246 PHYSIOLOGY 



CHAP. 



157) or with mechanical or chemical stimuli. The strength of 

 the muscular response, recorded on a revolving cylinder, is found 

 to be diminished when the nerve is stimulated in the region of the 

 anode, increased when excited near the kathode. 



A curve of the katelectrotonic and anelectrotonic alterations 

 of excitability corresponding with the kathodic and anodic regions 

 can be constructed by comparing the muscular responses obtained 

 by exciting different parts of the anodic and kathodic regions. 

 The form and height of the negative and positive excursions of 

 this curve alter, according to Pfliiger's comprehensive researches, 

 with the strength of the polarising current, and the degree of 

 excitability of the nerve experimented on. It is further found 

 that when the polarising current is weak the indifferent point in 

 the intrapolar tract lies near the anode, and in proportion as the 



FIG. 158. Diagram of electrotonic changes of excitability in the intra- and extra-polar portions of 

 the nerve. (Pflliger.) a, position of anode ; k, position of kathode ; a, k, intrapolar portion. 

 The three curves, y\, y%, 1/3, represent the electrotonic effects of weak, medium, or strong 

 currents. The .points, xi, zo, a*,, show the relative position of the indifferent points with the 

 three currents. The portions of the curves below the abscissa express the anelectrotonic 

 diminution of excitability ; the portions of the curves that rise above the abscissa express the 

 katelectrotonic increase of excitability. 



strength of the current increases it shifts towards the kathode. 

 All these facts are represented in the diagram of Fig. 158. 



Griinhagen's researches show that both the kathodic rise and 

 the anodic fall of excitability occur at the poles without any 

 appreciable delay after closure of the circuit. The electromotive 

 effects due to polarisation, on the contrary, appear in the im- 

 mediate vicinity of the poles at an interval of O'OOl sec. after 

 closure of a very brief current. 



On the strength of the facts at present known the electro- 

 motive effects and electrotonic alterations of excitability appear 

 not to be strictly synchronous. But seeing the parallelism of the 

 two classes of phenomena, it is natural to surmise that there is a 

 close connection between them, and probably a relation of cause 

 and effect. 



The alterations of excitability that occur on breaking the 

 polarising circuit must be regarded as the effects of recovered 

 equilibrium in the nerve. The anodic rise and kathodic fall of 

 excitability begin at the poles and spread thence to the peri- 



