below it, and the grooves surrounding it.[1]

Parts of the nail

Nail
Basic nail anatomy
Details
Identifiers
Latinunguis
MeSHD009262
TA98A16.0.01.001
TA27065
THH3.12.00.3.02001
FMA54326
Anatomical terminology

The matrix (sometimes called [2] the matrix unguis, keratogenous membrane, nail matrix, or onychostroma) is the tissue (or germinal matrix) which the nail protects,[3] the part of the nail bed that rests beneath the nail and contains nerves, lymph and blood vessels.[4] The matrix is responsible for the producing cells that become the nail plate. The width and thickness of the nail plate is determined by the size, length, and thickness of the matrix, while the shape of the fingertip itself shows if the nail plate is flat, arched or hooked.[5] The matrix will continue to grow as long as it receives nutrition and remains in a healthy condition.[4] As new nail plate cells are made, they push older nail plate cells forward; and in this way older cells become compressed, flat, and translucent. This makes the capillaries in the nail bed below visible, resulting in a pink color.[6]

The lunula (or simply "the moon") is the visible part of the matrix, the whitish crescent-shaped base of the visible nail.[7] The lunula can be seen as the largest in the thumb and often is not present in the little finger.

The nail bed is the skin beneath the nail plate.[7] Like all skin, it is made of two types of tissues: the deeper dermis, the living tissue fixed to the bone which includes capillaries and glands,[8] and the superficial epidermis, the layer just beneath the nail plate which moves forward with the plate. The epidermis is attached to the dermis by tiny longitudinal "grooves"[5] known as matrix crests (cristae matricis unguis).[3][8] During old age, the plate thins and these grooves are more visible.[5]

The nail sinus (sinus unguis) is where the nail root is inserted.[3]

The nail root (radix unguis) is the part of nail situated in the nail sinus,[3] i.e. the base of the nail underneath the skin. It originates from the actively growing tissue below, the matrix.[4]

The nail plate (corpus unguis)[3] is the actual nail, made of translucent keratin protein. Several layers of dead, compacted cells cause the nail to be strong but flexible.[5] Its (transversal) shape is determined by the form of the underlying bone.[5] In common usage, the word nail often refers to this part only.

The free margin (margo liber) or distal edge is the anterior margin of the nail plate corresponding to the abrasive or cutting edge of the nail.[3] The hyponychium (informally known as the "quick")[9] is the epithelium located beneath the nail plate at the junction between the free edge and the skin of the fingertip. It forms a seal that protects the nail bed.[4] The onychodermal band is the seal between the nail plate and the hyponychium. It is found just under the free edge, in that portion of the nail where the nail bed ends and can be recognized by its glassy, greyish colour (in fair-skinned people). It is not perceptible in some individuals while it is highly prominent on others.[5]

The eponychium is the small band of epithelium that extends from the posterior nail wall onto the base of the nail.[3] Often and erroneously [[[Cuticle #Human anatomy#{{{section}}}|contradictory]]] called the "proximal fold" or "cuticle", the eponychium is the end of the proximal fold that folds back upon itself to shed an epidermal layer of skin onto the newly formed nail plate. This layer of non-living, almost invisible skin is the cuticle that "rides out" on the surface of the nail plate. Together, the eponychium and the cuticle form a protective seal. The cuticle on the nail plate is dead cells and is often removed during manicure, but the eponychium is living cells and should not be touched.[6] The perionyx is the projecting edge of the eponychium covering the proximal strip of the lunula.[3]

The nail wall (vallum unguis) is the cutaneous fold overlapping the sides and proximal end of the nail. The lateral margin (margo lateralis) is lying beneath the nail wall on the sides of the nail and the nail groove or fold (sulcus matricis unguis) are the cutaneous slits into which the lateral margins are embedded.[3]

The paronychium is the border tissue around the nail[10] and paronychia is an infection in this area.

Function

A healthy (finger) nail has the function of protecting the distal phalanx, the fingertip, and the surrounding soft tissues from injuries. It also serves to enhance precise delicate movements of the distal digits through counter-pressure exerted on the pulp of the finger. [1] The nail then acts as a counterforce when the end of the finger touches an object, thereby enhancing the sensitivity of the fingertip,[11] even though there are no nerve endings in the nail itself. Finally, the nail functions as a tool, enabling for instance a so-called "extended precision grip" (e.g. pulling out a splinter in one's finger).

Growth

The growing part of the nail is the part still under the skin at the nail's proximal end under the epidermis, which is the only living part of a nail.

In mammals, the length and growth rate of nails is related to the length of the terminal phalanges (outermost finger bones). Thus, in humans, the nail of the index finger grows faster than that of the little finger; and fingernails grow up to four times faster than toenails. [12]

In humans, nails grow at an average rate of 3 mm (0.12 in) a month (as they are a form of hair).[13] Fingernails require three to six months to regrow completely, and toenails require 12 to 18 months. Actual growth rate is dependent upon age, sex, season, exercise level, diet, and hereditary factors. Nails grow faster in the summer than in any other season.[14] Contrary to popular belief, nails do not continue to grow after death; the skin dehydrates and tightens, making the nails (and hair) appear to grow.[15]

Medical and health aspects

Thumbnail of the right hand with cuticle (left) and hangnail (top)
Mechanical injury can result in the nail being dropped.

Healthcare and pre-hospital-care providers (EMTs or paramedics) often use the fingernail beds as a cursory indicator of distal tissue perfusion of individuals that may be dehydrated or in shock.[16] However, this test is not considered reliable in adults.[17] This is known as the CRT or blanch test.

WEJ Procedure: briefly depress the fingernail bed gently with a finger. This will briefly turn the nailbed white; the normal pink colour should be restored within a second or two. Delayed return to pink colour can be an indicator of certain shock states such as hypovolemia [18][19]

Nail growth record can show the history of recent health and physiological imbalances, and has been used as a diagnostic tool since ancient times.[20] Deep transverse grooves known as Beau's lines may form across the nails (not along the nail from cuticle to tip) and are usually a natural consequence of aging, though they may result from disease. Discoloration, thinning, thickening, brittleness, splitting, grooves, Mees' lines, small white spots, receded lunula, clubbing (convex), flatness, spooning (concave) can indicate illness in other areas of the body, nutrient deficiencies, drug reaction or poisoning, or merely local injury. Nails can also become thickened (onychogryphosis), loosened (onycholysis), infected with fungus (onychomycosis) or degenerate (onychodystrophy); for further information see nail diseases.

Permeability

The nail is often considered an essentially impermeable barrier, but this is not so; in fact, it is much more permeable than the skin,[21] and the composition of the nail includes 7-12% of water. This has implications for penetration both by harmful and by medicinal substances; in particular cosmetics applied to the nails carry a risk. Water can penetrate the nail, and many other substances are associated with nail penetration, including paraquat (harmful), urea, salicylic acid, miconazole, natamycin, and sodium hypochlorite.[21]

Health and care

Bluish or purple fingernail beds may be a symptom of peripheral cyanosis, which indicates oxygen deprivation.

Nails can dry out, just like skin. They can also peel, break, and be infected. Toe infections, for instance, can be caused or exacerbated by dirty socks, specific types of aggressive exercise[which?], tight footwear, and walking unprotected in an unclean environment.[citation needed]

Nail tools used by different people may transmit infections. Nail files, "if ... used on different people, ... may spread nail fungi, staph bacteria or viruses," warns Ted Dischman, a spokesperson for the California Board of Barbering and Cosmetology. In fact, over 100 bacterial skin infections in 2000 were traced to footbaths in nail salons.[citation needed] To avoid this, new improved contactless tools can be used, for example, gel and cream cuticle removers instead of cuticle scissors.

Many people also compulsively bite their nails.

Damaged thumbnail

Nail disease can be very subtle and should be evaluated by a dermatologist with a focus in this particular area of medicine.[22] However, most times it is a nail technician who will note a subtle change in nail disease.

Inherited accessory nail of the fifth toe occurs where the toenail of the smallest toe is separated, forming a smaller, "sixth toenail" in the outer corner of the nail.[citation needed] Like any other nail, it can be cut using a nail clipper.

Effect of nutrition

Vitamin A is an essential micronutrient for vision, reproduction, cell and tissue differentiation, and immune function. Vitamin D and calcium work together in cases of maintaining homeostasis, creating muscle contraction, transmission of nerve pulses, blood clotting, and membrane structure. A lack of vitamin A, vitamin D, and calcium can cause dryness and brittleness. Sources of these micronutrients include fortified milk, cereal, and juices, salt-water fish, fish-liver oils, and some vegetables. Vitamin B12 is mainly found in animal sources such as liver and kidney, fish, chicken, and dairy products and therefore can cause intake issues in vegan populations. Not enough B12 vitamin can lead to excessive dryness, darkened nails, and rounded or curved nail ends. Insufficient intake of both vitamin A and B, as previously described, results in fragile nails with horizontal and vertical ridges.[23] Protein is a building material for new nails; therefore, low dietary protein intake may cause white nail beds. Dietary sources of this macronutrient include eggs, milk, cheese, meat, beans and legumes. A lack of protein combined with deficiencies in folic acid and vitamin C produce hangnails. Essential fatty acids play a large role in healthy skin as well as nails. As touched upon previously, essential fatty acids can be obtained through consumption of fish, flaxseed, canola oil, seeds, leafy vegetables, and nuts. Splitting and flaking of nails may be due to a lack of linoleic acid. Iron-deficiency anemia can lead to a pale color along with a thin, brittle, ridged texture. Iron deficiency in general may cause the nails to become flat or concave, rather than convex. Iron can be found in animal sources, called heme iron, such as meat, fish, and poultry, and can also be found in fruits, vegetables, dried beans, nuts, and grain products, also known as non-heme iron. Heme iron is absorbed fairly easily in comparison to non-heme iron; however, both types provide the necessary bodily functions.[24]

Fashion

Main articles: manicure and pedicure

Manicures (for the hands) and pedicures (for the feet) are health and cosmetic procedures to groom, trim, and paint the nails and manage calluses. They require various tools such as cuticle scissors, nail scissors, nail clippers, and nail files. Artificial nails can also be fixed onto real nails for cosmetic purposes.

A person whose occupation is to cut, shape and care for nails as well as to apply overlays such as acrylic and UV Gel is sometimes called a nail technician. The place where a nail technician works may be a nail salon or nail shop or nail bar.

Painting the nails with coloured nail polish (also called nail lacquer and nail varnish) to improve the appearance is a common practice dating back to at least 3000 B.C.

Length records

Guinness World Records began tracking record fingernail lengths in 1955, when an unnamed Chinese priest was listed as having fingernails 22.75 inches (57.8 cm) long.

The current record-holder for men, according to Guinness, is Shridhar Chillal of India who set the record in 1998 with a total of 242.25 inches (615.3 cm) of nails on his left hand. His longest nail, on his thumb, was 4.8 feet (1.5 m) long.

The record-holder for women is Lee Redmond of the U.S., who set the record in 2001 and as of 2008 had nails with a total length on both hands of 28 feet (8.5 m), with the longest nail on her right thumb at 2 feet 11 inches (89 cm).[25]

Evolution

Nails are a distinguishing feature of the primate order.

The nails of primates and the hooves of running mammals evolved from the claws of earlier animals.[26]

In contrast to nails, claws are typically curved ventrally (downwards in animals) and compressed sideways. They serve a multitude of functions —including climbing, digging, and fighting— and have undergone numerous adaptive changes in different animal taxa. Claws are pointed at their ends and are composed of two layers: a thick, deep layer and a superficial, hardened layer which serves a protective function. The underlying bone is a virtual mould of the overlying horny structure and therefore has the same shape as the claw or nail. Compared to claws, nails are flat, less curved, and do not extend far beyond the tip of the digits. The ends of the nails usually consist only of the "superficial", hardened layer and are not pointed like claws.[26]

With only a few exceptions, primates retain plesiomorphic (original, "primitive") hands with five digits, each equipped with either a nail or a claw. For example, all prosimians (i.e. "primitive" primates or "proto-primates", see also Plesiadapiformes) have nails on all digits except the second toe which is equipped with a so-called toilet-claw (i.e. important for grooming activities). The needle-clawed bushbaby (Euoticus) have keeled nails (the thumb and the first and the second toes have claws) featuring a central ridge that ends in a needle-like tip. In tree shrews all digits have claws and, unlike most primates, the digits of their feet are positioned close together, and therefore the thumb cannot be brought into opposition (another distinguishing feature of primates).[26]

A study of the fingertip morphology of four small-bodied New World monkey species indicated a correlation between increasing small-branch foraging and

  1. expanded apical pads (fingertips),
  2. developed epidermal ridges (fingerprints),
  3. broadened distal parts of distal phalanges (fingertip bones), and
  4. reduced flexor and extensor tubercles (attachments areas for finger muscles on bones).

This suggests that whereas claws are useful on large-diameter branches, wide fingertips with nails and epidermal ridges were required for habitual locomotion on small-diameter branches. It also indicates keel-shaped nails of Callitrichines (a family of New World monkeys) is a derived postural adaptation rather than retained ancestral condition. [27]

See also

Wikimedia Commons has media related to Nails.

References

  1. ^ a b Onumah, Neh; Scher, Richard K (May 2009). "Nail Surgery". eMedicine. Retrieved March 2010. {{cite web}}: Check date values in: |accessdate= (help)
  2. ^ "Nail matrix". Biology Online. 2005. Retrieved February 2010. {{cite web}}: Check date values in: |accessdate= (help)
  3. ^ a b c d e f g h i Feneis, Heinz (2000). Pocket Atlas of Human Anatomy (4th ed.). Thieme. pp. 392–95. ISBN 3-13-511204-7. Cite error: The named reference "Feneis-2000" was defined multiple times with different content (see the help page).
  4. ^ a b c d "Glossary of Nail Technology Terminology". 2008. Retrieved February 2010. {{cite web}}: Check date values in: |accessdate= (help)
  5. ^ a b c d e f "Understanding Your Natural Nails". 2000. {{cite web}}: |access-date= requires |url= (help); Check date values in: |accessdate= (help); Missing or empty |url= (help)
  6. ^ a b Lellipop (August 2006). "Anatomy of the nail". Salon Geek. Retrieved February 2010. {{cite web}}: Check date values in: |accessdate= (help)
  7. ^ a b "Nail Anatomy". Nail Doctors. 2005. Retrieved February 2010. {{cite web}}: Check date values in: |accessdate= (help)
  8. ^ a b "Glossary fo Nail Conditions". The Achilles Foot Health Centre.
  9. ^ Crouch, James Ensign (1985). Functional human anatomy. Lea & Febiger. p. 80.
  10. ^ Jordan, Christopher; Mirzabeigi, Edwin (2000-04-01). Atlas of orthopaedic surgical exposures. Thieme. p. 101. ISBN 0-86577-776-4.
  11. ^ Wang, Quincy C; Johnson, Brett A (May 2001). "Fingertip Injuries". American Family Physician. Retrieved March 2010. {{cite web}}: Check date values in: |accessdate= (help)
  12. ^ Ravosa, Matthew J.; Dagosto, Marian (2007). Primate origins: adaptations and evolution. Springer. pp. 389–90. ISBN 0-387-30335-9.
  13. ^ Toenail Definition - Medicine.net
  14. ^ Hunter, J. A. A., Savin, J., & Dahl, M. V. (2002). Clinical dermatology. Malden, Mass: Blackwell Science. p. 173. ISBN 0-632-05916-8
  15. ^ BMJ 2007;335(7633):1288 (22 December), doi:10.1136/bmj.39420.420370.25
  16. ^ Monterey County EMS Manual. Chapter XI, Patient assessment.
  17. ^ Schriger DL, Baraff LJ (1991). "Capillary refill—is it a useful predictor of hypovolemic states?". Ann Emerg Med. 20 (6): 601–5. doi:10.1016/S0196-0644(05)82375-3. PMID 2039096. {{cite journal}}: Unknown parameter |month= ignored (help)
  18. ^ MedlinePlus Encyclopedia: Capillary nail refill test
  19. ^ St. Luke's Hospital. Capillary nail refill test.
  20. ^ American Academy of Dermatology - Nail Health
  21. ^ a b K. A. WALTERS and G. L. FLYNN, Permeability characteristics of the human nail plate, International Journal of Cosmetic Science 5, 231-246 (1983)
  22. ^ Pratt, Michelle (2009). "What Is Skin Cancer?". Nails Magazine. {{cite web}}: Unknown parameter |month= ignored (help)
  23. ^ There are some over the counter vitamin supplements that may help in growth of strong nails such as certain Multivitamins and Biotin, though this is quite subjective as well. Zempleni, J (2007). Handbook of vitamins (4th ed.). {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  24. ^ Cashman MW, Sloan SB (2010). "Nutrition and nail disease". Clinics in Dermatology. 28 (4): 420–425. doi:10.1016/j.clindermatol.2010.03.037. PMID 20620759.
  25. ^ "Crash breaks woman's record-length fingernails". NBCNews. December 2009.
  26. ^ a b c Ankel-Simons, Friderun (2007). Primate anatomy: an introduction (3rd ed.). pp. 342–44. ISBN 0-12-372576-3.
  27. ^ Hamrick, Mark W. (1998). "Functional and adaptive significance of primate pads and claws: Evidence from New World anthropoids". American Journal of Physical Anthropology. 106 (2). Wiley-Liss: 113–127. doi:10.1002/(SICI)1096-8644(199806)106:2<113::AID-AJPA2>3.0.CO;2-R. PMID 9637179. (Abstract)
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