THE SENSE ORGANS 481 



to evoke different qualities of sensation by different modes of stimulation 

 of nerve fibres, and it has therefore been concluded that the quality of any 

 sensation depends simply and solely on the termination of these nerves in 

 the central nervous system, and that where sensations of different quality 

 are produced there must be also difference of nerve fibres. This idea was 

 formulated by Miiller, and is often alluded to as Miiller's ' law of specific 

 irritability.' The law states that every sensory nerve reacts to one form of 

 stimulus and gives rise to one form of sensation only, though if under abnor- 

 mal conditions it be excited by other forms of stimuli, the sensation evoked 

 will still be the same. 



Although the different forms of sensation must be regarded as dependent 

 on the integrity of the brain, and of its connections with the peripheral 

 sense organs, sensations are not referred to the brain, but are localised as 

 proceeding from some part of the body or from some region outside of the 

 body. Thus the sensation of taste is always localised in the mouth ; 

 sensation of touch at the skin or surface of the body ; while the sensations 

 of hearing and of sight are ' projected,' i.e. are interpreted as coming from 

 the environment outside ourselves. Even the organic sensations of posture 

 or fatigue are referred to the peripheral reacting parts of the body and not 

 to the central nervous system. A sensation therefore cannot be interpreted 

 as a reproduction of external events, but as a symbol of these events evoked 

 by stimulation of the sense organs of the body. 



CONSCIOUSNESS. How the physiological excitatory process in nerve 

 fibres, with its concomitant chemical and electrical phenomena, is able on 

 arrival at the brain to excite a conscious sensation we are unable to decide, 

 or even to discuss, since we are dealing here with processes of two different 

 orders. We should not arrive any nearer to the solution of this riddle if 

 we were able to follow out the whole of the events occurring in the body as 

 the result of the application of any given stimulus to its surface. We might 

 under these circumstances be able to predict with certainty the behaviour 

 of any animal, if we knew its past history and the comparative resistance of 

 every path in its central nervous system which might possibly be traversed 

 by any given nerve impulse. Such knowledge would be purely objective 

 and could not be used to explain the ' epiphenomenon ' of consciousness. 

 One might in fact imagine a machine which would react like a living animal, 

 but would be perfectly devoid of self-consciousness, and we should be unable 

 in such a case to decide whether consciousness were or were not present. 

 Each one of us only knows consciousness as it exists in himself. 



It is indeed impossible by a purely intuitive study of sensations to arrive 

 at any correct idea of their origin or of the factors concerned in their produc- 

 tion. No sensation is the immediate and sole product of a stimulus applied 

 to the peripheral end of a nerve fibre, but the simplest sensation involves 

 a judgment, i.e. complex neural activities which are the resultant of innumer- 

 able past and present streams of nervous impulses aroused by peripheral 

 events and poured into the central nervous system. It is important there- 

 fore not to regard a sensation as in any way constituting an elementary 



3, 



