

i CUTANEOUS SENSIBILITY 3 



on the nature of psychical phenomena apart from any material 

 substrate. 



I. The sense-organs are the peripheral instruments of our 

 sensations. Most of the sensory nerves are so arranged at the 

 periphery as to present a surface on which the various changes 

 in the environment can operate. With this object the nerve- 

 endings of the sensory fibres are provided with specific mechanisms 

 which are partially non-neural (sense-organs in a strict sense), and 

 are so formed and adapted as only to allow certain definite external 

 alterations to act on and excite the corresponding nerve-fibres, 

 while absolutely or relatively excluding the action of any other 

 form of external stimulation. It is exclusively by means of the 

 sense-organ formed by the eyeball that we perceive those 

 rhythmical vibrations in the ether which we call light; it is 

 exclusively by the cochlea of the internal ear that we are aware 

 of the rhythmical vibrations in the pressure of the air that we 

 call sound] it is exclusively by the chemical excitation of the 

 olfactory epithelium of the nasal mucosa, or the gustatory 

 epithelium of the lingual mucous membrane, that we are aware 

 of smell or taste. 



The adequate stimulus for any given sense is that to which its 

 organ is specially adapted, so that it can receive it and be effectu- 

 ally excited by it ; all other kinds of stimuli are inadequate for 

 that sense-organ. Light, for instance, is the adequate stimulus 

 for the retina, sound for the cochlea, odoriferous and sapid sub- 

 stances for the organs of smell or taste. Electrical currents, and 

 physical and other mechanical means which can also excite these 

 sense-organs, are inadequate stimuli. 



Adequate stimuli are, as a rule, effective only when they act on 

 the peripheral sense-organ ; they are not always capable of exciting 

 the sensory nerve directly. The most vivid light fails to excite 

 visual sensation when it falls on the stump of the optic nerve ; 

 loud sounds are not perceived by the stump of the auditory nerve, 

 though to this there are some exceptions. Chemical, thermal, 

 and mechanical stimuli can take effect along the course of the 

 olfactory, gustatory, and tactile nerves; but they must be of 

 greater intensity than is required to evoke sensations of smell, taste, 

 temperature, and touch when they are applied to the peripheral 

 end-organ. Adequate stimuli therefore become effective only 

 when they act on the terminal sense-organs, which have presum- 

 ably been adapted to them by a long evolutionary development. 

 Inadequate stimuli, on the contrary (so far at least as we know), 

 can act on any part of the sensory nerve along its course, and are 

 less .effective, or even ineffective, when applied to the peripheral 

 sense-organ. 



Little is known at present about the specific arrangement of 

 the sense-organs, whereby they are specially excitable or sus- 



B I 



