THE SENSES 8gl 



nerve. And it is fair to conclude that in some manner this part of 

 the cerebral cortex is essential to the production of visual sensations. 

 But in what way the chemical or physical processes in the axis- 

 cylinders or nerve-cells are related to the psychical change, the inter- 

 ruption of the smooth and unregarded flow of consciousness which 

 we call a sensation of light, we do not know. To our reasoning, and 

 even to our imagination, there is a great gulf fixed between the 

 physical stimulus and its psychical consequence ; they seem incom- 

 mensurable quantities ; the transition from light to sensation of light 

 is certain, but unthinkable. 



Each kind of peripheral end-organ is peculiarly suited to 

 respond to a certain kind of stimulus. The law of ' adequate ' 

 or ' homologous ' stimuli is an expression of this fact. The 

 ' adequate ' stimuli of the organs of special sense may be divided 

 into (i) vibrations set up at a distance without the actual con- 

 tact of the object e.g., light, sound, radiant heat ; (2) changes 

 produced by the contact of the object e.g., in the production 

 of sensations of taste, touch, pressure, alteration of temperature 

 (by conduction). Midway between (i) and (2) lies the 

 adequate stimulus of the olfactory end-organs, which are excited 

 by material particles given off from the odoriferous body and 

 borne by the air into the upper part of the nostrils. 



The end-organs of the special senses all agree in consisting essen- 

 tially of modified ectodermic cells, but they occupy areas by no means 

 proportioned to their importance and to the amount of information 

 we acquire through them. The extent of surface which can be 

 affected by light in a man is not more than 20 sq. cm. ; the endings 

 of both nerves of hearing taken together do not at most expand to 

 more than 5 sq. cm. ; the olfactory portion of the mucous mem- 

 brane of the nose has an area of not more than 10 sq. cm. ; the 

 sensations of taste are ministered to by an area of less than 

 50 sq. cm. ; the end-organs of the senses of pressure, touch, and 

 temperature are distributed over a surface reckoned by square 

 metres. As the physiological status of the sensory end-organs 

 rises, their anatomical distribution tends to shrink. The organs of 

 comparatively coarse and common sensations are widely spread, 

 intermingled with each other, and seated in tissues whose primary 

 function may not be sensory at all. Even the nerve-endings of the 

 sense of taste are not confined to one definite and circumscribed 

 patch, but scattered over the tongue and palate ; and both tongue 

 and palate are at least as much concerned in mastication and 

 deglutition as in taste. The olfactory portion of the nasal mucous 

 membrane, although a continuous area with fairly distinct boun- 

 daries, is still a part of the general lining of the nostril. The 

 epithelial surfaces which minister to the supreme sensations of sight 

 and hearing the retina and the sensitive structures of the cochlea- 

 are the most sequestered of all the sensory areas, as the organs of 

 which they form a part are, of all the organs of sense, the most 

 highly specialized in function, and anatomically the most limited. 

 But although hidden in protected hollows, they are endowed, either 

 in virtue of their own movements or of those of the head, with the 

 power of receiving impressions from every side, and their actual size 

 is thus indefinitely multiplied. 



