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TEXT-BOOK OF PHYSIOLOGY. 



series of effects exhibited by a nerve through a portion of which a con- 

 stant galvanic current is passing. It appears desirable, 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 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 neigh- 

 borhood of the anode or positive pole and increased in the neigh- 

 borhood of the cathode or negative pole. These alterations in the 

 excitability are most marked in the immediate vicinity of the elec- 

 trodes, though they extend for some distance into both the extra- 

 polar and intra-polar regions, though with gradually diminishing 

 intensity, until they finally disappear. Between the electrodes there 



FIG. 52. SCHEME OF THE ELECTROTONIC EXCITABILITY. 



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

 neutral or indifferent point (Fig. 52). The extent to which the ex- 

 citability is modified as well as the position of the neutral point will 

 depend largely on the strength of the polarizing or galvanic current. 

 The electrotonic alterations of excitability and conductivity can 

 be experimentally demonstrated on the muscle-nerve preparation in 

 the following manner: 



i. Withajiescending current of medium strength. Previous to the 

 closure of the polarizing current, the nerve is stimulated first 

 in the extra-polar anodic region and the extra-polar cathodic 

 region with an induction shock of medium intensity and the 

 height of the contraction recorded. On repeating the stimulation 

 after closure of the polarizing current the contraction resulting 

 from stimulation of the anodic region will be enfeebled or may be 

 entirely wanting, while the contraction from stimulation of the 

 cathodic region will be decidedly increased. (See Fig. 53.) 



