90 EXPERIMENTAL PHYSIOLOGY 



are also seen not to extend far beyond the anode and kathode. For 

 a stronger polarising current the changes are represented by the 

 curve d ef. The indifferent point has moved towards the kathode ; 

 the changes are relatively greater and extend over a longer piece of 

 nerve, df. With a still stronger current the changes are represented by 

 the curve g him which the indifferent point is still nearer the kathode, 



Fig. 76. — Diagram Indicating the Changes of Excitability of a Nerve in 



Electrotonus. 



and there is a further increase in intensity of effect and in extent of 

 nerve involved. 



Immediately after opening the current these changes are for a 

 short time reversed, the excitability in that part of the nerve which 

 was in anelectrotonus is increased, in that part previously in katelec- 

 trotonus decreased. These reversed effects gradually disappear. 



2. Changes in conductivity. 



Experiment 10. — Arrange the apparatus as in fig. 75, but place the 

 polarising electrodes near the muscle and the exciting electrodes at the upper 

 end of the nerve. Tetanise the nerve, and while the contraction still con- 

 tinues close the polarising current. The tetanus ceases. Repeat, but 

 instead of tetanising apply a crystal of salt or a drop of strong salt solution to 

 the cut end of the nerve. The twitchings caused by the salt can be stopped 

 by throwing in the polarising current. The same effect is produced whether 

 the polarising current be ascending or descending. The polarising current has 

 acted as a block, i.e. the nerve at that part has had its conductivity depressed. 



The change of conductivity has also been studied by observation 

 of the rate of transmission of a nerve impulse along a piece of nerve 

 in anelectrotonus or in katelectrotonus, and further by observations 

 of the changes of strength of the negative variation * as it travels along 

 the nerve. Such experiments showed that for all currents, except 

 the weakest, conductivity was depressed in both the katelectrotonic 

 and anelectrotonic states. 



POLAR, EXCITATION OF NERVE 



When stimulating a muscle by the constant current we found 

 that the stimulation at make started from the kathode and at 

 1 For the explanation of the term negative variation see p. 146. 



