Anatomic considerations of the human nail.

The nail unit is distinctive for human beings. Developmentally, the fetus demonstrates cranial and caudal paired anlage of the upper and lower extremities. The leg bud develops a foot plate and eventually toe rays that are first notched, later webbed, and then develop into separate toes. Distal on the toe, the primitive nail unit appears and eventually moves dorsal. When developed fully, the nail unit consists of the nail plate, nail bed, ungualabia, eponychium, hyponychium, lunula, and nail matrix. These structures are associated intimately with the terminal or ungual phalanges of the foot's five toes.