DISPERSION 15/5 



part-functions, we can put no limit to the difference there may 

 be between the two sets of variables made active in the two 

 responses ; or in other words, there is, in general, no limit to 

 the degree of dispersion that may occur other than that imposed 

 by the finiteness of the mechanism. 



15/5. It will be proposed later that dispersion is used widely 

 in the nervous system. First we should notice that it is used 

 widely in the sense-organs. The facts are well known, so I can 

 be brief. 



The fact that the sense-organs are not identical enforces an 

 initial dispersion. Thus if a beam of radiation of wave-length 

 0-5 jli is directed to the face, the eye will be stimulated but not 

 the skin ; so the optic nerve will be excited but not the trigeminal. 

 But if the wave-length is increased beyond 0-8^, the excitation 

 changes from the optic nerve to the trigeminal. Dispersion has 

 occurred because a change in the stimulus has moved the excita- 

 tion (activity) from one set of anatomical elements (variables) 

 to another. 



The sense of taste depends on four histologically-distinguishable 

 types of receptors each sensitive to only one of the four qualities 

 of salt, sweet, sour, and bitter. If change from one solution to 

 another changes the excitation from one type of receptor to 

 another, then dispersion has occurred. 



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 4 pain ' type of receptor ; i.e. dispersion 

 occurs. 



In the cochlea, sounds differing in pitch vibrate different parts 

 of the basilar membrane. As each part has its own sensitive 

 cells and its own nerve-fibres, a change in pitch will shift the 

 excitation from one set of fibres to another. 



The three semicircular canals are arranged in planes mutually 

 at right-angles, and each has its own sensitive cells and nerve- 

 fibres. A change in the plane of rotation of the head will there- 

 fore shift the excitation from one set of fibres to another. 



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 



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