BY DR. MICHAEL FOSTER. 387 



breaking a still feeble but yet rather stronger descending and 

 ascending current. 



Proceed in this way, shifting the mercury cups by stages, 

 until they are brought to the other end of the board ; then 

 remove the plugs one by one, the removal of each plug mark- 

 ing a corresponding augmentation of the strength of the current 

 sent through the electrodes on the nerves. 



Wait some minutes between each observation to allow the 

 nerve to recover itself. Tabulate the results. They should be 

 such that, throwing the various intensities of current into four 

 categories, they illustrate the following law: 



Descending. Ascending 



Make. Break. Make. Break. 



Weakest Yes No No No 



Weak Yes No Yes No 



Moderate Y'es Yes Yes Yes 



Strong Yes No No Yes 



where " Y"es" means a contraction ; " No," none. 



The making of the descending current is the first to make 

 itself manifest by its effects, and maintains its pre-eminence 

 throughout the series as the most certain and strongest 

 stimulus. 



Next, the making of the ascending current also becomes effi- 

 cient; then the breaking of the descending; lastl} r , the break- 

 ing of the ascending ; so that with a certain intensity of current 

 which we here call " moderate," a contraction is called forth 

 both by making and breaking both ascending and descending 

 currents. 



With a further increase of intens ; ty, the contraction which 

 follows upon the making of the ascending current gets less, 

 and finally disappears altogether. The contraction due to 

 breaking the descending current suffers subsequently the same 

 fate, so that with a " strong" current we have only a single 

 contraction with each current; but it is a contraction on mak- 

 ing in the case of the descending, on breaking in case of the 

 ascending. 



We have seen that when a constant current is sent into a 

 nerve, katelectrotonus is established at the negative pole, ane- 

 lectrotonus at the positive. Both conditions remain during the 

 whole time of the passage, and both disappear (with more or 

 less rebound) when the current is broken. 



It is evident from the above observations that the rise of a 

 nervous impulse is connected with the transition of a nerve from 

 its ordinary condition into that of either katelectrotonus or 

 anelectrotonus, or both, or with its return from katelectrotonus 

 or anelectrotonus into its normal condition, and not with its 

 being or remaining in either katelectrotonus or anelectrotonus. 



