i6o PROTOPLASMIC ACTION AND NERVOUS ACTION 



interior of the cell, solely by altering the state of the 

 plasma membrane, and there is much additional evidence 

 that this is the case. The analytical results of Urano 

 and Fahr^ show that in normal vertebrate muscle Na 

 is almost entirely absent from the cell interior, although, 

 as just shown, it is essential to the maintenance of irrita- 

 bility. Urano kept frogs' sartorii for several hours in 

 isotonic sugar solution, and found that the muscle gave 

 off to the solution relatively much more Na than K. 

 Muscles thoroughly extracted in sugar solution were found 

 to contain only 2 per cent or less of the normal total Na- 

 content, but nearly all of the K and phosphate. The 

 conclusion seems certain that the Na is contained almost 

 entirely in the intercellular spaces of the tissue, and the 

 K and phosphate in the cells. Muscles kept for six 

 hours in sugar solution lose very little ash that may not 

 be accounted for by the salts present in the interstitial 

 lymph. Fahr, in an ash-analysis of the extract of fresh 

 uninjured muscles in isotonic sugar solution, found that 

 only 6 per cent of the original K of the tissue, but 90 

 per cent of the original Na, was thus recoverable. If 

 four-fifths of the total volume of the muscle be regarded 

 as consisting of muscle cells, and one-fifth of interstitial 

 tissue and lymph spaces, the Na-content of the tissue 

 is completely accounted for by the salts present in the 

 lymph. The latter must therefore exert their physio- 

 logical influence through their action upon the external 

 protoplasmic layer or plasma membrane. 



Hober's experiments on the influence of isotonic 

 solutions of neutral salts on the demarcation-current 

 potential of muscle^ also support Overton's view that 



^ Loc. cit. * Hober, Arch, ges. Physiol.^ CVI (1905), 599. 



