iv GENERAL PHYSIOLOGY OF NEEVOUS SYSTEM 223 



we shall examine the effects of galvanic currents upon nerve in 

 full detail. 



We have already seen (p. 19) that alternating currents of 

 high frequency (Hertz waves) and sufficient intensity to light 

 an electric lamp have no stimulating action upon nerve or muscle, 

 probably because they paralyse conductivity (D'Arsonval). 



The currents from a telephone are also capable of stimulating 

 nerve. Hlirthle succeeded in exciting a frog's nerve by the sounds 

 of a heart beating into a telephone. 



Lastly, the physiological electromotive phenomena of the 

 electrical organs of Torpedo (Marey), as well as the intrinsic currents 

 of voluntary muscles of the heart and the nerve itself, can also 

 be used as nerve stimuli (Hering). 



Whatever the nature of the agent employed as stimulus, the 

 excitation which it discharges given constant excitability in 

 the nerve is dependent both on the intensity of the stimulus 

 and on the rapidity with which its action begins and ceases, as 

 well as on its mode of action on the nerve. It is generally agreed 

 that the efficacy of a stimulus depends within certain limits upon 

 its intensity. But the method by which this law is deduced from 

 the muscular reaction, direct or reflex, is inaccurate. Waller 

 demonstrated (supra) that the only physical measure of the 

 activity of a nerve is its electrical variation, which is manifested 

 even when the stimulus is so weak that it fails to evoke any 

 muscular contraction, and which increases with the increase in the 

 strength of the stimulus, even when the muscular reaction is 

 already maximal. 



The relations between the excited state and the mode of 

 stimulating the nerve have been studied particularly for electrical 

 currents. When applied to motor nerves (Kitter and others) 

 these produce a maximum effect at the moment of incidence and 

 of disappearance, and evoke a contraction only at the instant of 

 making and breaking the current, and not during its passage, 

 provided there are no rapid positive and negative alternations 

 of its strength. On the basis of these facts Du Bois-Eeymond 

 formulated the law that currents stimulate in virtue not of their 

 absolute intensity but of the rapidity with which they arise and 

 disappear, or at which their intensity increases or diminishes. No 

 universal value can, however, be ascribed to this law. The 

 reaction of the muscle is not an exact index of the state of 

 activity of the nerve. Both during and after the passage of a 

 current, while the muscle is inactive, important changes are going 

 on in the nerve, which are not always, but only in given cases, 

 transmitted to the muscle. Further, under certain conditions, the 

 closure and opening of an electrical circuit connected with the 

 nerve will evoke a true tetanus instead of simple contractions. 

 Lastly, in sensory nerves there is, not only at the make and break 



