108 COMPARATIVE ELECTRO-PHYSIOLOGY 



this subject, then, demands that we first discover some means 

 of determining the relative excitabilities of different parts of 

 a tissue. 



~~As the simplest example of an anisotropic structure, we 

 may take a compound strip of ebonite and stretched india- 

 rubber, glued firmly together throughout their length. Of 

 these, the india-rubber is the more contractile, and when the 

 strip as a whole is subjected to periodic thermal stimulation, 

 response takes place by the greater contraction induced in 

 the india-rubber. If the strip be held, with the india-rubber 

 below, response will be by the induced concavity of the 

 lower side. In fig. 77 is shown a series of these responses of 

 the compound strip, taken on a smoked 

 surface by means of a recording lever. 



In anisotropic motile organs, such as 

 the pulvinus of Mimosa, response takes 

 place by differential contraction, the more 

 excitable side being that which under 

 diffuse stimulation becomes concave. 

 If we apply very moderate stimulus 



locall y on the u PP er half of the P^inus, 



sponse of Artificial we shall find that, by the excitatory con- 

 traction of this half, the leaf is raised. 

 A similar contractile effect, though of greater intensity, is 

 induced when the lower half of the pulvinus is stimulated 

 locally, the leaf in this case undergoing a depression. When 

 both upper and lower halves, then, are excited simultaneously, 

 the resulting fall of the leaf shows that the contraction of the 

 lower half must in this case be the greater, or, in other woids, 

 that this half is the more excitable of the two. This experi- 

 ment may be carried out very easily by using the stimulus of 

 light. Fig. 78 gives the results observed (a), showing the up 

 movement consequent on stimulation of the upper half; () 

 that caused by equal stimulation of the lower half; and (c) the 

 resultant fall when the two are excited simultaneously. In the 

 case of mechanical response, then, we find it true that response 

 is by the greater contraction of the more excitable. 



