116 Professor Huxley [March 7, 



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, infinitesimal 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 sounding 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'' 



Hartley, like Haller, had no conception of the nature and func- 

 tions of the grey matter of the brain. But, if for " white medullary 

 substance," in the latter paragraph, we substitute " grey cellular sub- 

 stance," Hartley's propositions embody the most probable conclusions 

 which are to be drawn from the latest investigations of physiologists. 

 In order to judge how completely this is the case, it will be well to 

 study some simple case of sensation, and, following the example of 

 Reid and of James Mill, we may begin with the sense of smell. 

 Suppose that I become aware of a musky scent, to which the name of 

 " muskiness " may be given. I call this an odour, and I class it 

 along with the feelings of light, colours, sounds, tastes, and the like, 

 among those phenomena which are known as sensations. 



The pure sensation of muskiness is almost sure to be followed 

 by a mental state which is not a sensation, but a belief, that there is 

 somewhere close at hand a something on which the existence of the 

 sensation depends. It may be a musk-deer, or a musk-rat, or a musk- 

 plant, or a grain of dry musk, or simply a scented handkerchief ; but 

 former experience leads us to believe that the sensation is due to the 

 presence of one or other of these objects, and that it will vanish if the 

 object is removed. In other words, there arises a belief in an external 

 cause of the muskiness, which, in common language, is termed an 

 odorous body. 



It is unnecessary for the present purpose to inquire into the 

 origin of our belief in external bodies, or into that of the notion of 



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



