264 SENSATION AND THE SENSIFEROUS ORGANS. 



stances as musk, has a good foundation. Infinitesimal 

 particles of musk fly off the surface of the odorous body, 

 and, becoming diffused through the air, are carried into 

 the nasal passages, and thence into the olfactory cham- 

 bers, where they come into contact with the filamentous 

 extremities of the delicate olfactory epithelium. 



But this is not all. The " mind " is not, so to speak, 

 upon the other side of the epithelium. On the contrary, 

 the inner ends of the olfactory cells are connected with 

 nerve fibres, and these nerve fibres, passing into the 

 cavity of the skull, at length end in a part of the brain, 

 the olfactory sensorium. It is certain that the integrity 

 of each, and the physical inter-connection of all these 

 three structures, the epithelium of the sensory organ, the 

 nerve fibres, and the sensorium, are essential conditions 

 of ordinary sensation. That is to say, the air in the 

 olfactory chambers may be 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. 

 Moreover, the epithelium may be said to be receptive, 

 the nerve fibres transmissive, and the sensorium sensi- 

 facient. For, in the act of smelling, the particles of the 

 odorous substance produce a molecular change (which 

 Hartley was in all probability right in terming a vibra- 

 tion) in the epithelium, and this change being trans- 

 mitted to the nerve fibres, passes along them with a 

 measurable velocity, and, finally reaching the sensorium, 

 is immediately followed by the sensation. 



