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TEXT-BOOK OF PHYSIOLOGY. 



begins to destroy the physical and chemic integrity of the nerve. The 

 electrotonic currents are strongest in the immediate neighborhood of the 

 electrodes, but gradually diminish in strength as the distance between the 

 polarized and led-off portions is increased. The distance to which the 

 electrotonic currents extend along the nerve will depend very largely upon 

 the strength of the polarizing current, though it is conditioned by the phys- 

 ical state of the nerve; for if it be ligated or injured beyond the polarized 

 portion, the electrotonic currents are abolished. The electrotonic currents 

 have no necessary connection with the natural nerve currents, nor are they 

 to be regarded as branchings of the galvanic current. They are in all 

 probability of artificial origin, due to an inner positive and negative polari- 

 zation of the nerve which extends for a variable distance on each side of 

 the poles, and due to the action of the polarizing or the galvanic current. 



3. An alteration in the excitability and conductivity of the nerve in the 

 neighborhood of the poles, whereby the results of nerve stimulation that 

 is, muscle contraction, sensation, and inhibition are increased or decreased 



N 



n\ e 



FIG. 53. SCHEME OF THE ELECTROTONIC EXCITABILITY. (Landois and Stirling.} 



according to the strength and direction of the current. To this condition 

 the term electro tonus was also given (Pfliiger). This word has thus been 

 employed to express two distinct series of effects exhibited by a nerve through 

 a portion of which a constant galvanic current is passing. It appears desir- 

 able, for the sake of clearness, to limit the term electrotonus to the electric 

 or electrotonic currents which can be led off from either extremity of the 

 nerve, and to apply to the modifications of irritability which accompany 

 electrotonus the expression, electrotonic alteration of excitability and con- 

 ductivity. 



During the passage of the current the excitability of the intra-polar as 

 well as the extra-polar regions undergoes a change which, as shown on 

 examination, is found to be diminished in the neighborhood of the anode or 

 positive pole and increased in the neighborhood of the kathode or negative 

 pole. These alterations in the excitability are most marked in the imme- 

 diate vicinity of the electrodes, though they extend for some distance into 

 both the extra-polar and intra-polar regions, though with gradually dimin- 

 ishing intensity, until they finally disappear. Between the electrodes 

 there is a point where the excitability is unchanged and known as the neutral 

 or indifferent point (Fig. 53). The extent to which the excitability is modi- 

 fied as well as the position of the neutral point will depend largely on the 

 strength of the polarizing or galvanic current. 



