ON THE PSYCHO-PHYSICAL VIEW OF NATURE. 481 



advance upon Newton. For the three primary colours 

 of the older opticians he substituted red, green, and 

 violet ; and for the remarkable fact that the simple 

 colours of the rainbow can be compounded out of these 

 three, he suggested a physiological reason viz., that the 

 eye possesses three distinct colour-sensations or three 

 distinct senses in relation to light, dependent upon some 

 peculiarity of nervous structure or function. Young 

 did not elaborate his ideas, but it is clear that in the 

 short passages in his ' Lectures on Natural Philosophy ' 

 and earlier papers, there were contained a variety of 

 definite problems and hints which were destined to 

 lead research for a long time after. 



The next great step in advance, which has revolu- 12. 



f Charles Bell. 



tionised and permanently fixed our ideas on the action 

 of the nervous system, was taken about the year 1810 

 by Charles Bell, who discovered the anatomical difference 

 between the anterior and posterior roots of the nerves 

 of the spine, and also went a long way towards show- 

 ing their different functions. The point as regards 

 functions was established by means of experiments on 

 living animals by Magendie, and independently by 

 Johannes Miiller. 1 Upon the combined labours of these 

 three masters of anatomy and experimental physiology 

 is based the distinction between sensory and motor 

 nerves namely, that the anterior nerves of the spine 

 are employed to carry the nervous stimulus outward 

 to the different organs (efferent or motor nerves), the 

 posterior and better protected nerves serving to carry 



1 On the respective merits of 

 Charles Bell, Msgendie, and Jo- 

 hannes Miiller, see the writings of 



Claude Bernard and Du Bois-Rey- 

 mond, referred to supra, p. 384 of 

 this volume. 



VOL. II. 2 H 



