The hard part of the fingernails and toenails contains a tough protein called keratin. Human nails (which actually are vestigial claws) have three parts: a root bed at the nail base, a body that’s attached to the fingertip, and a free edge that grows beyond the end of the finger or toe.
Heavily cornified tissue forms the nails from modified strata corneum and lucidum. A narrow fold of the stratum corneum turns back to form the eponychium, or cuticle. Under the nail, the nail bed is formed by the strata basale and spinosum.
At the base of the nail, partially tucked under the cuticle, the strata thicken to form a whitish area called the lunula (literally “little moon”) that can be seen through the nail. Beneath the lunula is the nail matrix, a region of thickened strata where mitosis pushes previously formed cornified cells forward, making the nail grow.
Under the free edge of the nail, the stratum corneum thickens to form the hyponychium. Nails are pinkish in color because of hemoglobin in the underlying capillaries, which are visible through the translucent cells of the nail.
On average, fingernails grow about 1 millimeter each week. Toenails tend to grow even more slowly. Nails function as an aid to grasping, as a tool for manipulating small objects, and as protection against trauma to the ends of fingers and toes.