536 PHYSIOLOGY 



auditory nerve or its intracranial endings gives rise to sensations 



of sound. 



Where the question has been investigated it has not been found 

 possible 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 termina- 

 tion 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 abnormal conditions it 

 be excited by other forms of stimuli, the sensation evoked will 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 environ- 

 ment 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. 



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 production. 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 innumerable past and present streams of 

 nervous impulses aroused by peripheral events and poured into the 

 central nervous system. It is important therefore not to regard a 

 sensation as in any way constituting an elementary unit, by the 

 aggregation of a number of which a conscious state is produced. As 

 we have seen, the primitive function of the whole nervous system is 

 reaction. The neural life of an animal is composed of a series of reac- 

 tions, some simple, some complex, and becoming ever more compli- 

 cated as we ascend the animal scale. The first reactions of a baby, 

 for instance, will be those by which it procures nourishment and satisfies 

 a need. The earliest event in its dawning consciousness will be, 

 not a sensation of sweetness or of colour, but that of a thing which can 



