94 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



ductivity is dependent on other properties of the nerve than irritability, and 

 there are some other facts pointing in the same direction ; for example, 

 regenerating nerves acquire the power to conduct before they recover their 

 irritability. The usual explanation of those who regard conduction as due 

 to the excitation of each succeeding part of the nerve by the one just pre- 

 ceding it is, that external excitation is a coarse affair as compared with the 

 normal internal excitation process, and the effect of the former may be lost 

 when the latter is still effective. 



(e) Effect of a Constant Battery Current. — A constant electric current, if 

 allowed to flow through a nerve or muscle, not only alters the irritability, but 

 also the conductivity. The change in the conductivity affects both the 

 strength and rate of the conduction process. Yon Bezold 1 found that weak and 

 medium currents have little effect on the conductivity, but that strong currents 

 completely destroy the power of the nerve to transmit the nerve-impulse. As 

 the strength of the current is increased, the first effect is observed at the anode, 

 and shows itself in a slowing of the passage of the exciting impulse. This 

 action is the greater the more of the nerve exposed to the current, the stronger 

 the current, and the longer it is closed. The loss of conduction power is asso- 

 ciated with changes at the place where the current enters and where it leaves 

 the nerve rather than with alterations within the intrapolar region. Engelmann, 

 in his experiments on the smooth muscle-fibres of the ureter, saw a decline of 

 power of conduction at the anode by weak currents, which when the strength 

 of the current was increased appeared also at the kathode; the conductivity 

 was wholly lost at both poles when the current was very strong. In the case 

 of a striated muscle, such as the sartorius of the frog, the kathode has been 

 found to become impassable after strong currents have flowed through a muscle 

 for a considerable time. The same is true of nerves. 



It is not surprising that a current which can greatly decrease the irritability 

 at the anode, and even inhibit a contraction which may be present when it is 

 applied, should be found to decrease the conductivity as well, but that the con- 

 ductivity should be decreased at the kathode, where the irritability is greatly 

 increased, was not to be expected. Rutherford -found that with weak currents 

 the rate of the conduction power at the kathode was increased rather than 

 diminished, and that it was only when strong currents acted a considerable 

 time that the conduction power lessened at the kathode. Biedermann explains 

 this on the ground that the increased excitability at the kathode leads in the 

 muscle to a condition of latent contraction and therefore to fatigue, and that 

 it is this which lessens the conductivity. The lessened power to conduct con- 

 tinues at the kathode alter the removal of the current. There is little doubt 

 that fatigue interferes with the conduction power of muscle, but this explana- 

 tion would hardly apply to nerves which are not known to fatigue at the point 

 of stimulation, i. e. if we limit the term fatigue to changes resulting from 

 physiological activity. Undoubtedly chemical and physical alterations may 



1 Unterauehungen iiber die elektrische Erregung der Nerval und Muskeln, Leipzig, 1861. 

 'Journal of Anatomy and Physiology, 1867, vol. 2, p. 87. 



