LONGiTUbiif aL transmission of effect of stimulus 273 



Physiologists have not been aware of the dual character 

 of the impulse generated by stimulns, and the term " trans- 

 mission of stimulus" is thus misleading since its effect 

 may be an expansion, or its very opposite, contraction. 

 It is therefore necessary to discriminate the effect of one 

 from the other : the impulse which induces an increase 

 of turgor, expansion, and galvanometric positivity will be 

 distinguished as positive, in the sense that it causes an 

 enhancement of turgor. The other, which induces diminution 

 of turgor and contraction, wilt be termed as the excitatory 

 impulse. Transmission of the latter is dependent on 

 conducting power of the tissue ; the positive impulse is 

 practically independent of the conducting power. 



In animal physiology again, there is no essential 

 difference between the effect of the direct and indirect 

 stimulation. In a nerve-and-muscle preparation, for ex- 

 ample, indirect stimulation at the nerve induces the same 

 contraction as the direct stimulation of the muscle. The 

 only difference lies in the latent period, which is found 

 to be longer under indirect stimulation by the time 

 interval necessary for the excitation to travel along the 

 conducting nerve. It is probable that stimulus gives rise 

 to dual impulses in the animal tissue as in the plant. 

 But the detection of the positive impulse in the animal 

 nerve is rendered exceedingly difficult on account of the 

 high velocity of conduction of excitation. I have explained 

 that the separate effects of the two impulses can only be 

 detected if there is a sufficient lag of the excitatory 

 negative behind the positive, so that the relatively sluggish 

 responding organ may exhibit the two impulses one 

 after the other. In a highly conducting tissue the lag is 

 very slight, and the negative will therefore mask the 

 positive by its predominant effect. In spite of the difficulty 

 involved in the problem, I have recently been successful in 

 demonstrating the dual impulses in the animal nerve. 



