482 ELECTRO-PHYSIOLOGY CHAP. 



cooling, and far more to the unavoidable mechanical excitation of 

 the mucosa on freeing the tongue from the palate, to which it 

 adheres in the natural position. 



The normal entering current also declines considerably under 

 similar conditions, and apparently from the same reason, and may 

 even be abolished. If the same point of the lingual mucosa, 

 which at first reacts strongly (in the direction of decline of 

 negativity) when gently rubbed with the tip of the electrode, is 

 repeatedly excited in the same way, the negative variation grows 

 weaker at each excitation, and at last there will be no reaction ; 

 the normal current of rest is unaltered in strength, notwith- 

 standing the excitation. The E.M.F. of the latter sometimes 

 appears to increase considerably in consequence of temporary, 

 local, mechanical excitation ; but the method employed is 

 hardly suited to the solution of this and other questions, and it 

 is advisable to employ some stimulus that can be better graduated 

 as regards intensity and duration. The electrical current in the 

 form of the tetanising alternating currents of an induction coil 

 is the best fitted for this purpose. 



If the secondary coil is connected with two electrodes of 

 platinum wire, which are then brought into contact, at a distance 

 of 3 - 5 mm., with the moist surface of a block of salt clay (as 

 used for testing the tongue current), while the one brush electrode 

 is in leading-off contact with the lateral surface, the other with 

 the upper surface of the block, between the two platinum wires, 

 no trace of deflection will be detected in the galvanometer in the 

 circuit, if the circuit of the secondary coil is closed by Wagner's 

 vibrating hammer, and the coil not pushed home ; but even in 

 the latter case there are, as a rule, only very weak effects on the 

 galvanometer, which in no way modify the consequences of ex- 

 citation to be described hereafter. Before testing these on the 

 living tongue, we ascertained of course by repeated experiments 

 that the results described with the clay block underwent no 

 alteration when a dead lingual preparation, incapable of yielding 

 electromotive action, was placed upon it. 



If, on the other hand, such excitation experiments are tried 



on normal tongue preparations set up and led off as above 



enormously marked effects may sometimes be seen, and almost 

 exclusively in the direction of a negative variation of the rest 

 current. Here again we see to a striking extent the dependence 



