STRUCTURE OF SENSIFEROUS ORGANS 295 



In truth, the theory of sensation, except in one 

 point, is, at the present moment, very much where 

 Hartley, led by a hint of Sir Isaac Newton's, left 

 it, when, a hundred and twenty years since, the 

 " Observations on Man: his Frame, his Duty, and 

 his Expectations," was laid before the world. 

 The whole matter is put in a nutshell in the 

 following passages of this notable book. 



" External objects impressed upon the senses occasion, 

 first on the nerves on which they are impressed, and then on 

 the brain, vibrations of the small and, as we may say, in- 

 finitesimal medullary particles. 



" These vibrations are motions backwards and forwards 

 of the small particles ; of the same kind with the oscillations 

 of pendulums and the tremblings of the particles of sound- 

 ing bodies. They must be conceived to be exceedingly short 

 and small, so as not to have the least efficacy to disturb or 

 move the whole bodies of the nerves or brain." * 



" The white medullary substance of the brain is also the 

 immediate instrument by which ideas are presented to the 

 mind ; or, in other words, whatever changes are made in this 

 substance, corresponding changes are made in our ideas ; and 

 vice versa" f 



Hartley, like Haller, had no conception of the 

 nature and functions of the grey matter of the 

 brain. But, if for "white medullary substance/ 5 

 in the latter paragraph, we substitute " grey 

 cellular substance," Hartley's propositions embody 



* Observations on Man, vol. i. p. 11. 



f Ibid, p. 8. The speculations of Bonnet are remarkably 

 similar to those of Hartley ; and they appear to have orig- 

 inated independently, though the Essaide Psychologic, (1754) 

 is of five years' later date than the Observations on Man (1 749). 



