292 COMPARATIVE ELECTRO-PHYSIOLOGY 



lower ends projecting pumice-stone cylinders of equal sec- 

 tions, soaked in normal saline. When the electrode E is 

 rotated, the mechanical friction induces local excitation of 

 the point A. B may also be subjected to similar isolated 

 excitation in the same way. In order that such successive 

 excitations may be quantitative and uniform, it is necessary 

 first that a definite area at A or B should be stimulated. In 

 other words, there must be no lateral slip. For this reason 

 the electrodes are passed through tubular holders which, from 

 the description presently to be given, will be seen to allow 

 rotation about a definite vertical axis. The extreme bases 

 of the pumice-stone cylinders are, as has been said, of equal 

 section. The glass electrode tube is tightly fixed, by means 

 of a cork, inside a brass rotating tube. The latter, again, 

 plays inside an outer brass tube, which is fixed. The inner 

 brass tube is provided with two collars, one below and one 

 above, by means of which rotation can take place without 

 up or down movement. A string is also wound round it, by 

 pulling which rotation is produced. The electrodes are 

 perpendicular to the plane of the platform which carries the 

 specimen. It will thus be seen that any variation of the 

 surface subjected to stimulation is prevented. 



The next difficulty to be overcome is that of liability 

 to variation in the pressure of the contact. It will be 

 remembered that the platform is hinged. It is further held 

 up against the electrodes by the tension of an elastic piece 

 of india-rubber or a spiral spring of steel. This pressure can 

 be regulated to a suitable value, and kept constant. 



The final difficulty is to apply successive stimuli of equal 

 value, and to render them capable also of graduation from 

 low to high values. This could be secured by rendering the 

 successive rotations of the exciting electrodes equal in number 

 and in time of execution. The intensity of stimulus might 

 then be increased by increasing the number of rotations or 

 the pressure of the electrodes on the specimen. 



In order to apply successive rotations of definite number, 

 one end of the string wound round the inner brass tube is 



