Each finger and toe on the human body has a flat plate commonly called a nail on its uppermost surface. Nails are epithelial cell structures that are composed of a fibrous protein called keratin. The same protein is also the constituent of wool, hair, horn, hoofs, and the quills of feathers.

The chemistry of keratin makes nail very resilient. The keratin protein is made up of large quantities of the amino acid cysteine. Adjacent cysteine molecules are able to link together by virtue of disulfide bonds between sulfur residues that are part of the cysteine structure. The formation of many of these disulfide bonds produces keratin chains that are tightly linked to each other, much like bundles of twigs lashed together. The result is a bundle of great rigidity that is resistant to breakage. When these linked polypeptides are on a flat plane, as in a nail, the...