292 COMPARATIVE PHYSIOLOGY OF THE BRAIN 



caused by the current. It is not impossible that the 

 so-called visual substances that is, the photo-sensi- 

 tive substances are electrolytes. We can thus under- 

 stand how the electric current produces sensations of 

 light and colour in the eye. But it is more difficult to 

 account for the fact that pressure or a blow on the 

 eyeball produces the sensation of a flash. Carey Lea 

 has found that on photographic plates pressure pro- 

 duces changes of the same character as weak light. 



The specific energy of the eye would accordingly 

 amount to nothing more than the fact that an increase 



O 



in the concentration of ions or certain other chemical 

 substances in the retina causes the sensation of light 

 and colour, no matter whether the changes are caused 

 by vibrations of the ether, by the electric current, or 

 by a blow on the eye. The stimuli which are trans- 

 mitted to the brain from the eye will hence show ex- 

 actly the variety and peculiarities which correspond 

 to the variety and peculiarities of the chemical pro- 

 cesses in the retina. 



The same holds good for the stimuli which are 

 transmitted to the brain from the organs of taste and 

 from the nose. The chemical nature of the causes 

 that produce the sensations of smell and taste is so 

 apparent as to require no proof. 



We find greater difficulty in dealing with the sense- 

 organs of the skin. Yet it is conceivable that a chem- 

 ical basis may also exist for the activity of these senses. 

 This idea finds support in a train of thought, by which 

 I attempted to explain the peculiar influence of grav- 



