THE ELECTROMOTIVE PHENOMENA 321 



cork. Lay the denuded jaw on the glass, and 

 turn the tongue forward with a glass rod until 

 the tip can be secured in the clamp. Avoid all 

 roughness. The normally upper surface of the 

 tongue will now rest on the clay. Bring one 

 non-polarizable electrode into contact with the 

 clay, and let the other touch the upper (normally 

 lower) surface of the tongue. Connect the elec- 

 trodes through an open key with the capillary 

 electrometer. Bring the meniscus into the field, 

 and note its position on the micrometer scale. 

 Close the key. 



A strong difference of potential will be shown. 

 The normal under surface is usually positive 

 towards the normal upper surface. 



The difference of potential thus demonstrated 

 is probably chiefly due to secreting glands in the 

 mucous membrane. If the "secretion current" 

 is compensated after the general compensation 

 method described on page 294, and the glosso- 

 pharyngeal nerve then stimulated, the electrom- 

 eter will show an electromotive force, in a 

 direction opposite to the original difference of 

 potential, in other words, a " negative variation." 



Negative Variation of Secretion Current. Place 



a frog curarized until voluntary motion is just 



paralyzed back uppermost on the frog board. 



Strip the skin from one thigh, and expose 



21 



