SENSATION AND PERCEPTION 119 



while that coming from an auditory receptor may give us sensation 

 of sound. So far as we know, every nervous impulse, in any 

 nerve whatever, '* sensory " or " motor," is the same kind of 

 molecular disturbance of the materials of the axis-cylinders, and 

 is always accompanied by dissipation of energy, just as in the 

 cases of other molecular processes of the same general category 

 — that is, some production of CO 2, however small, seems to be 

 a condition of the propagation of nervous impulses. 



It is, perhaps, significant that a nervous current passing along 

 a nerve should be constituted by unitary impulses succeeding 

 each other with a certain frequency. It may be significant that 

 this frequency of nerv^ous impulses appears to be different in 

 different nerves. We are reminded that the physical events that 

 stimulate receptors may also differ in frequency, thus low- 

 frequency radiation is felt as heat, higher-frequency radiation 

 as red to violet light, the colour varying with the frequency. 

 And even the material impulses that stimulate the auditory 

 receptors have varying frequencies and the pitch of the sound 

 sensations that follow these impulses depend on the frequencies 

 of the atmospheric vibrations. 



But again we have no notion at all why the qualities of the 

 sensations of light of various colour should depend on frequency 

 of nervous impulse, // that should indeed turn out to be the 

 case. 



43^. The Mechanism of Reception is not separable from 

 THE Mech.\nism of Behaviour. There is no such thing as a 

 receptor mechanism by itself. A receptor and its afferent nerve 

 always stand in structural connection with a series of synapses 

 that are in structural connection with a series of efferent nerves 

 and their endings in effector organs. That means that reception 

 itself does not exist in the animal organism but is always part 

 of the apparatus of action. And it may be doubted whether we 

 ever have sensation resulting from the stimulations of receptors 

 unless these stimuli, translated into nervous impulses, impinge 

 upon the nervT-centres and are follow^ed by action of some kind. 

 That action may be only virtual — the stimulation of the nerve- 

 centres " sets the points," so to speak, in order that something 

 may happen in the effector organs. It appears, from all that 

 we know of cerebral physiology, that the sensory system was 

 evolved for the " purposes " of behaviour and not merely for 



