GENERAL PHYSIOLOGY OF NERVE-TISSUE. 



107 



The electrotonic alterations of excitability and conductivity can be 

 experimentally demonstrated on the muscle-nerve preparation in the fol- 

 lowing manner: 



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- 



REGION OF 

 INCREASED EXCITABILITY 



SECONDARY COIL 



FIG. 54. DIAGRAM SHOWING THE REGION or INCREASED EXCITABILITY CAUSED BY THE 

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

 WHICH GIVES RISE TO INCREASED CONTRACTION. 



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 



REGION OF 

 DECREASED EXCITABILITY 



FIG. 55. DIAGRAM SHOWING THE REGION OF DECREASED EXCITABILITY CAUSED BY THE 

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

 WHICH GIVES RISE TO DECREASED CONTRACHON. 



stimulation of the kathodic region will be decidedly increased. (See 



Fi g- 540 



2. With an ascending current of the same strength. After preliminary 

 testing of the excitability and the subsequent closure of the polarizing 

 current, it will be found that stimulation of the extra-polar anodic 

 region will provoke a much less energetic contraction or perhaps none 



