490 ELECTRO-PHYSIOLOGY CHAP. 



place of the monophasic, negative variation, a diphasic, and 

 with very weak excitation, a monophasic, positive variation. 

 The excitation effects are then dissimilar in detail, and to a 

 certain extent very complicated. Generally speaking, it may 

 be said that the positive variation preponderates the more, in 

 proportion as the exciting, incoming mucosa current is ab initio 

 weaker, while on the other hand the negative effects become 

 more and more prominent with increasing strength of excita- 

 tion, so that when the coil is pushed nearly home there will in 

 the majority of cases be only, or at any rate chiefly, negative 

 variation, the more or less delayed entrance of which often 

 indicates the concealed presence of the opposite force. So too 

 the rapid back swing of the magnet to its position of rest and 

 even beyond it, as is often seen with lower excitation intensities ; 

 the primary, at first ingoing, negative variation always exceeds 

 the subsequent positive swing in magnitude in such a case, as 

 shown by the rapid reversal (when the exciting circuit is closed) 

 of the magnet deflected towards the negative variation : from 

 this there is but a step to the reaction in which the proportions 

 of the two antagonistic deflections are inverted, the negative 

 variation appearing only as a short prelude to the subsequent 

 positive swing, which often under these conditions attains 

 considerable amplitude, although the deflections caused by it 

 never equal those of the stronger negative variation. Finally 

 (with the lowest effective intensity of excitation), all direct 

 expression of the negative variation may be wanting, a more or 

 less definite retardation in the positive deflection being the sole 

 indication of its presence. On cooled preparations, where the 

 entering current was at zero, we have, even with the coil pushed 

 home, obtained only simple, positive deflections (in the direction 

 of the original E.M.F. of the current), and of no considerable 

 strength. It is perfectly easy to remove the objection that the 

 excitation effects described are caused by fallacies of any kind ; 

 it is only needful, as has already been described at length, to 

 render the mucosa incapable of electromotive action, or to 

 remove it entirely, in order to be certain that even on applying 

 strong currents, excitation effects are totally wanting. 



The non-ciliated cloacal mucosa and the skin of the leech 

 react to electrical excitation readily like the ciliated throat 

 mucosa. Here again, only perhaps in a still higher degree, the 



