GENERAL PHYSIOLOGY OF NERVE-TISSUE 



109 



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. 52). The extent to which the excitability is modi- 



\ REGION OF 

 ^INCREASED EXCITABILITY 



SECONDARY COIL 



FIG. 53. DIAGRAM SHOWING THE REGION OF INCREASED EXCITABILITY CAUSED BY THE 

 PASSAGE OF A GALVANIC CURRENT THROUGH A PORTION OF A NERVE, STIMULATION OF 

 WHICH GIVES RISE TO INCREASED CONTRACTION. 



fied 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 fol- 

 lowing manner: 



REGION OF 

 DECREASED EXCITABILITY 



FIG. 54. DIAGRAM SHOWING THE REGION OF DECREASED EXCITABILITY CAUSED BY THE 

 PASSAGE OF A GALVANIC CURRENT THROUGH A PORTION OF A NERVE, STIMULATION OP 

 WHICH GIVES RISE TO DECREASED CoNTRAcnoN. 



i. With a descending current of medium strength. Previous to the clo- 

 sure of the polarizing current, the nerve is stimulated first in the extra- 

 polar anodic region and the extra-polar kathodic region with an induc- 

 tion shock of medium intensity and the height of the contraction re- 

 corded. 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 



