322 COMPARATIVE ELECTRO-PHYSIOLOGY 



practically stable. The current of rest in the intact human 

 tongue is then found to be from the upper to the lower surface, 

 as in the frog. This, according to our previous results, would 

 indicate that the upper surface is the less excitable. This 

 inference finds independent verification, when we subject the 

 organ to the stimulus of equi-alternating shocks. A very 

 strong responsive current is now found to flow through the 

 tongue, from the lower to the upper surface. 



The tongue is so extremely sensitive that its characteristic 

 response can be evoked even with very feeble stimulus. I 

 have already explained that the alternating currents induced 

 by speaking before a telephone are not exactly equal and 

 opposite, the current being slightly stronger in one direction. 

 Hence, if such currents be made to play upon an organ in 

 which the excitability is only moderately differential, the 

 preponderance of one of the two elements of the alternating 

 shocks is then likely to mask the true excitatory effect. But 

 the differential excitability of the tongue is so great that the 

 responsive current is always from below to above, whether 

 the exciting current be made to act in a favourable or 

 unfavourable direction. Thus, if one speak, even in a very 

 ordinary voice, into an exciting telephone, which is in series 

 with the rest of the circuit, with its poles direct or reversed, 

 a definite lingual current is induced in response. This, as 

 already said, is always in direction from the lower surface to 

 the upper surely a curious instance of the speech of one 

 inducing lingual response in another, by direct, and not by 

 provocative action ! 



The results which have been described are the normal 

 effects given in response to stimulus of moderate intensity. 

 By moderate stimulus is here meant that intensity of current 

 which is obtained when the primary coil is slightly withii 

 the secondary. By feeble, on the other hand, is meant th< 

 intensity produced when the primary is at a distance froi 

 the secondary. Excessively strong stimulus again occur 

 when the primary is pushed fully within the secondary. I 

 shall now proceed to describe occasional variations which 



