NAIL, an elastic horny plate on the upper or dorsal surface of the end of a finger or toe, as in man and monkeys. Hoofs, claws, talons, sheath-horns and the bills of birds are analo goes. Nails and claws of all kinds are modi fications of the epidermis, identical in forma tion and mode of growth. The root of the nail rests in a matrix which is a fold of the dermis, particularly rich in vascular papillae from which the nail-cells are produced. The pink color of a healthy nail is due to the blood beneath. The little white area toward the root of a nail, called lunula from its crescentic shape, has less blood under it. When nails are de stroyed new ones will be formed if the matrix is uninjured. Nails are a support and a defense to the ends of the fingers and toes, assist in picking up small objects, and if healthy and in good condition add comeliness to the parts to which they are attached. To most animals possessing them they are of great importance, giving a needed rigidity to fingers and toes, and adapting them to a great variety of neces sary utilities, as firmly seizing and holding prey (perfected in the retractile apparatus of feline claws), scratching, digging, searching crevices for food and as formidable weapons. In horses, cattle and other ungulated animals, they enclose some or all the digits and are called hoofs (q.v.). In the sloths the nails assume a relative size and are used as a chief means in arboreal progression. In the Amphibia — as in some toads, efts, etc.— the nails are repre sented in their simplest form and appear as mere thickenings of the skin at the extremities of the digits.
In man the nails appear about the third month of fetal or embryonic life. After birth the nails of the hand grow at the rate of about one millimeter per week, those of the foot about one millimeter per month.
The health of the nails is affected, as is that of the skin, by local or general diseases. They may become thickened (hypertrophy) as the re sult of inflammation or a degeneration of nerves; diminished in size (atrophy) from trau matic influences; malformed as the result of disturbed function of the matrix; degenerated from faulty nutrition and discolored in various diseases, the discoloration probably being in the tissues beneath. Nail-biting is a bad habit not only leading to a morbid condition of the nails themselves but also sometimes being an indica tion of some types of nervous disease. The nails are also subject to parasitic diseases. A hang-nail is a sliver of skin attached at one end; it should be cut off close to the point of at tachment. The familiar white areas on the nails are considered to be injury received by the nails in being manicured. The painful af fection called ingrowing nail usually occurs in the great toe, not through alteration in the nail itself, but from constant pressure of the adjacent soft parts against its edge by a tight shoe. The irritation often results in the for mation of an ulcer, extremely sensitive, and sub ject to continual aggravation from the embedded nail, and in some cases necessitates a slight operation to excise a portion of the nail and the nail fold.