Nails

Nails, in humans, flattened, horny, protective coverings on the upper surface of the outer segments of the fingers and toes. The nails are made of dead cells containing the fibrous protein keratin. Each nail consists of a root, or part concealed within a fold of the 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. The matrix is thick and covered with highly vascular papillae, and its color is seen through the transparent horny tissue. Near the root the papillae are smaller and less vascular; thus, the portion of nail corresponding to this part is whiter. From its crescent form this portion is termed the lunula. The nail advances forward by the successive growth of new cells at the root and under the body of the nail. Homologous structures in animals include the hooves of horses and the claws of birds.