CURRENT OF INJURY AND NEGATIVE VARIATION l6l 



moderate stimulation, A often becomes more than normally 

 excitable. 1 It is thus seen how, after a recent injury, these 

 two factors of a hydro-positive effect at the injured, and of 

 increased excitability at the uninjured contact, in consequence 

 of moderate transmitted stimulation may act to enhance the 

 response. 



We have seen that the common effect of injury is to 

 induce a galvanometric negativity of the point injured. We 

 have further seen that in such a case the response to external 

 stimulus is by a negative variation of the current of injury. 

 We have next, then, to take up various instances which appear 

 highly anomalous, cases, that is to say, in which the injured 

 point, relatively to the uninjured, is, for some hitherto 

 unknown reason, galvanometrically positive. As a result 

 of this and other causes, there are, in addition to the cases 

 already described in a previous chapter, instances in which 

 response is found to take place, not by a negative, but by 

 a positive, variation of the current of rest or of injury as 

 the case may be. 



The first point to be considered in connection with such 

 abnormal responses is whether the experimental tissue is 

 physiologically isotropic, that is to say, of equal excitability 

 throughout, or anisotropic, possessed of unequal excitabilities 

 at different points. The discussion of the first of these 

 cases, the isotropic, I propose to defer to the following chapter. 

 The anisotropic will be touched upon here, though its detailed 

 consideration will be entered upon in the next. 



As an example of the anisotropic organ, we may take 

 the pulvinus of Mimosa, in which the lower side is more 

 excitable than the upper. In animal tissues also, such aniso- 

 tropy is not uncommon. For example, we may have a 

 muscular tissue terminating in a glandular. Owing to this 

 anisotropy, the muscular and glandular surfaces are unequally 

 excitable, and it will be shown in a later chapter that, 

 generally speaking, it is the glandular which exhibits more 

 intense excitatory galvanometric negativity. When such a 



1 For further details, see Chapter XLII. 



M 





