688 COMPARATIVE ELECTRO-PHYSIOLOGY 



positivity. This is followed by the more slowly transmitted 

 wave of true excitation, which on its arrival gives rise to the 

 normal response of contraction and galvanometric negativity. 

 The two responsive effects can thus be exhibited separately, 

 when one lags behind the other. When the intervening 

 tract is short, or the conductivity great, the excitatory 

 negativity masks the hydro-positive effect. But this hydro- 

 positive may again be unmasked, by various forms of 

 selective physiological block, which depress the conduction 

 of the true excitatory, without interfering to any appreciable 

 extent with the passage of the hydraulic wave. In this way, 

 the positive may be separated from the contained negative, 

 the response being thus rendered diphasic positive followed 

 by negative. Or, by the complete suppression of the 

 excitatory negative wave, a response originally negative may 

 be converted into purely positive (figs. 45, 47, and 49). 



As response is an expression of molecular derangement, 

 it is the extent of this which determines its amplitude. The 

 character of response is also modified by the molecular 

 condition of the responding substance, and the different 

 molecular conditions through which a substance may pass 

 are indicated by the characteristic curve. From the study of 

 such a characteristic curve we find that these molecular 

 transformations are not specific, but of general occurrence 

 alike in inorganic and living tissues. When the energy of 

 the responding 'substance is for any reason below par, that is 

 to say, when it is in the extremely sub-tonic A condition, 

 external stimulus will be absorbed without evoking the 

 normal excitatory expression. Response will then be 

 abnormal, or of opposite sign to the true excitatory effect. 

 By the absorption of impinging stimulus the substance now 

 passes into the next stage B, where molecular transformation 

 proceeds at a rapid rate. At this stage, the previous 

 abnormal response is not only reversed to normal, but 

 successive responses e'xhibit a staircase increase. At the 

 next or C stage, the responses are uniform. Following this, 

 we arrive at the maximally distorted position D. Stimulus 



