DESIGN FOR A BRAIN 13/17 



In the skin are histologically-distinguishable receptors sensitive 

 to touch, pain, heat, and cold. If a needle on the skin is changed 

 from lightly touching it to piercing it, the excitation is shifted 

 from the ' touch ' to the ' pain ' type of receptor; i.e. dispersion 

 occurs. 



Whether a change in colour of a stimulating light changes 

 the excitation from one set of elements in the retina to another 

 is at present uncertain. But dispersion clearly occurs when the 

 light changes its position in space; for, if the eyeball does not 

 move, the excitation is changed from one set of elements to 

 another. The lens is, in fact, a device for ensuring that disper- 

 sion occurs: from the primitive light-spot of a Protozoon dis- 

 persion cannot occur. 



It will be seen therefore that a considerable amount of dis- 

 persion is enforced before the effects of stimuli reach the central 

 nervous system: the different stimuli not only arrive at the 

 central nervous system different in their qualities but they often 

 arrive by different paths, and excite different groups of cells. 



13/17. The sense organs evidently have as an important function 

 the achievement of dispersion. That it occurs or is maintained 

 in the nervous system is supported by two pieces of evidence. 



The fact that neuronic processes frequently show threshold, 

 and the fact that this property implies that the functioning 

 elements will often be constant (S. 12/15) suggest that dispersion 

 is bound to occur, by S. 12/16. 



More direct evidence is provided by the fact that, in such cases 

 as are known, the tracts from sense-organ to cortex at least 

 maintain such dispersion as has occurred in the sense organ. 

 The point-to-point representation of the retina on the visual 

 cortex, for instance, ensures that the dispersion achieved in the 

 retina will at least not be lost. Similarly the point-to-point 

 representation now known to be made by the projection of the 

 auditory nerve on the temporal cortex ensures that the dispersion 

 due to pitch will also not be lost. There are therefore good 

 reasons for believing that dispersion plays an important part in 

 the nervous system. 



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