NAIL (AS. mega, OlIG. nagal, Ger. Nagel, nail, Goth. to nail; connected with ()Church Slay. nogtii, Lith. nagas, Skt. nakhu, nail). The terms nail, claw, and hoof are popu lar names for different developments of the same epidermal formation. All are horny secretions formed from the outer layer of the skin, pri marily to protect the tip of the digits in the higher vertebrates. Nails have retained this function, while claws have become specialized as weapons of offense or defense, and hoofs are specialized to protect the whole of the terminal joint of the digit. Hoofs are usually thick, heavy, and blunt, and are best developed where the number of digits is reduced; they are char acteristic of the great group of herbivorous animals known as Ungulata, and reach their highest development in the horse. Nails are flattened. usually thin, and more or less rounded at the end. They are present in some reptiles, in certain birds (as the grebes), and in many mammals. especially the monkeys and man. Com pa re lIcmx.
Itt•mAN NAILS. These are the elastic, horny plates placed as protective coverings on the dorsal surface of the terminal plmlanges of the fingers and toes. Each nail consists of a root, or part concealed a fold of skin; a body, or exposed part attached to the surface of the skin; and a free anterior extremity called the edge. The skin below the root and body of the nail is termed the matrix, from its being the part from \Odell the nail is produced. This is thick and covered with highly vascular papilla., and its color is seen through the transparent horny tissue. Near the root the papilla' are smaller and less vascular: hence the portion of nail corresponding to this part is of a whiter color; from its form, this portion is termed the 11111111a. The chemical composition of the
nails is given in the article Hoax. According to the observation of Beau, the finger-nails grow at the rate of about two-tifths of a line in a week, while the toe-nails only grow with about one-fourth of that rapidity. The general state of health influences the growth of nails, and after illness a deficient formation of horny matter may result, shown by the production of a groove across the nail. The breadth and position of this groove indicate roughly the period and duration of the illness. An inflammation of the matrix of the nail is vaned onychia, and an acute inflammation of the tissues about the nail is termed poronychio or whitlow. When a nail has lwen removed by violence, or has been thrown off in consequence of the formation of matter (pus) beneath it. a new nail is speedily formed, provided the matrix. has not been seriously in jured.
There is a very common and troublesome affec tion popularly known as ingrowing nail. Its most usual scat. is the great toe. It does not in reality arise from any alteration of the nail. but from the adjacent soft parts being constantly pressed by the use of tight shoes against its edge. These parts swollen and inflamed; suppuration ensues, and an intensely sensitive ulcer is formed, in which the nail is imbedded. In obstinate cases, it is not infrequently necessary to re move a portion of the nail, an operation which may be done tinder local amesthesia without pain.