536 



NERVOUS SYSTEM. 



one that has been divided or ligated may present these phenomena under 

 the stimulation of a very powerful current and then only to a slight degree 

 when the condition depends upon the purely physical properties of the nerve 

 as a conductor ; but these phenomena are not to be compared with those ob- 

 served in nerves that retain their physiological properties. 



As stated above, the electrotonic condition is not restricted to that por- 

 tion of the nerve included between the poles of the battery. The condition 

 of the portion between the poles is called intrapolar electrotonus, and the 

 condition of the nerve outside of the poles is called extrapolar electrotonus. 



When a portion of a nerve is subjected to a moderately strong constant 

 current, the conditions of the extrapolar portions corresponding to the two 

 poles of the battery are entirely different. Near the anode the excitability 

 of the nerve and the rate of nervous conduction are diminished. If, how- 

 ever, a galvanometer be applied to this portion of the nerve, its electromo- 

 tive power, measured by the deflection of the galvanometric needle, is in- 

 creased. On the other hand, near the cathode the excitability of the nerve 

 is increased, as well as the rate of nervous conduction, but the electromo- 

 tive power is diminished. 



The anelectrotonic condition, on the one hand, and the catelectrotonic 

 condition, at the other pole of the battery, are marked in extrapolar portions 

 of the nerve and are to be recognized, as well, in that portion through 

 which the current is passing ; but between the the poles, there is a point 

 where these conditions meet, as it were, and where the excitability is un- 

 changed. This has been called the neutral point. When the galvanic cur- 

 rent is of moderate strength, the neutral point is about half-way between the 

 poles. "When a weak current is used, the neutral point approaches the 

 positive pole, while in a strong current, it approaches 

 the negative pole. In other words, in a weak cur- 

 rent the negative pole rules over a wider territory 

 than the positive pole, whereas in a strong current 

 the positive pole prevails " (Rutherford). 



The conditions of extrapolar excitability vary 

 with the direction of the current applied to the 

 nerve. A convenient stimulus with which to meas- 

 ure this excitability is a solution of common salt, 

 which excites more or less powerful tetanic contrac- 

 tions of the muscles. These variations are illus- 

 trated in Fig. 189. 



In Fig. 189, A, a descending constant current is 

 applied to the nerve. When the circuit is open, the 

 salt applied to the nerve at R produces contrac- 

 tions of the muscle. If the circuit be closed, the 

 contractions either become much less vigorous 



or 



Fro. 189. Method of testing the 



excitability in electrotonus 



(Landois). 

 The positive poles are + and 



the negative poles are ; 



b ' ?ne^kuneSiHtion Xcited cease > on account of the diminished excitability near 



the anode. This is called descending extrapolar an- 



electrotonus. If the salt be applied at R 1} the contractions are increased in 



