674 COMPARATIVE ELECTRO-PHYSIOLOGY 



We shall next study the various effects induced by the 

 variation ^of conductivity. We have already seen that a 

 moderately intense stimulus gives rise to two waves, one 

 positive and the other negative, the former having the 

 greater velocity of the two. Thus these two waves, starting 

 from the receptive and reaching the distant responding point, 

 will give rise to two different responsive effects, separated 

 from one another by an appreciable interval. The separation 

 depending on the lag of one wave behind the other, will be 

 greater the greater the length of the conducting tract. The 

 first to arrive at the perceiving centre being the positive 

 impulse, we shall have there a positive sensation of mere 

 touch or contact. The later-arriving negative wave gives 

 rise, according to the nature of the indicator, to mechanical 

 contraction, galvanometric negativity, or a painful sensatio^ 

 as the case may be. If the conducting tract be not 

 sufficiently long, the two waves will be superposed, or 

 indistinguishable, one masking the other. But they may be 

 analysed, or separated, by anything which diminishes the 

 conductivity of the intervening tract, and this is rendered 

 possible by the fact that the positive wave is not much 

 affected by changes of conductivity. On the other hand, the 

 velocity and intensity of the negative wave are both 

 diminished by anything that diminishes the conductivity. 

 Hence a diphasic response the sensation of touch followed 

 by pain may be expected, wherever the intervening con- 

 ducting tract between receptive and responsive points is 

 sufficiently long. In other cases, where the tract is shorter, 

 a sufficiently strong stimulation will give rise to a single 

 sensation of negative or painful tone. This negative 

 sensation, however, is complex in its character, masking as it 

 does, a contained positive element If then the conductivity 

 of this transmitting nerve be in any way diminished to a 

 moderate extent, the complex sensation will be found to be 

 analysed, the negative being made to lag behind the positive. 

 There will thus be a dissociation of the dual elements of the 

 complex sensation, and the result will be a diphasic response 



