PHYSICAL BASIS OF SENSATION 663 



this the positive declines, and passes over the line of trans- 

 ition into the region of the negative or painful. 



The fact, again, that a moderate stimulus, such as is 

 efficient to induce the positive sensation, will, if tetanically 

 applied, become negative or painful, finds satisfactory explana- 

 tion from the peculiar characteristics of the molecular curve. 

 For we have seen, under tetanisation, that the curve is raised 

 from the positive region, below the point of transition, into 

 the negative, above it (cf. fig. 383). 



We have thus seen that by molecular transformation 

 the excitability of the nervous tissue may be enhanced or 

 depressed. There are other conditions also under which the 

 conducting power of the nerve, as well as its excitability, 

 appear to be modified by the exercise of will. Thus, by 

 attention, a stimulus which was previously scarcely per- 

 ceptible may be raised to sensory prominence. The reaction - 

 time, again, may be diminished by attention. The very 

 opposite of these effects, again, are induced by inhibition. 

 As an example of the latter may be mentioned the inhibi- 

 tion by the will of the muscular movements natural under a 

 given stimulus. It was at one time thought that this inhibi- 

 tion of movement was brought about by the in-nervation of 

 antagonistic muscles. This theory, however, is held to be 

 disproved by the fact that such inhibitory effects are seen, 

 even where antagonistic muscles are not present. It would 

 thus appear that the nerve is susceptible of being thrown 

 into certain conditions, in response to internal action, of an 

 unknown character, by which the transmission of excitation 

 through it is either accelerated or inhibited. This power 

 has been said to be ' unique and mysterious.' 



That such opposite dispositions of the nerve may actually 

 result from opposite incipient distortions of the molecules is 

 well shown, however, by the action of electrotonus. Here 

 we have seen that by the influence of one pole an incipient 

 molecular distortion is brought about, which facilitates the 

 transmission of the true excitatory wave, while by the oppo- 

 site this transmission is hindered or blocked. Thus an 



