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TEXT-BOOK OF PHYSIOLOGY 



are at the same time increased and decreased at opposite extremities of the 

 nerve according to the direction of the polarizing current. 



To this changed condition of the electromotive forces in a nerve the 

 term electrotonus was given (du-Bois Reymond). The currents them- 

 selves are known as electrotonic currents; from their relation to the anode 

 and kathode, they are termed anelectro tonic and katelectro tonic currents. 

 The condition of the nerve around the poles both in the intra-polar and 

 extra-polar regions is known as anelectrotonus and katelectrotonus. 



The electrotonic currents vary considerably in strength and extent, 

 according to the intensity of the polarizing current, increasing steadily 

 with the intensity of the latter up to the point at which the polarizing cur- 

 rent 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 



A 



+ 



J^ 



a d 



/ 



FIG. 52. SCHEME OF THE ELECTROTONIC EXCITABILITY. (Landois and Stirling.} ' 



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 

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

 the term electrotonus was also given (Pfluger). 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 electro- 

 tonus the expression, electrotonic alteration of excitability and conductivity. 



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 



