Nail (anatomy)

Nail (anatomy)

Overview
A nail is a horn-like envelope covering the dorsal aspect of the terminal phalanges of fingers and toes in humans, most non-human primate
Primate
A primate is a mammal of the order Primates , which contains prosimians and simians. Primates arose from ancestors that lived in the trees of tropical forests; many primate characteristics represent adaptations to life in this challenging three-dimensional environment...

s, and a few other mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

s. Nails are similar to claw
Claw
A claw is a curved, pointed appendage, found at the end of a toe or finger in most mammals, birds, and some reptiles. However, the word "claw" is also often used in reference to an invertebrate. Somewhat similar fine hooked structures are found in arthropods such as beetles and spiders, at the end...

s, which are found on numerous other animals.

Fingernails and toenails are made of a tough protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

 called keratin
Keratin
Keratin refers to a family of fibrous structural proteins. Keratin is the key of structural material making up the outer layer of human skin. It is also the key structural component of hair and nails...

, as are animals' hooves and horns. The mammalian nail, claw, and hoof are all examples of unguis [plural ungues].

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

The matrix (synonyms: matrix unguis, keratogenous membrane, nail matrix, onychostroma) is the tissue (or germinal
Germ layer
A germ layer, occasionally referred to as a germinal epithelium, is a group of cells, formed during animal embryogenesis. Germ layers are particularly pronounced in the vertebrates; however, all animals more complex than sponges produce two or three primary tissue layers...

 matrix
Matrix (biology)
In biology, matrix is the material between animal or plant cells, in which more specialized structures are embedded, and a specific part of the mitochondrion that is the site of oxidation of organic molecules. The internal structure of connective tissues is an extracellular matrix...

) upon which the nail rests, the part of the nail bed that extends beneath the nail root and contains nerves, lymph and blood vessels.
The matrix is responsible for the production of the cells that become the nail plate.
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Encyclopedia
A nail is a horn-like envelope covering the dorsal aspect of the terminal phalanges of fingers and toes in humans, most non-human primate
Primate
A primate is a mammal of the order Primates , which contains prosimians and simians. Primates arose from ancestors that lived in the trees of tropical forests; many primate characteristics represent adaptations to life in this challenging three-dimensional environment...

s, and a few other mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

s. Nails are similar to claw
Claw
A claw is a curved, pointed appendage, found at the end of a toe or finger in most mammals, birds, and some reptiles. However, the word "claw" is also often used in reference to an invertebrate. Somewhat similar fine hooked structures are found in arthropods such as beetles and spiders, at the end...

s, which are found on numerous other animals.

Fingernails and toenails are made of a tough protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

 called keratin
Keratin
Keratin refers to a family of fibrous structural proteins. Keratin is the key of structural material making up the outer layer of human skin. It is also the key structural component of hair and nails...

, 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.

Parts of the nail


The matrix (synonyms: matrix unguis, keratogenous membrane, nail matrix, onychostroma) is the tissue (or germinal
Germ layer
A germ layer, occasionally referred to as a germinal epithelium, is a group of cells, formed during animal embryogenesis. Germ layers are particularly pronounced in the vertebrates; however, all animals more complex than sponges produce two or three primary tissue layers...

 matrix
Matrix (biology)
In biology, matrix is the material between animal or plant cells, in which more specialized structures are embedded, and a specific part of the mitochondrion that is the site of oxidation of organic molecules. The internal structure of connective tissues is an extracellular matrix...

) upon which the nail rests, the part of the nail bed that extends beneath the nail root and contains nerves, lymph and blood vessels.
The matrix is responsible for the production of the 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 determines if the nail plate is flat, arched or hooked. The matrix will continue to grow as long as it receives nutrition and remains in a healthy condition.
As new nail plate cells are incubated, they emerge from the matrix round and white to push older nail plate cells forward; and in this way yet older cells become compressed, flat, and translucent, making the pink colour of the capillaries in the nail bed below visible.

The lunula
Lunula (anatomy)
The lunula, or lunulae , is the crescent-shaped whitish area of the bed of a fingernail or toenail. The lunula is the visible part of the nail matrix The lunula, or lunulae (pl.) (little moon in Latin), is the crescent-shaped whitish area of the bed of a fingernail or toenail. The lunula is the...

(occasionally called simply "the moon") is the visible part of the matrix, the whitish crescent-shaped base of the visible nail. The lunula is largest in the thumb and often absent in the little finger.
The nail bed is the skin beneath the nail plate.
Like all skin, it is composed of two types of tissues: the deeper dermis
Dermis
The dermis is a layer of skin between the epidermis and subcutaneous tissues, and is composed of two layers, the papillary and reticular dermis...

, the living tissue fixed to the bone which contains capillaries and glands, 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" known as the matrix crests or crests of nail matrix (cristae matricis unguis). During old age, the plate thins and these grooves are made evident in the structure.

The nail sinus (sinus unguis) is the deep furrow into which the nail root is inserted.

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

The nail plate
Nail plate
The nail plate is the hard and translucent portion of the nail, composed of keratin.- See also :* Runner's toe, repetitive injury seen in runners...

or body of nail (corpus unguis) is the actual nail, and like hair and skin, made of translucent keratin
Keratin
Keratin refers to a family of fibrous structural proteins. Keratin is the key of structural material making up the outer layer of human skin. It is also the key structural component of hair and nails...

 protein made of amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...

s. In the nail it forms a strong flexible material made of several layers of dead, flattened cells. The plate appears pink because of the underlying capillaries. Its (transversal) shape is determined by the form of the underlying bone. 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.
The hyponychium (informally known as the "quick") is the epithelium
Epithelium
Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. Functions of epithelial cells include secretion, selective...

 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.
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.

The eponychium
Eponychium
In human anatomy, the eponychium [Greek epi + onychion ], also known as the cuticle, is the thickened layer of skin surrounding fingernails and toenails. Beneath the cuticle is a thin layer of a membrane known as the pterygium. The function is to protect the area between the nail and epidermis...

is the small band of epithelium
Epithelium
Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. Functions of epithelial cells include secretion, selective...

 that extends from the posterior nail wall onto the base of the nail. Often and erroneously called the "proximal fold" or "cuticle
Cuticle
A cuticle , or cuticula, is a term used for any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticles" are non-homologous; differing in their origin, structure, function, and chemical composition...

", 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. The perionyx is the projecting edge of the eponychium covering the proximal strip of the lunula.

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.

The paronychium
Paronychium
Paronychium or the "lateral nail fold" is the term for the soft tissue surrounding the border of the fingernail or toenail. -Clinical significance:...

is the border tissue around the nail and paronychia
Paronychia
The nail disease paronychia , commonly misidentified as a synonym for whitlow or felon, is an often-tender bacterial or fungal hand infection or foot infection where the nail and skin meet at the side or the base of a finger or toenail...

 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.

The nail then acts 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.
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.
In humans, nails grow at an average rate of 3 mm (0.118110236220472 in) 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
Dehydration
In physiology and medicine, dehydration is defined as the excessive loss of body fluid. It is literally the removal of water from an object; however, in physiological terms, it entails a deficiency of fluid within an organism...

 and tightens, making the nails (and hair
Hair
Hair is a filamentous biomaterial, that grows from follicles found in the dermis. Found exclusively in mammals, hair is one of the defining characteristics of the mammalian class....

) appear to grow.

Medical aspects




Healthcare and pre-hospital-care providers (EMT
Emergency medical technician
Emergency Medical Technician or Ambulance Technician are terms used in some countries to denote a healthcare provider of emergency medical services...

s or paramedic
Paramedic
A paramedic is a healthcare professional that works in emergency medical situations. Paramedics provide advanced levels of care for medical emergencies and trauma. The majority of paramedics are based in the field in ambulances, emergency response vehicles, or in specialist mobile units such as...

s) often use the fingernail beds as a cursory indicator of distal tissue perfusion
Perfusion
In physiology, perfusion is the process of nutritive delivery of arterial blood to a capillary bed in the biological tissue. The word is derived from the French verb "perfuser" meaning to "pour over or through."...

 of individuals that may be dehydrated
Dehydration
In physiology and medicine, dehydration is defined as the excessive loss of body fluid. It is literally the removal of water from an object; however, in physiological terms, it entails a deficiency of fluid within an organism...

 or in shock. However, this test is not considered reliable in adults. This is known as the CRT or blanch test
Capillary refill
Capillary refill is the rate at which blood refills empty capillaries. It can be measured by holding a hand higher than heart-level , pressing the soft pad of a finger or toe until it turns white, and taking note of the time needed for the color to return once pressure is released. Normal refill...

.

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
Hypovolemia
In physiology and medicine, hypovolemia is a state of decreased blood volume; more specifically, decrease in volume of blood plasma...

 

Nail growth record can show the history of recent health and physiological imbalances, and has been used as a diagnostic tool since ancient times. Deep transverse grooves known as Beau's lines
Beau's lines
Beau's lines are deep grooved lines that run from side to side on the fingernail. They may look like indentations or ridges in the nail plate. This condition of the nail was named by a French physician, Joseph Honoré Simon Beau , who first described it in 1846.Beau's lines are horizontal, going...

 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
Mees' lines
Mees' lines or Aldrich-Mees' lines are lines of discoloration across the nails of the fingers and toes.- Causes :Mees' lines appear after an episode of poisoning with arsenic, thallium or other heavy metals, and can also appear if the subject is suffering from renal failure.- Presentation :They are...

, small white spots
Leukonychia
Leukonychia , also known as white nails, is a medical term for white discoloration appearing on nails. It is derived from the Greek words leuko and onyx...

, receded lunula, clubbing (convex), flatness, spooning
Spooning
Spooning or choreic hand is flexion and dorsal arching of the wrists and hyperextension of the fingers when the hands are extended sideways palms down....

 (concave) can indicate illness in other areas of the body, nutrient
Nutrient
A nutrient is a chemical that an organism needs to live and grow or a substance used in an organism's metabolism which must be taken in from its environment. They are used to build and repair tissues, regulate body processes and are converted to and used as energy...

 deficiencies, drug reaction or poisoning, or merely local injury. Nails can also become thickened (onychogryphosis
Onychogryphosis
Onychogryphosis is a hypertrophy that may produce nails resembling claws or a ram's horn, possibly caused by trauma or peripheral vascular disorders, but most often secondary to neglect and failure to cut the nails for extended periods of time...

), loosened (onycholysis
Onycholysis
Onycholysis refers to the detachment of the nail from the nail bed, starting at its distal and/or lateral attachment. It is said to occur particularly on the ring finger but can occur on any of the fingernails. The most common cause of onycholysis is psoriasis. It can also occur in thyrotoxicosis...

), infected with fungus
Fungus
A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds , as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants, animals, and bacteria...

 (onychomycosis
Onychomycosis
Onychomycosis means fungal infection of the nail. It is the most common disease of the nails and constitutes about a half of all nail abnormalities....

) or degenerate
Degenerative disease
A degenerative disease, also called neurodegenerative disease, is a disease in which the function or structure of the affected tissues or organs will progressively deteriorate over time, whether due to normal bodily wear or lifestyle choices such as exercise or eating habits...

 (onychodystrophy); for further information see nail disease
Nail disease
Nail diseases are distinct from diseases of the skin. Although nails are a skin appendage, they have their own signs and symptoms which may relate to other medical conditions. Nail conditions that show signs of infection or inflammation require medical assistance...

s.

Health and care


Bluish or purple fingernail beds may be a symptom of peripheral cyanosis
Cyanosis
Cyanosis is the appearance of a blue or purple coloration of the skin or mucous membranes due to the tissues near the skin surface being low on oxygen. The onset of cyanosis is 2.5 g/dL of deoxyhemoglobin. The bluish color is more readily apparent in those with high hemoglobin counts than it is...

, which indicates oxygen deprivation.

Nails can dry out, just like skin
Skin
-Dermis:The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane. It also harbors many Mechanoreceptors that provide the sense of touch and heat...

. They can also peel, break, and be infected. Toe infections, for instance, can be caused or exacerbated by dirty
Dirty
Dirty is the seventh album by the American alternative rock group Sonic Youth, originally released on July 21, 1992 by DGC Records. It was deemed best album of 1992 by Entertainment Weekly.-Recording:...

 sock
Sock
A sock is an item of clothing worn on the feet. The foot is among the heaviest producers of sweat in the body, as it is able to produce over of perspiration per day. Socks help to absorb this sweat and draw it to areas where air can evaporate the perspiration. In cold environments, socks decrease...

s, specific types of aggressive exercise, tight footwear
Footwear
Footwear consists of garments worn on the feet, for fashion, protection against the environment, and adornment. Being barefoot is commonly associated with poverty, but some cultures chose not to wear footwear at least in some situations....

, and walking unprotected in an unclean environment
Natural environment
The natural environment encompasses all living and non-living things occurring naturally on Earth or some region thereof. It is an environment that encompasses the interaction of all living species....

.

Nail tools used by different people may transmit infections. Nail file
Nail file
A nail file is a tool used to gently grind down and shape the edges of nails. They are often used in manicures and pedicures after the nail has been trimmed using appropriate nail clippers...

s, "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
Skin
-Dermis:The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane. It also harbors many Mechanoreceptors that provide the sense of touch and heat...

 infections in 2000 were traced to footbaths in nail salons. 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
Nail biting
Onychophagia, or nail biting, is a common oral compulsive habit in children and adults.-Classification:Nail biting is considered an impulse control disorder in the DSM-IV-R; the ICD-10 classifies it as "other specified behavioral and emotional disorders with onset usually occurring in childhood and...

.
Nail disease can be very subtle and should be evaluated by a dermatologist with a focus in this particular area of medicine. 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. Like any other nail, it can be cut using a nail clipper
Nail clipper
A nail cutter is a hand tool used to trim fingernails and toenails.-Design:Nail cutters are usually made of metal but can also be made of plastic. Two common varieties are the plier type and the compound lever type. A third variety of the push-button type has also been introduced. All are common...

.

Nutrition for healthy nails


Vitamin A
Vitamin A
Vitamin A is a vitamin that is needed by the retina of the eye in the form of a specific metabolite, the light-absorbing molecule retinal, that is necessary for both low-light and color vision...

 is an essential micronutrient for vision, reproduction, cell and tissue differentiation, and immune function. Vitamin D
Vitamin D
Vitamin D is a group of fat-soluble secosteroids. In humans, vitamin D is unique both because it functions as a prohormone and because the body can synthesize it when sun exposure is adequate ....

 and calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...

 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
Vitamin B12
Vitamin B12, vitamin B12 or vitamin B-12, also called cobalamin, is a water-soluble vitamin with a key role in the normal functioning of the brain and nervous system, and for the formation of blood. It is one of the eight B vitamins...

 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. Protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

 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
Folic acid
Folic acid and folate , as well as pteroyl-L-glutamic acid, pteroyl-L-glutamate, and pteroylmonoglutamic acid are forms of the water-soluble vitamin B9...

 and vitamin C
Vitamin C
Vitamin C or L-ascorbic acid or L-ascorbate is an essential nutrient for humans and certain other animal species. In living organisms ascorbate acts as an antioxidant by protecting the body against oxidative stress...

 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
Linoleic acid
Linoleic acid is an unsaturated n-6 fatty acid. It is a colorless liquid at room temperature. In physiological literature, it has a lipid number of 18:2...

. 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.

Fashion


Manicures and pedicures are health and cosmetic procedures to groom, trim, and paint the nails and manage callus
Callus
A callus is an especially toughened area of skin which has become relatively thick and hard in response to repeated friction, pressure, or other irritation. Rubbing that is too frequent or forceful will cause blisters rather than allow calluses to form. Since repeated contact is required, calluses...

es. They require various tools such as cuticle scissors
Scissors
Scissors are hand-operated cutting instruments. They consist of a pair of metal blades pivoted so that the sharpened edges slide against each other when the handles opposite to the pivot are closed. Scissors are used for cutting various thin materials, such as paper, cardboard, metal foil, thin...

, nail scissors, nail clipper
Nail clipper
A nail cutter is a hand tool used to trim fingernails and toenails.-Design:Nail cutters are usually made of metal but can also be made of plastic. Two common varieties are the plier type and the compound lever type. A third variety of the push-button type has also been introduced. All are common...

s, and nail file
Nail file
A nail file is a tool used to gently grind down and shape the edges of nails. They are often used in manicures and pedicures after the nail has been trimmed using appropriate nail clippers...

s. Artificial nails
Artificial nails
Artificial nails, also known as fake nails, false nails, fashion nails, nail enhancements, or nail extensions, are coverings placed over fingernails as fashion accessories. Some artificial nails attempt to mimic the appearance of real fingernails as closely as possible, while other designs may...

 can also be appended 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
Nail salon
A nail salon or nail bar is a beauty services establishment that primarily offers nail care services such as manicures, pedicures, and nail enhancements. Often, nail salons also offer skin care services. There are approximately 9,900 nail salons in the U.S., up 23% from 2007 according to the ....

 or nail shop or nail bar.

Painting the nails with nail polish
Nail polish
Nail polish, or nail varnish, is a lacquer applied to human fingernails or toenails to decorate and/or protect the nail plate.-History:...

 (also called nail lacquer and nail varnish) is a common practice dating back to at least 3000 B.C.

Non-human anatomy



The nails of primate
Primate
A primate is a mammal of the order Primates , which contains prosimians and simians. Primates arose from ancestors that lived in the trees of tropical forests; many primate characteristics represent adaptations to life in this challenging three-dimensional environment...

s and the hooves
Hoof
A hoof , plural hooves or hoofs , is the tip of a toe of an ungulate mammal, strengthened by a thick horny covering. The hoof consists of a hard or rubbery sole, and a hard wall formed by a thick nail rolled around the tip of the toe. The weight of the animal is normally borne by both the sole...

 of running mammals evolved from the claw
Claw
A claw is a curved, pointed appendage, found at the end of a toe or finger in most mammals, birds, and some reptiles. However, the word "claw" is also often used in reference to an invertebrate. Somewhat similar fine hooked structures are found in arthropods such as beetles and spiders, at the end...

s of reptile
Reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...

s.

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
Taxon
|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...

. 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 prosimian
Prosimian
Prosimians are a grouping of mammals defined as being primates, but not monkeys or apes. They include, among others, lemurs, bushbabies, and tarsiers. They are considered to have characteristics that are more primitive than those of monkeys and apes. Prosimians are the only primates native to...

s (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
Toilet-claw
A toilet-claw is the specialized claw or nail on the foot of certain primates, used for personal grooming. All prosimians have a toilet claw, but the digit that is specialized in this manner varies. Tarsiers have a toilet claw on toe two and toe three...

 (i.e. important for grooming activities). The needle-clawed bushbaby
Needle-clawed bushbaby
The needle-clawed bushbabies are the two species in the genus Euoticus, which is in the family Galagidae. Galagidae is sometimes included as a subfamily within the Lorisidae .* Genus Euoticus...

 (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).

A study of the fingertip morphology of four small-bodied New World monkey
New World monkey
New World monkeys are the five families of primates that are found in Central and South America: Callitrichidae, Cebidae, Aotidae, Pitheciidae, and Atelidae. The five families are ranked together as the Platyrrhini parvorder and the Ceboidea superfamily, which are essentially synonymous since...

 species indicated a correlation between increasing small-branch foraging and
  1. expanded apical pads,
  2. developed epidermal ridges (fingerprint
    Fingerprint
    A fingerprint in its narrow sense is an impression left by the friction ridges of a human finger. In a wider use of the term, fingerprints are the traces of an impression from the friction ridges of any part of a human hand. A print from the foot can also leave an impression of friction ridges...

    s),
  3. broadened distal parts of distal phalanges
    Distal phalanges
    The distal or terminal phalanges are the terminal limb bones located at the tip of the digits...

     (fingertip bone), and
  4. reduced flexor and extensor tubercles.

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.

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