536 



NER VE. 



but obviously due to an increased and decreased susceptibility of the 

 nerve to the production of negative variations. The secondary augment- 

 ing effect of C0 2 is so pronounced, that when this gas is combined with 

 chloroform it tends to neutralise the depressing effect produced by that 

 vapour. 



Influence of solutions of certain salts. — The investigation of this 

 influence has been carried out by Waller in the following way. The 

 excised sciatic of the frog is soaked in 06 per cent. NaCl, and the 

 negative variations produced by short periods of faradisation is re- 

 corded by the photographic method previously described. The normal 

 saline is now replaced by solutions containing definite percentages of 

 some other salt. 



Neutral salts. — The immersion of the nerve in decimolecular solu- 

 tions of these salts profoundly modifies the amount of the electrical 

 changes. Potassium salts are markedly depressant ; calcium and 

 strontium salts, on the other hand, augment the variations ; sodium 



salts are slightly 

 depressant, but the 

 depressing action 

 is in no way com- 

 parable with that 

 of potassium. The 

 nature of the acid 

 moiety modifies 

 the result ; thus 

 potassium bromide 

 is rather more de- 

 pressant than the 

 chloride. 



Salts of various 

 alkaloids. — Most 

 alkaloids appear 

 to exercise a de- 

 pressing action 

 upon the negative 

 variation. Aconitin and veratrin in 1 per cent, solution rapidly abolish 

 the effect ; curarin, digitalin, and morphin diminish its amount, whilst 

 strychnin causes an augmentation. 



The method of soaking the nerve in solutions of the salts produces 

 an alteration in electrical resistance, which need not invalidate the 

 conclusions derived from the photographic records of the responses, 

 since any change due to this cause can be ascertained from the records 

 of excursions caused by a known electromotive force before and after 

 the alteration. 



Influence of previous activity. — A short succession of stimuli may be 

 repeated over and over again in the case of the frog's sciatic nerve, 

 without altering perceptibly the extent of the negative variations thus 

 evoked. At the commencement of such a series, comprising short 

 periods of excitation with longer intervals of repose, the effect increases 

 in amount, then it attains a maximum and continues undiminished. 

 The negative variation in a nerve trunk thus shows no sign of fatigue 

 in consequence of the repetition of the external stimulus. This inde- 

 fatigability has been already alluded to as a characteristic of the excita- 



CMoro/'orm, 



jEther 



O 



Fig. 275. — A T =level of normal excitability. = level of zero 

 excitability. The curves show alteration in excitability, 

 produced by continuance of the three vapours indicated. 

 Ordinates indicate amount of negative variation. Abscissa; 

 indicate time after exposure to vapour. — After Waller. 



