of animals, are a form of the outer layer of the skin. In man the nails do not enclose the ends of the fingers and toes, but in the horse and other animals the nails assume the form of protective coverings and are then known as "hoofs." Nails may be produced to form "claws," as in birds and flesh-eating animals, while in the sloths they are large enough to aid in climbing trees. The human nails consist of a root, which is hidden below the skin, and an exposed part, which is attached to the skin. Both are produced from the true skin. They grow in length about one-thirtieth of an inch in a week on the fingers and more slowly on the toes. If a nail be removed by accident, it will grow again, provided the cells which secreted it have not been injured. The light spot at the base of the nail is called the lunula.