762 THE PHYSIOLOGY OF CONSCIOUS STATES [CH. LII. 



depends not upon the nature of the stimulus, but upon the nature 

 of the sensory apparatus on which the stimulus acts. Johannes 

 Miiller expressed this conception in what is known as the law of 

 specific nervous energy. He supposed that every sensory apparatus 

 had its own " specific energy," and that that energy was evoked by 

 any stimulus so long as the stimulus was at all effective. We have, 

 however, no physiological evidence that the nerve impulses passing, 

 say, along the optic fibres, are different in " energy," or in any other 

 character, from those which are transmitted, say, by the auditory 

 fibres. Indeed, the experiments of Langley and others on nerve- 

 crossing (p. 161) would seem to indicate that the nervous impulse is 

 an identical process in all nerves. It may be that the "specific 

 energy " of sensations resides in the various sensory centres of the 

 brain. But if that be so, it is important to realise how dependent 

 that "energy" is for its development on the corresponding end- 

 organs. A person whose visual or auditory end-organs have been 

 f unctionless from birth, can never know what it is to see or hear ; 

 he can never think or dream in terms of visual or auditory imagery. 



Whether qualitatively different sensations involve separate end- 

 organs, or whether they are the outcome of different kinds of 

 activity in one and the same end-organ, is at present far from certain. 

 Probably there are a few " primary sensations " for each sense organ, 

 and the many different qualities of sensation obtainable are due to 

 various combinations of such elements. 



We know, generally speaking, that sensations differ in quality 

 according to the rate of vibration of the stimulus. Sound waves of 

 rapid and slow vibration give rise to sensations of high and low 

 pitch respectively. Light waves of rapid and slow vibration 

 give rise to sensations of blue and red respectively. Differences in 

 intramolecular vibration probably give rise to qualitative differences 

 in olfactory, gustatory, and thermal sensations. 



The strength of the stimulus (e.g. the amplitude of vibration) 

 determines a third character in which sensations may differ from 

 one another, namely, in intensity (for instance, the loudness of a 

 sound, or the brightness of a light). 



Yet another character of many sensations is extensity, or " spread- 

 outness." Smell and taste and some other sensations seem to be 

 devoid of extensity. It is best developed in visual and cutaneous 

 sensations, and these possess yet another characteristic, local signa- 

 ture. Every point stimulated on the retina or skin has its local 

 sign, in virtue of which we are able to localise the stimulus at that 

 point and to distinguish the sensation from those produced by the 

 stimulation of neighbouring points. On the basis of extensity and 

 local signature is built up our perception of extension, form, and 

 spatial relations generally. 



