772 A MANUAL OF PHYSIOLOGY 



media of the eyeball, see it focussed on the retina, lead off the 

 current of action set up in that membrane, which, doubtless, gives 

 token of the passage of nervous impulses into and up the optic nerve. 

 We can even follow the nervous impulses to a definite portion of the 

 cortex of the occipital lobe, and determine that if this is removed 

 no sensation of sight will result from 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 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 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, altera- 

 tion 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 epiblastic 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 



