NEGATIVE VARIATION 491 



more or less powerful tetanic contractions of the muscles. These varia- 

 tions are illustrated in Fig. 113. 



In Fig. 1 13, A, a descending constant current is applied to the nerve. 

 When the circuit is open, the salt applied to the nerve at R produces 

 contractions of the muscle. If the circuit .is closed, the contractions 

 either become much less vigorous or cease, on account of the dimin- 

 ished excitability near the anode. This is called descending extrapolar 

 anelectrotonus. If the salt is applied at R, the contractions are in- 

 creased in vigor by closing the circuit, on account of the increased 

 excitability of the nerve near the cathode. This is called descending 

 extrapolar catelectrotonus. 



In Fig. 113, B, the conditions are reversed. The polarizing current 

 here must be very weak, as a strong current may destroy the conduc- 

 tivity of the intrapolar portion of the nerve and thus prevent the con- 

 duction of the stimulus to the muscle when the salt is applied at R. On 

 closing the circuit, there is ascending extrapolar catelectrotonus at R, 

 and ascending extrapolar anelectrotonus at R v 



Within certain limits, the greater the strength of the constant cur- 

 rent 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 

 movements produced by the action of a stimulus applied to these 

 nerves. The variations in excitability of inhibitory nerves are indicated 

 by increase or diminution in the inhibitory action. These phenomena 

 are analogous to those observed in motor nerves. The influence of a 

 constant current on the muscle-current is distinct though feeble, produc- 

 ing 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 distributed, the normal or tranquil nerve-current (current of rest) is 

 overcome, and a galvanoscopic needle applied to the nerve, which was 

 first deviated by the nerve-current, will be observed to retrograde and 

 may return to zero (Du Bois-Reymond). This may also be observed to 

 a slight degree under the influence of mechanical or chemical stimulation 

 of the nerve, the natural nerve-current being diminished, but usually not 

 abolished. The variation of the needle under the influence of these con- 

 ditions has been called negative variation. It is observed in greater or 

 less degree when impulses of any kind are passing through a nerve, this 

 conduction constituting what is known as the current of action. These 

 phenomena are analogous to the negative variation of muscle-currents, 

 which has already been described. 



