kEVlEW OF RESPONSE OF ANlSOtROPIC ORGANS 70! 



As an example of a differentially excitable organ we may 

 take the pulvinus of Mimosa, in which the lower half is more 

 excitable than the upper. In this case, strictly localised 

 stimulation of either the upper or the lower evokes con- 

 traction and galvanometric negativity of that particular half, 

 the effect in the lower half being the greater. But if the 

 stimulus be diffused, whether internally or externally, the 

 response will be differential^ by the greater contraction or 

 galvanometric negativity of the more excitable. From this 

 we arrive at the general law of the electrical response of 

 anisotropic organs. 



1. On simultaneous excitation of two points A and B, the 

 responsive current flows in the tissue from the more to the less 

 excited, 



2. Conversely, if under simultaneous excitation, the responsive 

 current be from B to A, B is the more excitable of these two 

 points, 



The second of these two laws enables us to determine 

 the relative excitabilities of any two points. As a simple 

 example of the anisotrorjy induced in a tissue by the unequal 

 action of the natural stimuli of the eriviroment, we may take 

 a tubular organ, such as the hollow peduncle of Uriclis lily. 

 Here the exposed outer surface, constantly subjected to 

 external stimuli such as light, becomes as it were fatigued, 

 and reduced in excitability. Other histological modifications 

 follow on this, the external cells becoming thus cuticularised 

 and protoplasmically defective. Owing to the depression of 

 excitability on this epidermal surface, the intensity of its 

 normal excitatory change by galvanometric negativity is 

 decreased, a change which, in the case of certain skins, 

 culminates in responsive positivity. The inner surface of the 

 hollow peduncle, which may be regarded as epithelial, being, 

 on the other hand, protected, remains normally excitable 

 and is thus more so than the outer surface. The outer sur- 

 face, however, probably by reason of the action of the external 

 stimuli to which it is constantly exposed, is naturally 

 negative, relatively to the protected and more excitable inner 



