490 NER VE. 



time after the application of the salt, the ordinates the minimal 

 exciting intensity of an induced current applied to the upper end of 

 the nerve. 



It is seen that sodium fluoride, 2'1 per cent., produces a very rapid 

 rise in excitability, which soon subsides ; and that sodium bromide 5*14 

 per cent., and chloride 2*92 per cent., each causes a rise which subsides 

 comparatively slowly, whilst sodium iodide, 7*14 per cent., causes a fall. 

 The middle and lower ends of the nerve are not so rapidly affected, and 

 the rise when produced lasts longer ; moreover, in these situations, all 

 four salts cause a rise. Unlike sodium salts, all potassium salts lower 

 the excitability for such stimuli as induced currents — potassium 

 iodide more than potassium bromide, and potassium chloride least. 

 Injurious effects are produced by any salt of sufficient concentration. 

 These may amount to irreparable loss of excitability, or to an abeyance 

 of this, from which the nerve may be recovered by immersion in 

 isotonic indifferent liquids, blood serum or physiological saline solution. 

 This loss of excitability is produced by some salts more rapidly than by 

 others, the effect, as far as the bases are concerned, being in the follow- 

 ing order : — K, Eub, Coes, Ca, Sr, Ba, Na. As regards the acid moieties 

 of the neutral salts, the injurious action of iodine is more rapid than that 

 of chlorine. It thus appears that initial rise of excitability to stimulation 

 by induced currents is caused by appropriate solutions of sodium salts, 

 and initial fall by potassium salts. In the section dealing with chemical 

 stimulation, remarkable facts were referred to which showed that sensory 

 nerves are more readily excited by potassium than by sodium salts, and 

 it appears that the same difference is true as regards excitability, though 

 the evidence on this point is more ambiguous. 



The effect of salts upon the propagation of the excitatory state has 

 not been so thoroughly investigated, owing to technical difficulties ; 

 the points of chief interest are those which show that conductivity 

 is not affected to the same degree as excitability. It does not appear 

 that an excitatory state is ever augmented during its propagation 

 through a portion treated with NaCl ; on the other hand, a gradual 

 diminution in the intensity of the transmitted effect is often observed, 

 and after prolonged action of the chemical reagent all propagation 

 may cease. The final injurious influences upon conductivity, due to 

 different salts, appears to be produced in the same order as those 

 previously referred to in connection with excitability ; in motor nerves 

 potassium salts are more powerful in effecting them than sodium salts, 

 and iodides than chlorides. All these chemical agencies, whether neutral 

 salts, acids, alkalies, or other compounds, which, by their continued action, 

 lower and finally abolish nerve excitability, will, if their action is further 

 prolonged, lower and abolish conductivity ; but, in most instances, ex- 

 citability begins to fail some time before conductivity is appreciably 

 affected. Thus the susceptibility to internal stimulation, which lies at 

 the basis of nerve propagation, remains practically unaffected when, by 

 the constant operation of a chemical substance, the susceptibility to 

 stimulation by an external agent has partially or even wholly disappeared. 

 A remarkable exception is that in which the sciatic nerve is treated in 

 the middle of its course by such an agent as Nal 61 per cent. It 

 can retain its diminished excitability to the induced current at this 

 point when excitation of the central portion is quite ineffectual to 

 evoke the muscular response ; here conductivity appears to have 



