Page:Life in Motion.djvu/104

84 experiment on a muscle, the line joining the vertical lines that represent the stretchings is not straight but it is a curve, the curve of a hyperbola, in mathematical language; that is to say, a curve which constantly tends to become parallel with a horizontal line in the same plane but never reaches this condition.

The fact that muscles can be stretched, and that they return to their first length when the stretching force ceases to act, is of great importance to their mode of action. In the body, the muscles are always partially on the stretch. They are never "on the slack," to use a familiar phrase, and they are ready to exert a pull on the bones to which they are attached, the instant they begin to contract. Thus no power is wasted. Further, as has been ingeniously shown by Professor Marey, the elasticity breaks the force of the shock produced by the sudden contraction of the muscle, and the energy of the contraction is expended more gradually and effectively than if the muscle had been non-elastic. Suppose a horse drawing a cart along a rough road by non-elastic ropes or chains attached to its