X.] SENSATION AND THE SENSIFEROUS ORGANS. 257 



the epidermis covers the skin. In the case of the olfac- 

 tory sense, there can be no doubt that the Democritic 

 hypothesis, at any rate for such odorous substances 

 as musk, has a good foundation. Infinitesimal par- 

 ticles of musk fly off from the surface of the odorous 

 body, and, becoming diffused through the air, are 

 carried into the nasal passages, and thence into the 

 olfactory chambers, 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 con- 

 nected with nerve fibres, and these nerve fibres, pass- 

 ing 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 sen- 

 sorium 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 sensifacient. 

 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 vibration) in 

 the epithelium, and this change being transmitted to 



