The Physical Properties of Nail**From the Department of Dermatology. Massachusetts General Hospital and Harvard Medical School, Boston, Massanchusetts 02114.

The nails and claws of mammals contain α-type fibrous protein filaments. In the human nail the filaments are aligned perpendicular to the growth axis, while nails from other animals show different types of orientation depending on their shape. Studies of water content and diffusion suggest that nail is similar in structure to hair and quite different than epidermis. The lipids which have been observed in nail by x-ray diffraction analysis may be in part fatty acids salts derived from soaps. Measurement of the modulus of elasticity of nail by two different techniques gives values similar to that of hair and suggests that connections between cells contribute to the strength of nail tissue