MECHANICAL RESPONSE TO STIMULUS 1 7 



it is difficult by this means to ensure the uniformity of 

 successive stimuli, inasmuch as the wire cannot be heated 

 repeatedly to the same temperature, or made to touch the 

 same point, many times in succession, with an equally effec- 

 tive contact. This difficulty is removed by means of what 

 I have named the ELECTRO-THERMIC STIMULATOR. This 

 consists of a thin M-shaped wire of platinum, with thick 

 copper leads. It is slipped over the petiole which carries the 

 sensitive leaflets. By now sending through it a current of 

 definite intensity and duration, we can raise its temperature 

 to any point we wish, and thus secure the application of a 

 known intensity of stimulus at will. The elasticity due to 

 the peculiar form of the thermal stimulator gives a definite 

 and constant pressure 

 of contact (fig. 13). 



The observer ap- 

 plies the stimulus with 

 his left hand, by press- 

 ing a tapping -key 

 which is interposed in 



an electric circuit, for 



1 c >. 1 , • Fig. 13. The Electro-thermic Stimulator 



a definite short time- J 



With his right hand he records on the revolving drum the 

 exact moment of this application. This mode of thermic 

 stimulation is, as will be shown presently, very efficient. 



Electric stimulation. — I have been able, however, to 

 employ a mode of stimulus still more perfect, that, namely, of 

 the electrical discharge from a condenser. Other forms of elec- 

 trical stimulation may be used, such as those given by means 

 of constant or induction currents. But these are liable, not 

 only to cause more or less permanent internal changes by 

 polarisation, but also to induce fatigue of the tissue. It will 

 be shown in a later chapter that, on making the circuit, excita- 

 tion takes place at the point where the current leaves the 

 tissue — that is to say, at the kathode — and not at the anode, 

 or point of entrance. By appropriate connections shown in 

 the diagram (fig. 14), the point to be excited can be made 



C 



