= Nail
{disambiguate=anatomy}
A <nail (anatomy)> is a specialized structure of the <epidermis> at the tip of a finger or toe. Its visible <nail plate> is a compact layer of dead <keratinocytes> rich in <keratin>. Living proliferating cells in the <nail matrix> produce the material that advances towards the free edge. The <nail plate>, its producing <nail matrix> and the supporting <nail bed> are therefore different parts of the apparatus.<Anatomy and Physiology 2e by J. Gordon Betts and others>[Section 5.2, Nails and Figure 5.13: keratinized cells, matrix growth and underlying nail bed]{ref}
The firm <nail plate> protects the fingertip and supplies a backing surface when the finger manipulates a small object. The pink appearance mainly comes from the vascular tissue underneath, rather than blood vessels within the dead plate. Growth requires cell production at the base; the exposed distal material does not lengthen itself by dividing.<Anatomy and Physiology 2e by J. Gordon Betts and others>[Section 5.2, Nails: protective/back-support function and vascular nail bed]{ref}
A direct growth experiment follows a mark rather than repeatedly measuring the free edge, which can be clipped or worn away. <S. Yaemsiri> and colleagues studied 22 healthy young adults who used a supplied nail file to mark near the proximal nail fold. They recorded distance from the fold and repeated the measurement one to three months later. The observed mean rates were $3.47\,\mathrm{mm/month}$ for fingernails and $1.62\,\mathrm{mm/month}$ for toenails.<Growth rate of human fingernails and toenails in healthy American young adults by S. Yaemsiri and others>[Abstract, Methods and Results]{ref} These are study averages, not universal rates for every person or digit.
For example, a mark moving from $1.0$ to $7.0\,\mathrm{mm}$ from the chosen fold over 60 days gives a mean advance $(7.0-1.0)/60=0.10\,\mathrm{mm/day}$. This constructed calculation measures displacement over that interval. Interpreting a chemical measurement from a clipping as a dated exposure record requires more: the person's growth history, the clipping's position and possible external contamination all matter. A single current growth-rate estimate does not establish an exact date for every part of the <nail plate>.
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