156 PROTOPLASMIC ACTION AND NERVOUS ACTION 



An important section of Overton's work has reference 

 to the influence of salts on the transmission across the 

 myoneural junctions and through reflex arcs. Locke^ 

 (1894) had found that when the frog's sartorius, with 

 nerve attached, was placed in a pure isotonic NaCl 

 solution the tissue within fifteen to twenty minutes 

 lost irritabihty through the nerve for single induction 

 shocks, while it remained directly irritable for some 

 hours; the addition of a little CaCla (0.02 per cent) to 

 the solution restored indirect irritability in a few minutes; 

 a second return to pure NaCl again aboHshed stimulation 

 through the nerve, and the effect could be again reversed 

 by CaCla. Muscles perfused with salt solution show 

 the same phenomenon;^ and other cases of innervation, 

 such as inhibition of the heart through the vagus, are 

 similarly dependent on the salts of the medium.^ 



Overton^ found that the addition of KCl to the NaCl 

 solution greatly accelerated the junctional paralysis; 

 salts of Rb and Cs acted similarly, also salts of NH3 

 and organic ammonium compounds, which have long 

 been known to exhibit this curare-like action. When the 

 concentration of NaCl was 0.7 per cent, the addition of 

 .05 per cent KCl was found to shorten the period of 

 irritabihty through the nerve to an eighth or a tenth 

 of its duration in the pure solution; with a lower concen- 

 tration of NaCl less KCl was required. This blocking 

 effect of potassium is antagonized by calcium; there is, 



* Locke, Zentralbl. f. Physiol., VIII (1894), 166. 



^ Gushing, American Journal of Physiology, VI (1901), 77. 



3 Cf. Hober, op. cit., p. 539; Howell, American Journal of Physiology, 

 XV (1906), 280; Howell and Duke, ibid., XXXV (1907), 131. 



4 Loc. cit. 



