STRUCTURE OF SENSIFEROUS ORGANS 303 



charged with particles of musk; but, if either the 

 epithelium, or the nerve fibres, or the sensorium 

 is injured, or if they are physically disconnected 

 from one another, sensation will not arise. More- 

 over, the epithelium may be said to be receptive, 

 the nerve fibres transmissive, and the sensorium 

 sensifacient. For, in the act of smelling, the par- 

 ticles of the odorous substance produce a mole- 

 cular change (which Hartley was in all probability 

 right in terming a vibration) in the epithelium, 

 and this change being transmitted to the nerve 

 fibres, passes along them with a measurable 

 velocity, and, finally reaching the sensorium, is 

 immediately followed by the sensation. 



Thus, modern investigation supplies a repre- 

 sentative of the Epicurean " simulacra " in the 

 volatile particles of the musk; but it also gives us 

 the stamp of the particles on the olfactory epithe- 

 lium, without any transmission of matter, as the 

 equivalent of the Aristotelian "form"; while, 

 finally, the modes of motion of the molecules of 

 the olfactory cells, of the nerve, and of the cere- 

 bral sensorium, which are Hartley's vibrations, 

 may stand very well for a double of the " inten- 

 tional species" of the Schoolmen. And this last 

 remark is not intended merely to suggest a fanci- 

 ful parallel; for, if the cause of the sensation is, 

 as analogy suggests, to be sought in the mode of 

 motion of the object of sense, then it is quite 

 possible that the particular mode of motion of the 



