_Influence of Muscular Contraction on the Intramuscular Osmotic Pressure._--When a muscle is immersed in an isotonic salt solution it does not change in weight. In a hypertonic solution it loses weight in consequence of a loss of water, which passes from the muscle into the solution to equalize the osmotic pressure. It gains weight in a hypotonic solution, the water current setting towards the point of higher concentration. It is easy, therefore, to tell whether the osmotic pressure in a muscle is above or below that of a given solution, by observing whether the muscle gains or loses weight when immersed in it. Thus we may measure the osmotic pressure in a muscle by finding a salt solution in which the muscle neither gains nor loses weight. In this way we have been able to prove that the osmotic pressure of a tired muscle is higher than that of the normal muscle. Our experiments were carried out on the muscles of frogs. After having pithed the frog, one of the hind legs is removed by a single stroke of the scissors. The leg is skinned, dried with blotting paper, and weighed. It is then placed in a salt solution whose freezing point is -.53deg C. At 15deg C. such a solution has an osmotic pressure of 6.6 atmospheres. We next proceed to determine the osmotic pressure of the corresponding leg after it has been tired by muscular work. For this it is stimulated by an intermittent faradic current passing once a second for five minutes. The leg is then skinned, dried, weighed, and placed in the same salt solution. After eight hours' immersion the legs are weighed again. The following are the results of six experiments, the numbers representing fractions of the original weight:-- Change of weight of untired leg-- After 8 hours -.000. After 16 hours -.000. After 24 hours -.006. Change of weight of stimulated leg-- After 8 hours +.050. After 16 hours +.080. After 24 hours +.101. {54} This result shows that muscular work provoked by electric stimulation noticeably increases the osmotic pressure of the muscle. In order to discover the exact osmotic pressure in the stimulated muscles we repeated the series of experiments, using more and more concentrated solutions. In a solution whose freezing point was -.57deg, we obtained the following values:-- Change of weight of untired leg-- After 8 hours -.000. After 16 hours -.004. After 24 hours -.006. Change of weight of stimulated leg-- After 8 hours +.039. After 16 hours +.072. After 24 hours +.099. Finally, in a solution freezing at -.72deg, _i.e._ with an osmotic pressure at 15deg C. of 9.176 atmospheres, we obtained the following mean values for the untired leg:-- After 8 hours -.04. After 16 hours -.05. After 24 hours -.05.