THE SENSES IN GENERAL 907 



any excitation of retina or optic nerve. And it is fair to conclude that 

 in some manner this part of the cerebral cortex is essential to the pro- 

 duction 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 interruption of the smooth and unregarded flow of conscious- 

 ness 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 ' homol- 

 ogous ' stimuli is an expression of this fact. The ' adequate ' 

 stimuli of the organs of special sense may be divided into (i) vibra- 

 tions set up at a distance without the actual contact 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). Mid- 

 way 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 essentially 

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

 portioned 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 membrane 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 com- 

 paratively coarse and common sensations are widely spread, inter- 

 mingled 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 boundaries, 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 in- 

 definitely multiplied. 



