In this solution, freezing at -.72deg C., some of the stimulated muscles showed no diminution in weight, while others showed a very small diminution, and others again a slight augmentation, the maximum increase being .085 of the initial weight. The solution is therefore practically isotonic with the stimulated muscle. In this case the elevation of the intramuscular osmotic pressure produced by the electrical excitation and the muscular contractions was therefore 2.5 atmospheres, or more than 2.6 kilogrammes per square centimetre of surface. I made further experiments in order to discover whether the variation in osmotic pressure depended on the duration of {55} the muscular contraction. For this purpose I used a solution freezing at -.53deg C. and immersed in it untired muscles, and muscles which had been electrically excited for two, four, and six minutes respectively. The following are the results:-- Untired muscles. Muscles stimulated once a second during 2 Minutes. 4 Minutes. 6 Minutes. .000 +.026 +.084 +.094 +.001 +.034 +.065 +.093 +.005 +.045 +.079 +.097 .000 +.037 +.070 +.095 .000 +.032 +.072 +.096 Mean of all the observations-- +.0012 +.0348 +.074 +.095 These experiments show clearly that the osmotic intramuscular pressure rises in proportion to the duration of the electrical stimulation. In order to determine the influence of the work accomplished by the muscle on the elevation of the osmotic pressure, I made the following experiment. The two hind legs of a frog were submitted to the same electrical excitation, one leg being left at liberty, and the other being stretched by a hundred-gramme weight, acting by a cord and pulley. After exciting them electrically for five minutes, the legs were immersed for twenty-four hours in a saline solution freezing at .53deg C. The free limb showed an augmentation of .085 of the initial weight, and the stretched limb an increase of .106 of the initial weight. It is evident, therefore, that the osmotic pressure increases with the amount of work done by a muscle. Briefly, then, the results of our experiments are as follow:-- 1. Muscular contraction electrically produced causes an increase of the osmotic pressure in a muscle. 2. The intramuscular osmotic pressure may reach, or even exceed, 2.5 atmospheres, or 2.6 kilogrammes per square centimetre of surface. 3. When a muscle is made to contract once a second, the {56} elevation of the osmotic pressure increases with the number of contractions. 4. The intramuscular osmotic pressure increases with the work done by the muscle. 5. Fatigue is caused by the increase of osmotic pressure in a contracting muscle. [Illustration: FIG. 3.--Fields of diffusive force.