ELECTROTONUS. 537 



vigor by closing the circuit, on account of the increased excitability of the 

 nerve near the cathode. This is called descending extrapolar catelectronus. 



In Fig. 189, B, the conditions are reversed. The polarizing current here 

 must be very weak, as a strong current may destroy the conducting power of 

 the intrapolar portion of the nerve and thus prevent the conduction of the 

 stimulus to the muscle when the salt is applied at E. On closing the cir- 

 cuit, there is ascending extrapolar catelectronus at R, and ascending extra- 

 polar anelectronus at RJ. 



Within certain limits, the greater the strength of the constant current 

 applied to the nerve and the greater the length of nerve included between 

 the poles of the battery, the greater is the deflection of the galvanoscopic 

 needle, by which the electrotonic condition is measured. 



Electrotonic conditions in sensory nerves are measured by reflex move- 

 ments produced by the action of a stimulus applied to these nerves. The 

 variations in excitability of inhibitory nerves, produced by a constant current, 

 are indicated by increase or diminution in the inhibitory action. The phe- 

 nomena in sensory and inhibitory nerves are analogous to those observed in 

 motor nerves. The influence of a constant current upon the muscle current 

 is distinct though feeble, producing a kind of electrotonic condition of 

 muscle. 



Negative Variation. When a rapidly interrupted current is applied to a 

 nerve so as to produce a tetanic condition of the muscles to which it is dis- 

 tributed, the normal or tranquil nerve-current is overcome, and a galvano- 

 scopic needle applied to the nerve, which was first deviated by the nerve- 

 current, will be observed to retrograde and will finally return to zero (Du 

 Bois-Reymond). This may also be observed to a slight degree under the in- 

 fluence of mechanical or chemical stimulation of the nerve, the proper nerve- 

 current being diminished, but generally not abolished. The variation of the 

 needle under the influence of the tetanic condition has been called negative 

 variation. It is not known that this has any important physiological or 

 pathological significance. 



