A nail is a protective plate characteristically found at the tip of the digits (fingers and toes) of all primates, corresponding to the claws in other tetrapod animals.

Fingernails and toenails are made of a tough rigid protein called alpha-keratin, a polymer also found in the claws, hooves, and horns of vertebrates.

The matrix will continue to produce cells as long as it receives nutrition and remains in a healthy condition.

The epidermis is attached to the dermis by tiny longitudinal "grooves" called matrix crests (cristae matricis unguis).

[3] The hyponychium (informally known as the "quick")[6] is the epithelium located beneath the nail plate at the junction between the free edge and the skin of the fingertip.

The cuticle is the semi-circular layer of almost invisible dead skin cells that "ride out on" and cover the back of the visible nail plate.

The hyponychium is the area of epithelium, particularly the thickened portion, underlying the free edge of the nail plate.

A healthy fingernail 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.

Finally, the nail functions as a tool enabling a so-called "extended precision grip" (e.g., pulling out a splinter in one's finger), and certain cutting or scraping actions.

In mammals, the growth rate of nails is related to the length of the terminal phalanges (outermost finger bones).

Actual growth rate is dependent upon age, sex, season, exercise level, diet, and hereditary factors.

This permeability has implications for penetration by harmful and medicinal substances; in particular cosmetics applied to the nails can pose a risk.

Water can penetrate the nail as can many other substances including paraquat, a fast acting herbicide that is harmful to humans; urea which is often an ingredient in creams and lotions meant for use on hands and fingers; several fungicidal agents such as salicylic acid, miconazole branded Monistat, natamycin; and sodium hypochlorite which is the active ingredient in common household bleach (but usually only in 2–3% concentration).

[15] Healthcare and pre-hospital-care providers (EMTs or paramedics) often use the fingernail beds as a cursory indicator of distal tissue perfusion of individuals who may be dehydrated or in shock.

[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.

Discoloration, thinning, thickening, brittleness, splitting, grooves, Mees' lines, small white spots, receded lunula, clubbing (convex), flatness, and spooning (concave) can indicate illness in other areas of the body, nutrient deficiencies, drug reaction, poisoning, or merely local injury.

Nails can also become thickened (onychogryphosis), loosened (onycholysis), infected with fungus (onychomycosis), or degenerate (onychodystrophy).

Toe infections, for instance, can be caused or exacerbated by dirty socks, specific types of aggressive exercise (long-distance running), tight footwear, and walking unprotected in an unclean environment.

[citation needed] Common organisms causing nail infections include yeasts and molds (particularly dermatophytes).

Vitamin D and calcium work together in cases of maintaining homeostasis, creating muscle contraction, transmission of nerve pulses, blood clotting, and membrane structure.

[29][30] Manicures (for the hands) and pedicures (for the feet) are health and cosmetic procedures to groom, trim, and paint the nails and manage calluses.

The current record-holder for men, according to Guinness, is Shridhar Chillal from India who set the record in 1998 with a total of 20 feet 2.25 inches (615.32 cm) of nails on his left hand.

The underlying bone is a virtual mold of the overlying horny structure and therefore has the same shape as the claw or nail.

[38] 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, nearly all living strepsirrhine primates have nails on all digits except the second toe which is equipped with a grooming claw.

[39] The needle-clawed bushbaby (Euoticus) has keeled nails (the thumb and the first and the second toes have claws) featuring a central ridge that ends in a needle-like tip.

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

[40] An alternative theory is that the nails of primates evolved to enable silent movement through trees while stalking prey, replacing noisier claws to make ambush hunting more effective.