Nails do not keep growing after death, and they soak up more than skin
Nails are plates of alpha-keratin, the same protein found in claws, hooves and horns, and almost every primate has them except marmosets. Fingernails grow about 3.5 millimetres a month, toenails roughly half that, and despite the old story they do not keep growing after death.
A nail has four parts: the plate, the matrix, the bed beneath and the grooves around it. The matrix, under the skin at the base, is the only living, growing part; it makes cells that harden as they are pushed out to form the plate, and its size and thickness set the plate’s width and thickness. Its visible tip is the lunula, the pale crescent clearest on the thumb and often hidden on the little finger, which looks white because light reflects where matrix and bed meet.
The plate itself is layers of dead, compacted keratin cells, strong but flexible, with no nerves or blood vessels, and its curve follows the bone underneath. It rests on the nail bed, whose nerves and vessels feed the whole unit. Seals guard the edges: the hyponychium, or quick, under the free edge, and the cuticle, made by a skin fold called the eponychium, at the base. The soft border along the sides, the paronychium, is where hangnails, ingrown nails and infections start.
Nails shield the fingertip and press back against the finger pad, sharpening touch even though they feel nothing themselves, and they let us grip finely enough to pull out a splinter. Growth rate tracks the length of the end finger bone, so the index nail outpaces the little finger’s and finger nails can outgrow toe nails fourfold. A fingernail regrows fully in three to six months, a toenail in twelve to eighteen. The longest female nails on record totalled 8.65 metres.
Nails hold 7 to 12 percent water and are far more permeable than skin, so substances such as the herbicide paraquat can pass through. Doctors have read them since ancient times: Beau’s lines, deep grooves across the nail, often come with age but can signal illness.
Source: Nail (anatomy)
