INORGANIC SALTS 163 



concerned in normal irritability;^ a striking example is 

 the marked hypersensitivity induced in frog's muscle 

 and nerve by Na salts whose anions precipitate calcium 

 or form Ca salts of limited solubility. Muscles treated 

 for a few minutes with isotonic solutions of these salts 

 (sulphate, phosphate, tartrate, citrate, oxalate, etc.) 

 become highly sensitive to the contact of foreign sub- 

 stances or media, and contract vigorously when exposed 

 to air.^ These phenomena of sensitization are produced 

 so rapidly as to leave little doubt that they depend on 

 alterations in the surface-films of the cells. ^ 



Direct evidence that changes in the permeability of 

 the plasma membranes play an essential part in processes 

 of stimulation will be cited later. Apparently all 

 substances that alter the physical or chemical state of 

 the plasma membrane (salts, acids, alkalis, narcotic 

 agents) modify the stimulation-process; this influence 

 may be in the direction either of facilitation or of repres- 

 sion, according to conditions."* 



The precise means by which salts produce these 

 effects has been much debated and is still imperfectly 



^ Cf. the experiments of A. J. Clark, cited on page 185. 

 ^ J. Loeb, American Journal of Physiology, V (1901), 362. 



3 Other effects dependent on surface changes in the cells are related 

 to those just described; e.g., changes in the surface-films of blood 

 corpuscles and bacteria, affecting the critical concentrations of haemolysis 

 or agglutination by H ions, are produced by the addition of proteins; 

 cf. the recent papers of Coulter, Jour. Gen. Physiol., Ill (1921), 309, and 

 IV (1922), 403; Northrop and De Kruif, ibid., IV (1922), 655; also 

 Eggerth and Bellows, ibid., p. 669. 



4 Such effects evidently imply an influence on cell metabolism. 

 Warburg's observations on sea-urchin eggs show that the oxygen con- 

 sumption may be increased several times by the addition of alkali which 

 shows no evidence of penetrating the cell: Z. physiol. Chem., LXVI 

 (1910), 305. 



