12 COMPARATIVE PHYSIOLOGY 



how the appUcation of acid to a colloidal system may give rise 

 to a considerable quantity of mechanical energy. Thin strands 

 of catgut immersed in acid undergo quick and extensive 

 shortening ; the process is completely reversible when the 

 acid is removed, and can be repeated indefinitely. Shortening 

 in the length of a muscle-fibre could occur without a change in 

 volume by increased curvature of the surface ; as regards the 

 muscle as a whole, this as a matter of fact is what does happen. 

 Now Hartridge and Peters have shown that increase of hydrogen 

 ion concentration increases the surface tension at an oil-water 

 interface, and Neuerschloss finds that analogous phenomena 

 occur with lecithin sols. There is very good reason, based 

 on the penetration of dyes into living cells, to bear out the 

 conclusion that lipoid substances accumulate at the cell surfaces 

 and interfaces. Hill has shown that, provided that the muscle 

 is allowed to shorten before maximum tension has been 

 developed, the heat-production of a muscle which is released 

 after stimulation so as to shorten (without lifting a weight) 

 is less than in a rigidly isometric twitch ; this is so if the muscle 

 shortens while the processes which give rise to tension are 

 still at work, but if the muscle is liberated after maximum 

 tension has been attained there is no diminution of heat- 

 production. Were the heat-production uniformly distributed 

 through the substance of the muscle one can see no reason why 

 this should occur, since the muscle does not change its volume. 

 But if the heat production is located at definite interfaces, it 

 must tend to become smaller if the area of these interfaces is 

 reduced, as it presumably must be, when the muscle shortens. 

 (e) Electrical Phenomena in Muscle. — When the cut end of a 

 muscle is connected with one lead of a delicate galvanometer 

 and the uninjured surface with the other, there is found to be 

 a difference of potential between the two surfaces, the cut end 

 being negative to the uninjured surface of the muscle. When 

 the latter is stimulated, there is a diminution of this potential. 

 This diminution or '' negative variation " is referred to as 

 current of action^ and flows in the opposite direction to the 

 normal or demarcation current which is from the cut to the 

 uninjured surface of the resting muscle. It is not a specific 



