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 ner 

 is increased, as well as the rate of nervous conduction, but the electronic 

 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 portioi 

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

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

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

 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 approache 

 the negative pole. In other words, in a weak 

 rent the negative pole rules over a wider territor 

 than the positive pole, whereas in a strong current 

 the positive pole prevails " (Rutherford). 



The conditions of extrapolar excitability va 

 with the direction of the current applied to the 

 nerve. A convenient stimulus with which to mt 

 ure this excitability is a solution of common s; 

 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 

 applied to the nerve. When the circuit is open, 

 salt applied to the nerve at R produces cont 

 tions of the muscle. If the circuit be closed, 

 contractions either become much less vigorous or 

 cease, on account of the diminished excitability nen 

 the anode. This is called descending extrapolar 

 electrotonus. If the salt be applied at R t , the contractions are increased ii 



FIG. 189. Method of testing the 

 excitability in electrotonus 

 (Landois). 



The positive poles are + and 

 the negative poles are ; 



