292 ELECTRO-PHYSIOLOGY CHAP. 



a strong closure tetanus) that there is a deflection of more than 

 12 degrees of the scale for the negative fore-swing. The 

 positive effect quickly reaches its maximum, and at once declines 

 again (sometimes even to zero). 



On opening the exciting circuit there is usually a negative 

 deflection, the magnitude of which .depends essentially upon the 

 duration of the previous passage of current ; this declines slowly. 



If the galvanometer electrodes are approximated (at unaltered 

 distance) to the exciting tract, the intermediate portion of nerve 

 being thereby shortened, the deflections caused by closure of the 

 ascending current are rapidly augmented, independent of any pre- 

 existing P.D., and soon exceed the negative variation produced 

 with the same position of the leading-off electrodes by closure of 

 the descending current. 



The negative fore-swing which is generally present, or at 

 least indicated, in the transverse lead-off is nearly always absent 

 in leading off from the continuity of the nerve, so that mono- 

 phasic, positive variations alone ensue, the magnitude of which 

 diminishes the less during closure, in proportion as the distance 

 between galvanometer and exciting tract is reduced. There is 

 even a perceptible increase in the vicinity of the anode during 

 closure. On opening the exciting circuit, there is usually a more 

 or less pronounced heterodromous (negative) effect, with longi- 

 tudinal as with transverse lead-off. This is at all events the rule 

 in the vicinity of the exciting tract. At more distant points the 

 appearance or failure of a negative opening variation seems, like 

 the negative closure effect, to be conditioned essentially by the 

 existence of a P.D. between the two contacts. The experimental 

 tables quoted above contain evidence for all that has been said 

 with regard to the galvanic alterations of the extrapolar region 

 of the nerve, on the side of the anode. 



The interpretation of these facts relative to manifesta- 

 tions of negativity at make and break of the ascending current 

 can hardly be doubtful. The agreement with the corresponding 

 phenomena in non-medullated molluscan nerve is here so striking 

 that the same explanation, as the consequence of closing or opening 

 excitation, is obvious. The frequent absence of the negative 

 initial swing with ascending stimulation of medullated nerve, 

 and its insignificance when present, is hardly surprising, when 

 we remember that the effect depends, on the one hand, upon a 



