The nail (from the Latin ungula), a hard and translucent blade, covers the tips of the fingers of the hand and of the toes. In humans, there are 20 of them. Formed of keratin, it protects the fingertips and allows the grip of objects.

The nail, or ungal tablet, is located on the dorsal surface of the fingers and toes. It is transparent and almost colorless although it usually sports a pinkish tinge due to the presence of many blood vessels under the skin. Smooth in appearance, it is approximately 1 to 2mm thick.

The nail is a modification of the epidermis that corresponds to the hoof or claw of animals. It is made up of different parts:

• The body of the nail, attached and visible part, and its free edge,
• The root of the nail, part buried under the skin,
• Le rampart of the nail, skin fold that covers the edges. The thick part that borders the base of the nail is a flexible membrane commonly called the cuticle.
• The nail bed, layer of skin on which the nail rests and adheres,
• The nail matrix, a layer located under the skin and composed of epithelial cells which are responsible for the growth of the nail,
• The lunula, a light-colored area in the shape of a crescent moon on the base of each nail.

In the fetus, the nail appears from the 4th month of pregnancy. It is at the level of the nail matrix that its growth takes place: the nail cells are produced there, then they thicken more and more by a phenomenon of keratinization and die. The nail is then composed of a very resistant protein, keratin, which gives it its protective role. We find the same principle for hair growth.

The nail grows throughout life at a rate of 0,1mm per day for the fingernails and half as fast for the toenails. The renewal of a nail of the hand is done between 4 and 6 months while for the foot it generally takes 12 to 18 months.

Age and many factors (hormonal, nutritional, medicinal, vascular, etc.) can influence its growth.

Role

The nails perform several functions:

• Protection: obviously, they protect the tips of the fingers of the hand and toes from shocks or other external aggressions.
• Stiffness: by making the fingertips rigid, they can catch small objects.
• Sense of touch: richly innervated and vascularized, they help refine our sense of touch at the extremities.
• Practical: they are useful for scratching, tearing or even scratching.

Ingrown nail (or onychocryptosis): appears when one of the upper corners of the nail penetrates the flesh that surrounds it. This causes inflammation, pain and sometimes infection.

Panaris : infection which is located in two thirds of cases on the periphery or on the underside of the nail. Most often, it is caused by a germ of the Staphylococcus aureus type, but it can also be streptococcus, enterococcus, etc.

Black nail : it is the appearance of a hematoma following a shock. It is the effusion of blood under the nail that forms a black spot.

Onychomycose : infection of the nails with a fungus that thrives in hot, humid and dark environments, such as shoes. These fungi feed on keratin, the protein that makes up the nail.

Onycholysis: detachment of the nail from its bed from the free edge. It can be caused, for example, by psoriasis, eczema or handling chemicals.

Onychophagie : Compulsive attitude to biting nails that usually reflects a feeling of stress or anxiety. Appears during childhood and may persist into adulthood.

Melanoma : The appearance under the nail of a brown or black band in the direction of the length can be the sign of a melanoma of the skin.

Paronychie : inflammation of the soft tissues around the nail, such as the cuticle. 

Many factors can influence the shape and appearance of the nails (3).

First, genetic factors determine the shape and natural appearance of the nail. Diet also plays an important role in the appearance of the nail. Deficiencies or diets can impact the health of the nail. A balanced diet rich in nutrients is beneficial to them. The white spots that sometimes dot the nails, however, are not linked to a lack of calcium even if calcium has been shown to be important for the quality of their structure.

Certain behaviors impact the nails. Smoking is known to turn fingernails yellow for example. Wearing artificial nails can involve the risk of allergies to products, weakening or even infections. Several studies6,7 have shown that false nails are conducive to the appearance of fungal infections and other bacterial infections, thus increasing their spread. Healthcare personnel and those working in the food industry are therefore not recommended to wear false nails. For the simple application of colored varnish, it is advisable to apply a protective base beforehand.

For people who bite their nails, applying bitter-tasting varnish (8) can in some cases suppress the urge to bite them. If this may dissuade some, note that often wearing your nails to the mouth is conducive to bacterial and viral contaminations which can for example cause gastroenteritis.

Trying to replace this habit with another may also be advisable. If the attitude is too compulsive, psychological follow-up is possible.

Throughout the ages, the fingernail has been used for various purposes. In Antiquity and the following times, the nail was an integral part of certain beliefs. It is, for example, added in love potion recipes or in gris-gris to identify the desired people (9). It is also used in potions to cure certain illnesses. History does not tell us if this ingredient was very effective …

Today, the nail is the subject of special attention in forensic medicine (10). A potential source of information, it is studied by the medical examiner during an examination of the body. The fingernails, fingers and toes can carry the trace of cyanosis (corresponds to a blue tint of the tissues which indicates an insufficient transport of oxygen by the blood). In practice, it can be assumed that the victim already lacked oxygen before dying. This is the case, for example, of people hanged, suffocated or died by a toxic product.

The nails are also cleaned because they can be carriers of clues: the victim having been able to scratch his attacker or tear off clothes, one would find there samples of skin or fibers which would help in the identification of the culprit.