RESPONSE OF ANIMAL AND VEGETAL SKINS 293 



attached to a piece of stretched india-rubber, which is fixed 

 by its other end to the apparatus. The second end of the 

 string is tasselled, after being passed through a fixed ring. 

 This position of the string is adjusted by means of a knot, so 

 that the india-rubber at its other end is already in a state 

 of tension. When the tassel is now suddenly pulled and let 

 go, it gives rise to a number of rotations in the positive, 

 followed by an equal number of rotations in the opposite 

 direction, the latter work being performed by the stretched 

 antagonistic spring. It should be remembered that a 

 mechanical rotation, whether plus or minus^ gives rise to the 

 same excitatory reaction. Next, to make the number of 

 rotations definite, let us suppose the inner brass tube to have 

 a circumference say of I cm. If the string be now pulled 

 through a distance of 5 cm., and let go, there will be five 

 rotations in the positive, followed by five rotations in the 

 negative direction. A second knot in the string, at the 

 distance of 5 cm. from the first, exactly limits the length of 

 the pull ; and increase or decrease in the intensity of the 

 stimulus can be brought about by a change in the distance 

 of the regulating knot. 



The distance between the two electrodes being always 

 the same, the resistance of the interposed tissue remains 

 approximately constant. To nullify any accidental variation, 

 a high and constant external resistance is interposed in the 

 galvanometer circuit. When the excitatory electro-motive 

 variation of the specimen is very great, it is possible to use 

 an external resistance as high as one million ohms. It should, 

 however, be remembered that even if there be any unavoid- 

 able variation of resistance, it will not in any way affect the 

 discrimination of sign of the characteristic electro-motive 

 response. For the excitatory effect at either electrode may 

 be tested by repeating the experiment with the other. The 

 experiments which will be described afforded definite and 

 characteristic records, which were found capable of repetition. 

 The physiological character of such responses was further 

 demonstrated by repeating the experiment, after killing the 





