ORGANS OF THE SENSORI-MOTOR SYSTEM 105 



are affected by these waves, but they can be made to transform 

 so that the final resuh is sound waves of audible frequency. 



39c. The Physical Nature of Stimuli. Those agencies in 

 nature that affect the receptor organs are gravitation ; contact 

 with material things ; chemical substances (salt, sugar, quinine, 

 etc.) ; electric currents and charges ; perhaps magnetic fields ; 

 radiation of a certain frequency (for light) ; radiation of much 

 higher frequency (X-rays) ; radiation of low frequency (heat) ; 

 actual contact with cold or hot material bodies and so on. 



Thus we can speak of gravity-receptors, tactile-receptors, 

 sound-receptors, chemical receptors (in the taste and smell 

 organs), pressure-receptors, electric receptors and so on. 



39^. '' Reception " in General. It has become customary 

 to speak of a sense-organ " receiving " stimuli, but what happens 

 is that the nerve-terminations in a receptor organ simply react 

 with some other agency. Thus when salt is placed on the tongue 

 there is a chemical reaction between the sodium chloride and 

 the materials in the nerve-termination ; when light falls on the 

 retina the radiation does much the same thing as light does 

 when it impinges on a photographer's sensitive plate, that is, 

 some definite chemical reactions occur ; when a prawn stands 

 on its head gravity causes the otoliths, or liquids to impinge on 

 different nerve-terminations than when the animal is the right 

 way up. And so on, the stimulation of a receptor organ is not 

 merely something " received " or " impressed " by or on the 

 animal (as when water is poured into a vessel, or when a stamp 

 makes a device on plastic wax). The materials of the receptor 

 participate in a positive reaction with something in the environ- 

 ment. 



Essentially a vertebrate visual receptor does not differ from 

 a photographer's camera ; an auditory organ is physically the 

 same kind of thing as a microphone and a taste-bud on the 

 tongue is comparable with a slip of litmus paper. The animal 

 body simply reacts with the things in its neighbourhood in 

 essentially the same ways that any other physical thing does. 

 But, because of the extraordinary physical complexity of the 

 system called an animal body, the variety of the reactions, and 

 their delicacy as regards the quantities of energy involved, 

 transcend most inanimate reactions. Thus the smell of, say, 

 chlorine can be experienced when the concentration of the gas 



