490 



NERVOUS SYSTEM 



either not observed or is feeble. These facts show that the phenomena 

 of electrotonus depend on the physiological integrity of nerves. A dead 

 nerve or one that has been divided or ligated may present these phe- 

 nomena under the stimulation of a powerful current and then only to 

 a slight degree when the condition depends on the purely physical 

 properties of the nerve as a conductor; but these phenomena are not 

 to be compared with those observed in nerves that retain their physio- 

 logical properties. 



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

 portion of the nerve included between the poles of the battery. The 

 condition of the portion between the poles is called intrapolar electroto- 

 nus, 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 condi- 

 tions of the extrapolar portions corresponding to 

 the two poles of the battery are different. Near 

 the anode the excitability of the nerve and the 

 rate of nervous conduction are diminished. If, 

 however, a galvanometer is applied to this portion 

 of the nerve, its electromotive power, measured 

 by the deflection of the galvanometric needle, is 

 increased. On the other hand, near the cathode 

 Fig. 113. -Method of test- the excitability of the nerve is increased, as well 

 ing excitability in electrotonus as the rate of nervous conduction, but the electro- 

 (Landois) ' motive power is diminished. 



The positive poles are + .-m i T 



and the negative poles are - ; The anelectrotomc condition, on the one 



R,R^R,RI, points excited by hand, and the catelectrotonic condition, at the 



the saline solution. 



other pole of the battery, are marked in extra- 

 polar portions of the nerve and are to be recognized, as well, in that 

 portion through which the current is passing ; but between the poles, 

 there is a point where these conditions meet, as it were, and where 

 the excitability is unchanged. This is called the neutral point. When 

 the galvanic current is of moderate strength, the neutral point is 

 about midway 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 current 

 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 

 measure this excitability is a solution of common salt, which excites 



