Scientific Concepts: Nail and Skin Structure, Anatomy, and Chemistry
The nail unit, layer by layer
The nail plate is a translucent sheet of keratin with no nerves or blood vessels. It is produced by the matrix, the only part of the unit that creates new plate; the matrix's length and curve determine plate thickness and shape. The nail bed lies beneath the plate and carries the blood and nerve supply - it does not make nail. The lunula is the visible portion of the matrix, which is why it is whitish. The eponychium is the living skin overlapping the matrix; the cuticle is the dead tissue clinging to the plate, which is what a technician may loosen and gently remove. The hyponychium seals the free edge, the perionychium is the tissue around the nail, and the lateral folds border the sides.
Growth: fingernails average about 3 mm (1/8 inch) per month and replace in 4–6 months; toenails grow more slowly and take 9–12 months. Growth is fastest on the middle finger and on the dominant hand, and faster in summer than winter. Nutrition, illness, pregnancy and trauma all alter growth.
Skin: epidermis (outermost, with the keratin-rich stratum corneum and melanocytes), dermis (nerves, vessels, glands), subcutaneous layer. Hand anatomy: 14 phalanges, 5 metacarpals, 8 carpals; flexor and extensor muscles; digital arteries and nerves running along the sides of each finger - which is why nipping a sidewall can bleed heavily or hurt sharply.
Chemistry: skin pH is about 4.5–5.5 and the nail plate about 5, so alkaline products swell and soften the plate. Nail products work by polymerization (monomers joining into polymers) or solvent evaporation; adhesion depends on a clean, dehydrated, lightly etched surface.
Worked example / trap: damage to the matrix causes a permanent ridge or groove; damage to the bed causes only temporary disturbance. Any question blaming the bed for a lasting deformity is wrong.