chemical composition increases the surface tension of the cell sufficiently to provoke its contraction into a more spherical form. Ostwald has introduced a very useful conception for the study of this question of surface energy. A liquid surface contains a quantity of energy equal to its surface tension multiplied by its area, hence any variation either of area or of tension corresponds to a variation of its energy. This novel conception constitutes a valuable addition to the experimental study of the physiology of muscular action, since it gives us some idea of the mechanism by which chemical energy may be transformed into muscular contraction. Whatever the mechanism of transformation in the animal machine, we have to consider the same quantities as in other motor machines. These are: (1) the efficiency; (2) the potential energy; (3) the power; (4) the energy given up to the medium under the form of heat; (5) the temperature. Muscles, then, are merely transformers which change chemical energy into mechanical work, the diminution of stored-up energy in a muscle being expressed by the sensation of fatigue. A muscle may be studied in four different phases: (1) in repose; (2) in a state of tension; (3) when doing positive work; (4) when work is being done on it. When a muscle is in a state of tension, as when a weight is sustained by the outstretched arm, the muscle is producing no external work. The entire work done is converted into heat; just as it is in a dynamo or steam engine which is prevented from turning by a brake. Muscular contraction produces fatigue even when it does no external work. It is impossible for the muscle to support even the weight of the outstretched arm itself for any considerable time. A muscle is doing positive work when it is raising a weight or moving a body from one point to another. {110} The fourth state of muscular contraction is when the muscle is doing negative work, _i.e._ when work is being done on it, as for instance when we go downstairs, or when a descending weight forces down the opposing arm which attempts to support it. In this case the muscles receive a portion of the energy lost by the descending weight, and this energy shows itself in the muscle in the form of heat. This increase of heat in a muscle doing negative work has been clearly demonstrated by the calorimetric experiments of Hirn and the thermometric experiments of Beclard. Hirn's observations on muscular calorimetry show a production of heat corresponding to 150 calories per hour when in repose, 248 calories per hour during positive work, and 287 during negative work. Beclard's thermometric measurements also show that the temperature of a muscle rises each time that it contracts, and that the rise of temperature is greatest when the muscle is doing negative work, least during positive work, and intermediate when in a state of tension.