180 GEORGE EALPH MIXES. 



production and sliortening in muscle disproves the hypotliesis that 

 the shortening is an effect due to the action of local rise of temperature 

 on some constituent of the muscle structure. The muscle is certainly 

 not a heat engine. 



The so-called osmotic theory of contraction — which supposes that as 

 a result of the liberation of a high concentration of electrolytes within 

 certain membranous compartments of a particular shape, an increase of 

 hydrostatic pressure is set up within tliem causing them to bulge and 

 shorten, thus producing the shortening of the whole muscle — meets 

 with the objections that the osmotic pressure which would be necessary 

 to give rise to mechanical effects of the magnitude encountered in 

 muscular contraction would necessitate extraordinarily high concentra- 

 tions of electrolytes, and such changes could scarcely be completed in 

 the very brief interval which often suffices for the completion of a 

 muscle twitch. 



The sort of explanation which has most to recommend it is that 

 which refers the act of shortening to a mechanism in which the 

 alteration of surface tension at the junctions of different pliases is the 

 motive power. Anyone who has studied the behaviour of a globule of 

 mercury, lying in a dish of dilute sulphuric acid to wliich a trace of 

 potassium chromate has been added, when touched by a clean iron 

 wire, will recognize in the sudden drawing together of the globule 

 movements which recall more forcibly than any other artificial scheme 

 the movements of living contractile tissue. Here, too, the evolution of 

 a considerable amount of kinetic energy is the result of a very small 

 amount of chemical change. 



As to the mechanism by which surface tension alterations can be 

 supposed to play a part in the muscle, there are two chief hypotheses. 

 Both suppose that as a result of the process of excitation in the muscle 

 there is a liberation of excess of some electrolyte in the tissue. 

 According to one view, an ion of this electrolyte combines chemically 

 with some protein constituent of the muscle to form a new ion-protein 

 compound possessing different physical properties — the protein being 

 so disposed in the muscle that an alteration in its surface tension will 

 exert a pull on the whole structure and lead to shortening. 



Another view is that the electrolyte is liberated in a system pre- 

 senting surfaces possessed of differential permeability towards the ions 

 and that thus the liberated electrolyte sets up difierences of electrical 

 potential across these surfaces resulting in mechanical strain and 

 alteration in the contour of these surfaces. This last view, though it 

 is at present of necessity somewhat vague, has on the whole more to 

 recommend it than either of the others. The electrical phenomena 



