290 COMPARATIVE PHYSIOLOGY OF THE BRAIN 



counting and locating the telephone connections in a 

 big city. 



If we are anxious to develop a dynamics of the 

 various life-phenomena, we must remember that the 

 colloidal substances are the machines which produce 

 the life-phenomena. But the physics of these sub- 

 stances is still a science of the future. The new 

 methods and conceptions created by physical chemis- 

 try give us the hope that a physics of the colloidal 

 substances may be looked for in the near future. At 

 present we can only consider data of secondary im- 

 portance for the mechanics of associative memory. 

 The first group of these data is furnished by the 

 study of the functions of the sense-organs. 



Helmholtz emphasised the fact that our senses only 

 furnish us symbols of the external world. Every 

 physical process that affects a sense-organ produces 

 changes in the organ. These changes are determined 

 by the peripheral structure or by the specific " energy " 

 of the sense-organs, as physiologists since Johannes 

 Miiller call it. Whether a blow, an electric current, 

 or ether-vibrations of about 0.0008-0.0004 mm. wave- 

 length stimulate the retina, the sensation is always a 

 specific one, namely, light, while a blow or an electric 

 current produces sensations of sound in the ear. 

 This so-called law of the specific energy of the sense- 

 organs is not peculiar to the sense-organs ; it applies, 

 as was emphasised by Sachs, to all living matter; it 

 even holds good for machines. It is in reality only 

 another expression for the fact that the eye, the ear, 



