90 HUMAN PHYSIOLOGY. 



a change from an alkaline to an acid reaction, from the development of 

 sarcolactic acid ; a disappearance of the natural muscle currents, which 

 undergo a negative variation in the "latent period," just after the nervous 

 impulse reaches the termination of the nerve, and before the appearance 

 of the muscular contraction wave. 



Electrical Properties of Nerves. When a galvanic current is made 

 to flow along a motor nerve from the centre to the periphery, from the 

 positive to the negative pole, it is known as the direct, descending or centri- 

 fugal current. When it is made to flow in the reverse direction it is known 

 as the inverse, ascending or centripetal current. 



The passage of a direct current enfeebles the excitability of a nerve ; the 

 passage of the inverse current increases it. The excitability of a nerve 

 may be exhausted by the repeated applications of electricity ; when thus 

 exhausted it may be restored by repose, or by the passage of the inverse 

 current if the nerve has been exhausted by the direct current or vice versa. 



During the actual passage of a feeble constant current in either direction 

 neither pain nor muscular contraction is ordinarily manifested; if the cur- 

 rent be very intense the nerve may be disorganized and its excitability 

 destroyed. 



Electrotonus. The passage of a direct galvanic current through a por- 

 tion of a nerve excites in the parts beyond the electrodes a condition of 

 electric tension or electrotonus, during which the excitability of the nerve 

 is decreased near the anode or positive pole, and increased near the kathode 

 or negative pole ; the increase of excitability in the kathelectrotonic area, 

 that nearest the muscle, being manifested by a more marked contraction of 

 the muscle than the normal, when the nerve is irritated in this region. The 

 passage of an inverse galvanic current excites the same condition of 

 electrotonus ; and the diminution of excitability near the anode, the anelec- 

 trotonic area, that now nearest the muscle, being manifested by a less 

 marked contraction than the normal when the nerve is stimulated in this 

 region. Between the electrodes is a neutral point where the kathelectro- 

 tonic area emerges into the anelectrotonic area. If the current be a strong 

 one, the neutral point approaches the kathode ; if weak, it approaches the 

 anode. 



When a nervous impulse passes along a nerve, the only appreciable 

 effect is a change in its electrical condition, there being no change in its 

 temperature, chemical composition or physical condition. The natural 

 nerve currents, which are always present in a living nerve as a result of its 

 nutritive activity, in great part disappear during the passage of an impulse, 

 undergoing a negative variation. 



