Anatomy and Physiology of the Hand, Arm, and Nail
Skin, the foundation
The nail is an appendage of the integumentary system. Skin has three layers: the epidermis (outermost, no blood vessels), the dermis (nerves, glands, blood vessels), and the subcutaneous layer (fat). This matters when you judge how aggressively to exfoliate, file, or massage.
Nail structures
The nail plate is the hard keratinized covering. The nail bed is the skin beneath it, rich in nerves and blood vessels. The matrix extends from the base to the lunula and produces the plate, so injury there permanently deforms the nail. The lunula is simply the visible portion of the matrix; it looks whiter because it is thicker. The cuticle is the dead tissue sealing the base of the plate, while the eponychium is the living skin overlapping it. The hyponychium is the skin under the free edge. The perionychium includes the nail folds and surrounding side tissue, and the mantle hides the matrix at the base.
Growth
Fingernails grow about 1/8 inch (roughly 3 mm) per month; toenails grow much more slowly. A fingernail replaces itself in about 4 to 6 months, a toenail in about 9 to 12 months. Growth speeds up in summer, in younger clients, during pregnancy, and on the dominant hand.
Bones, muscles, and circulation
There are 27 bones in the wrist and hand: 8 carpals, 5 metacarpals, and 14 phalanges. The thenar and hypothenar muscles control the thumb and little finger. The radial, ulnar, and median nerves supply sensation, and the radial and ulnar arteries supply blood.
Trap to remember
The nail plate has no nerve endings or blood supply, so filing it does not hurt. The moment a client feels pain, you have reached living tissue in the bed or sidewalls. Stop.