484 ELECTRO-PHYSIOLOGY CHAP. 



The behaviour of the cooled lingual mucosa, giving opposite 

 electromotive action, is of interest ; here, too, there is normally a 

 negative variation, i.e. diminution of E.M.F. in the outgoing 

 current, but this effect is generally much less, and therefore 

 demands much stronger currents, than in the normal incoming 

 rest currents. While this is the usual result with weak alternating 

 currents, approximation of the coil under otherwise uniform con- 

 ditions will often cause a positive variation after a first negative 

 swing of greater or less amplitude, i.e. a temporary increase of the 

 outgoing current occurring in the return process. 



As regards, finally, the time-relations of the variation, these, 

 with an entering current of rest, are highly characteristic. 

 Without employing any finer artificial means, a latent period 

 (" stage of latent electromotive action," Engelmann) may invari- 

 ably be determined, its duration being essentially conditioned by 

 the strength of excitation, in the sense that it decreases inversely 

 with increasing strength of current. The deflection begins 

 slowly at first, and rapidly attains its full value later ; as a rule, 

 the return swing of the magnet begins while the excitation is still 

 in progress, and if the secondary circuit is left closed, runs in a 

 zigzag course, sometimes interrupted by short backward move- 

 ments in the direction of the negative variation. If, on the 

 contrary, the excitation ends as soon as the deflection has reached 

 its maximum, there will always be a rapid and uniform return 

 swing of the magnet, during which the E.M.F. of the current not 

 only regains its original proportions, but nearly always exceeds 

 them to a marked extent, so that we are justified in saying that the 

 negative variation further entails a weaker positive variation, which 

 is relatively slower in its development, and still more in its decline. 



If a sufficiently long interval is allowed between each pair of 

 excitations, the experiments may be repeated with uniform results. 

 Sometimes, however, the amplitude of the negative variation 

 suddenly diminishes, and with each excitation there is a negative 

 after-effect, which finally causes a permanent diminution of E.M.F. 

 in the current. It is also important to avoid too rapid fatigue of 

 the preparation, by an undue length of each individual excitation ; 

 protracted tetanus soon weakens the entering current perma- 

 nently. We have already shown that the negative variation of the 

 ingoing rest current is in a marked degree dependent on the 

 strength of the latter, and diminishes very rapidly with the fall 



