REVIEW OF RESPONSE OF ISOTROPIC ORGANS 693 



The first way of doing this is to localise the stimulus at one 

 of the two contacts. This may be done by interposing a 

 physiological block between the two, so that the excitation 

 of one does not reach the other. The second method is to 

 select an experimental specimen which is anisotropic, whether 

 naturally or artificially. Artificial anisotropy is induced by 

 injuring one of the two contacts, and so bringing about a 

 relative depression of excitability at that point. 



The method of resistivity variation which I had pre- 

 viously employed, in observing the response of inorganic 

 substances, proved capable of sufficient perfectibility for the 

 study of similar phenomena in living tissues also. The 

 main difficulty in applying this method had hitherto lain 

 in the disturbing electro-motive variation, consequent on 

 a-symmetrical excitation, or the differential excitability 

 of the structure. A detailed account of the means by 

 which this method was rendered reliable will be found in 

 Chapter XXXVII., where it will be seen that records of 

 the excitatory variation obtained by it are in every way 

 similar, to those made by other methods. All the different 

 modes of taking records which have been enumerated are, it 

 must be remembered, independent expressions of a common 

 fundamental molecular change. Thus, on physically restrain- 

 ing that mechanical movement in a motile organ which- is 

 due to excitatory change, the electromotive response of 

 galvanometric negativity continues to be given. Similarly, 

 in a tissue in which, under the experimental arrangements, 

 there can be no resultant electro-motive change and no con- 

 tractile movement, the excitatory change may, nevertheless, 

 be observed by means of the resistivity variation (p. 548). 



For the obtaining of the electromotive response, the 

 electrical mode of stimulation, unless special precautions 

 are taken, is subject to various disturbing influences, such as 

 current-escape and the occurrence of polarisation. For this 

 reason it was desirable to devise some non-electrical form of 

 stimulation which should be capable of quantitative appli- 

 cation ; and this I have been able to secure by no less than 



