INFLUENCE OF NEUTRAL SALTS. 489 



A further point in connection with both these agencies is significant. 

 Having first increased excitability, and then actually excited, they 

 generally end by permanently impairing functional activity. This final 

 extinction occurs rather suddenly, the excitability being first gradually 

 diminished, and then rapidly disappearing, whilst conductivity is not 

 notably affected for some little time afterwards. We have here another 

 link in the chain of evidence which supports the distinction between 

 these two forms of nerve activity. 



Although there is but little doubt as to the general nature of the 

 modus operandi of NaCl in augmenting nerve excitability, it is possible 

 that other neutral salts may affect nerve by causing more specific 

 chemical change than an alteration in the water content of the tissue, 

 since this view, first put forward by Eckhard in regard to nerve, and 

 extended by Kunkel to the action of 1 neutral salts on muscle, does not 

 explain the peculiar effect of certain salts. 



Thus, in the case of KN0 3 , though the effect described by Eanke is 

 an initial increase with subsequent impairment of excitability, the latter phase 

 is really dependent upon the nature of the stimulus. 2 Biedermann found 

 that if the sciatic nerve of the frog is treated with a 1 per cent, solution of 

 this salt, its excitability is lowered for most stimulating agencies. The passage 

 of induced currents, or the closure of a galvanic current, evokes a far less 

 pronounced muscular response than that produced by the same intensity of 

 stimulating current before the treatment. On the other hand, the opening of 

 the current now causes an excitation producing an opening muscular twitch, 

 with a current intensity which, in the normal condition of the nerve, was 

 quite inadequate to evoke an opening response. 3 



By placing the nerve in physiological NaCl (0*6 per cent.) for a few 

 minutes, the conditions are again changed, and the nerve excitability restored 

 to its normal condition. This remarkable opening effect will be alluded to 

 again in dealing with the phenomena of polar excitation. It is introduced 

 here to show that chemical agencies which lower excitability for one form of 

 stimulation may raise it for another. 



The comparative effects of various chemical substances upon nerve 

 excitability has formed the subject of prolonged investigations carried 

 out by Griitzner. It has been _ _ • 

 already pointed out in connec- «■ 

 tion with nerve excitation 

 that such comparison can only 

 be made when equimolecular 

 solutions of the various salts 

 are employed. The results 

 of most interest as regards 

 motor nerves are those relat- 



mg to the action of neutral tfaFt JVai Wa/ir jV«cc 



salts — their acid moieties, and 

 their basic moieties. 



Influence of neutral salts. — The alterations in local excitability of 

 motor nerves through the action of neutral sodium salts is best 

 expressed by the diagram (Fig. 257), in which the abscissas indicate the 



Fig. 257. 



Eckhard, Ztschr. f. rat. Med., 1851 (2), Bd. i. ; Kunkel, Arch. f. d. ges. Physiol., 

 Bonn, 1885, Bd. xxxvi. S. 353. 



2 " Lebensbedingungen der Nerven," 1868. 



3 Biedermann, "Elektrophysiologie," 1895, S. 593. 



