Nail (anatomy)

In human anatomy, a nail is a horn-like envelop covering the dorsal aspect of the terminal phalanges of fingers and toes. Nails are also present in several other primates. Although not exclusive to primates, the development of nails is extremely rare in other mammals.

Nails are similar to claws, which are found on numerous other animals. In common usage, the word nail often refers to the nail plate only.

Fingernails and toenails are made of a tough protein called keratin, as are animals' hooves and horns. Along with hair and teeth they are an appendage of the skin.

Parts of the nail
The parts of the nail are:
 * Matrix: The only living part of the nail. It is situated behind and underneath the nail fold and produces the keratin which makes up the nail plate. If the matrix is damaged, growth of the nail plate is affected.
 * Eponychium: Dead skin that forms around the cuticle area. It can be lifted and trimmed during a professional manicure treatment. Tends to be more prominent on males.
 * Paronychium: The 'live' skin that folds around the cuticle area, giving protection to the matrix.
 * Hyponychium: The area of attachment between the nail plate and nail bed that lies underneath the free edge. Anatomical terms of location: proximal and distal, end of the nail.
 * Nail plate: The hard and translucent part of the nail composed of layers of keratin.
 * Nail bed: Tissue underneath the nail plate, responsible for the pink colour of the nail. It also determines what shape the nail will grow. It is informally referred to as "the quick", especially the end nearest the fingertip.
 * Lunula: The visible part of the matrix, a whitish crescent shape around the base of the nail plate. Tends to only be visible in larger nails.
 * Nail fold: A fold of hard skin overlapping the base and sides of a fingernail or toenail
 * Free edge or Distal edge: The part of the nail that extends past the finger, beyond the nail plate. There should always be a free edge present to prevent infections.
 * Nail groove: Grooves that guide the direction of nail growth. They are located down the sides of the nail fold.

Function
Nails act as a counterforce when the end of the finger touches an object, thereby enhancing the sensitivity of the fingertip, even though there are no nerve endings in the nail itself.

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 grow faster than that of the little finger; and fingernails grow up to four times faster than toenails.

In humans, nails grow at an average rate of 3 mm a month (as they are a form of hair). Fingernails require 3 to 6 months to regrow completely, and toenails require 12 to 18 months. Actual growth rate is dependent upon age, gender, season, exercise level, diet, and hereditary factors. Nails grow faster in the summer than in any other season. 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.

Medical aspects
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. However, this test is not considered reliable in adults. 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

Nail growth record can show the history of recent health and physiological imbalances, and has been used as a diagnostic tool since ancient times. Major illness will cause a deep transverse groove to form across the nails (not along the nail from cuticle to tip). 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.

Evolution
The nails of primates and the hooves of running mammals evolved from the claws of reptiles.

In contrast to nails, claws are typically curved ventrally (downwards in animals) and compressed sideways, and 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.

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") 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 (primate-like rodents) 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 features of primates.

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, (2) developed epidermal ridges (colloquially known as fingerprints), (3) broadened distal parts of distal phalanges (fingertip bone), and (4) reduced flexor and extensor tubercles. This indicates 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.