668 PHYSIOLOGY. 



With ascending current: 1. If the current is strong, the intra- 

 polar anelectrotonic part of the nerve loses its conductivity, the 

 stimulus at the kathode at the make is not transmitted to the nerve, 

 and no contraction follows. Loeb explains electrotonus by an in- 

 creased and diminished concentration of the ions of calcium and 

 magnesium at the kathode and anode. At the breaking of the cur- 

 rent the anelectrotonus disappears, stimulation is produced at the 

 anode, and the muscle contracts. 



2. If the current is moderate, the conductivity of the anelectro- 

 tonic part of the nerve is not much affected and the stimulus pro- 

 duced at the opening and closing of the current is transmitted to 

 the muscle, which contracts. 



3. With weak currents, the stimulation is only active at the 

 point farthest from the muscle and the closing produces contraction. 



With descending current: 1. With strong currents the stim- 

 ulus at the kathode at the make produces a contraction, as kathode 

 is nearest the muscle, but the stimulation of break at anode is not 

 conducted on account of the lowered conductivity of the intrapolar 

 anelectrotonic part and the kathodic part is not immediately passable 

 after a strong current. 



2. With moderate current, contraction ensues on the opening 

 and closing of the current for the same reasons as in the case of tho 

 ascending current. 



3. With weak current, the onset of katelectrotonus is a more 

 powerful stimulant than the disappearance of the anelectrotonus; 

 the effect of the latter is too slight to manifest any action. 



The same law is applicable in the electrotonus of muscle. 



Katelectrotonus diminishes electromotivity, while anelectrotonus 

 increases it. 



CONTRACTION LAWS IN MAN (WALLER). A pair of electrodes 

 cannot be applied to a nerve in man so as to send a current in at 

 one and out at another point; so you cannot have ascending and 

 descending currents. One electrode must be applied to a nerve, the 

 second, where convenient, to some other part of the body. If the 

 electrode be the anode of a current, the latter enters the nerve by 

 a series of points and leaves it by a second series of points; the 

 former series of points forms the polar zone or region, the latter or 

 distal series of points the peripolar zone or region. In such case the 

 polar region is the seat of entrance of current into the nerve, that 

 is, the anode ; the peripolar region is the seat of exit of current from 



