Anatomy and Physiology of the Nail, Hand, and Skin
The nail unit
The nail plate is the visible keratin structure produced by the matrix, the tissue at the proximal end beneath the proximal nail fold. The lunula is the whitish visible portion of the matrix; injury there can permanently distort growth. The nail bed lies beneath the plate and carries blood vessels and nerves, while the plate itself has no nerves or blood supply - which is why filing does not hurt but a nick to the bed does. The eponychium is the living fold of skin at the base; the cuticle is the thin dead tissue sealing plate to fold. The hyponychium is the seal under the free edge; the perionychium includes the lateral folds and surrounding tissue. Fingernails grow roughly 0.1 mm per day (about 3 mm per month), faster in youth, in summer, and on the dominant hand; toenails are much slower.
Skin, bone, and vessels
Skin has epidermis, dermis, and subcutaneous layers. The nail sits over the distal phalanx, supplied by digital arteries and nerves and moved by extensor and flexor tendons. Nails hold about 15–20% water and are far more permeable than skin, which is why solvents penetrate and why repeated wet-dry cycling causes splitting.
Disorders to recognize
Onychomycosis (fungal), paronychia (bacterial fold infection), onycholysis (separation), leukonychia (white spots, usually minor trauma), Beau's lines (growth arrest), pterygium, koilonychia, and green discoloration from Pseudomonas are common in questions. The technician recognizes and refers; never diagnoses or treats.
Rule of thumb: matrix = growth, bed = attachment, eponychium = living, cuticle = dead.