PHYSICAL BASIS OF SENSATION 665 



of painful or negative tone. The particular relation which 

 exists between stimulus and sensation has been shown to be 

 an expression of the peculiar characteristics of the molecular 

 curve. The ascent in the curve, being rapid immediately 

 above the point of transition, and slow later, equal increments 

 of stimulation cause in this region an increase of sensation 

 which is at first rapid and then slow. Below the point of 

 transition responses are positive. But tetanisation, inducing 

 a molecular transformation, carries the curve above the point 

 of transition. Hence moderate stimulus, inducing positive 

 sensation, is converted, when tetanically applied, into negative 

 or painful. The positive response is simple, but the negative 

 is complex, containing a masked positive ; and we shall see 

 in the next chapter how this complex sensation can be 

 analysed into its component parts. 



We have also seen that, in addition to moderate stimula- 

 tion, there are other agencies by which the tonus of nerves 

 may be altered. In a magnetic substance, the incipient 

 molecular distortions of one sign induced by K-tonus, enhance 

 the excitability and conductivity, while those due to A-tonus 

 depress them. Strong K-tonus, again, inhibits conduction. 

 By such polar actions, then, an impulse is either accelerated 

 or inhibited. Similar polar changes are seen in the nerve } 

 moreover, under the action of an- and kat-electrotonus. In 

 like manner, by the exercise of the internal stimulus of will, 

 the tonus of the nerve may be so varied that, under different 

 circumstances, the transmission of excitatory impulses is 

 accelerated or inhibited. 



