254 PHYSIOLOGY CHAP. 



Erb's reaction, since it survives for a long time, and is, in ad- 

 vanced stages of disease, the sole means of proving the existence of 

 degenerative processes. 



In order to determine more exactly what changes in the con- 

 ductivity of nerve accompany the electrotonic alterations of 

 excitability, Novi with Brugia (1890) carried out a series of 

 investigations on the latent period by direct stimulation of motor 

 nerves in a state of electrotonus. These experiments were per- 

 formed on the exposed sciatic of the dog by Chauveau's unipolar 

 method. A second series of investigations was made by Brugia 

 on man with the object of determining the degree in which the 

 electrotonic alterations of excitability affect the conductivity of 

 motor nerves left in normal relation with the surrounding tissues. 

 The results are as follows : 



(a) In the isolated nerve of the dog anelectrotonus produces a 

 considerable delay in the rate of transmission ; katelectrotonus, on 

 the contrary, accelerates the transmission of excitation, excepting 

 for strong currents, when it is retarded, though to a less extent 

 than for anelectrotonus. 



(b) In the nerve of man left in its normal relations with the 

 surrounding tissues, both katelectrotonus and anelectrotonus, but 

 the latter more especially, produce a considerable delay in the 

 rate of conductivity. 



(c) In the nerve both of dog and man a progressive increase 

 in polarisation increases the latent period proportionately; but 

 while a certain degree of anelectrotonus blocks the conduction of 

 the impulse completely, katelectrotonus may become very pro- 

 nounced before it abolishes the conductivity of the nerve. 



(d) While the delay in the muscular response ends almost 

 simultaneously with the cessation of katelectrotonus, there is, on 

 the contrary, both in dog and man, a very long interval before the 

 nerve regains its full conductivity in anelectrotonus. 



(e) Increased strength of stimulus has hardly any effect on the 

 anelectrotonised nerve, while it compensates the difficulty of con- 

 duction for the nerve produced by katelectrotonus. 



(/) In nerves which have begun to degenerate, i.e. in the state 

 in which faradic and galvanic excitability are merely diminished, 

 the electrotonic delay in conduction is more pronounced than 

 under normal conditions ; at a more advanced stage of degenera- 

 tion even katelectrotonus is capable of prolonging the latent 

 period, and all the various phenomena of electrotonus are slower 

 and more feebly developed. 



IX. How far is it possible from the whole of the facts before 

 us to construct a general theory of the genesis and intrinsic 

 mechanism of nervous activity ? Before replying to this question 

 we must review the various hypotheses that have been brought 

 forward. 



