Measurements of the extensibility of muscles and other tissues were first made, as far as the authors are aware, by Wertheim in 1846. He proposed for them a hyperbolic law of elasticity of the form: y2 = ax2 + bx. Further measurements were made at much later dates by Marey and by Howell, who did not attach to them any mathematical law of elasticity.The present writers have investigated the elasticity of muscles again, and have found that their own data, as well as those of previous investigators, conform to the logarithmic law,[Formula: see text]where E is the extension, W the stretching force, and k and c are constants.This law is found to hold for muscles both striped and plain, and for nerve tissue. It seems to be true for all tissues except bone which, according to Wertheim, follows Hooke's law of elasticity for inorganic elastic matter.