CONDUCTIVITY AND EXCITABILITY 219 



point. The intermediate row, C, is the conductor of disturb- 

 ance ; and the last molecule, R, which may be provided with 

 an index, or a reflecting mirror, by means of which the dis- 

 turbance can be made conspicuous, represents the motile 

 responder. 



We shall next observe how the extent of the distortion of 

 each of the molecules from the position of equilibrium by a 

 given force — that is to say, the amplitude of its response — 

 is modified by the factor of molecular mobility. Under the 

 action of certain agencies the freedom of molecular movement 

 may be retarded, by variation"^ of elasticity or of viscosity. 

 We may, with our model, imitate the resultant molecular 

 sluggishness, by means of dampers, which are seen in the 

 diagram, attached to each* sphere. The extent of damping is 

 capable of increase by immersion of the damper in a viscous 

 fluid. The response-curve of this particular sphere may now 

 be taken by the usual method of a reflected spot of light. 

 The curves thus obtained will show, firstly, that, the disturb- 

 ing force remaining the same, diminished molecular mobility 

 is attended by diminution of amplitude of response ; secondly, 

 that this diminution may become so marked that visible 

 response may disappear ; thirdly, that though, with a given 

 moderate disturbance, response may thus be in abeyance, yet 

 it may be restored if the disturbing force be made sufficiently 

 strong ; and fourthly, that the sluggishness thus induced may 

 also be exhibited by delay in the initiation of response, that 

 is to say, by the prolongation of the latent period. 



From such considerations, it is clear that if an agency 

 which reduces molecular mobility be applied on the receptive 

 area, then, inasmuch as the initiation of excitation is prevented, 

 there will be no response exhibited by the motile organ, 

 although the conducting power of the intervening tissue, 

 and the motility of the responding organ, remain unchanged. 

 Again, if the intervening conducting tissue be subjected to 

 loss of molecular mobility by any means, the power of 

 conduction will be either very much retarded, or abolished, 

 the receptivity and excitability of the terminal points 



