118 Professor Huxley [March 7, 



objects, or the " forms " of sensible things, pass straight to the mind ; 

 on the contrary, it stands as a firm and impervious barrier, through 

 which no material particle of the world without can make its way to 

 the world within. 



Let us consider the olfactory sense organ more nearly. Each of 

 the nostrils leads into a passage completely separated from the other 

 by a partition, and these two passages place the nostrils in free com- 

 munication with the back of the throat, so that they freely transmit 

 the air passing to the lungs when the mouth is shut, as in ordinary 

 breathing. The floor of each passage is flat, but its roof is a high 

 arch, the crown of which is seated between the orbital cavities of the 

 skull, which serve for the lodgment and protection of the eyes, and 

 therefore lies behind the apparent limits of that feature which in ordi- 

 nary language is called the nose. From the side walls of the upper 

 and back part of these arched chambers, certain delicate plates of 

 bone project, and these, as well as a considerable part of the partition 

 between the two chambers, are covered by a fine, soft, moist membrane. 

 It is to this Schneiderian, or olfactory, membrane that odorous bodies 

 must obtain direct access if they are to give rise to their appropriate 

 sensations ; and it is upon the relatively large surface which the 

 olfactory membrane offers that we must seek for the seat of the organ 

 of the olfactory sense. The only essential part of that organ consists 

 of a multitude of minute rod-like bodies, set perpendicularly to the 

 surface of the membrane, and forming a part of the cellular coat, or 

 epithelium, which covers the olfactory membrane, as the epidermis 

 covers the skin. In the case of the olfactory sense, there can be no 

 doubt that the Democritic hypothesis, at any rate for such odorous 

 substances as musk, has a good foundation. Infinitesimal particles 

 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 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 senso- 

 rium, 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 senso- 

 rium are injured, or physically disconnected from one another, sensa- 

 tion will not arise. Moreover, the epithelium may be said to be 

 receptive, the nerve fibres transmissive, and the sensorium sensifa- 

 cient. For, in the act of smelling, the particles of the odorous sub- 

 stance produce a molecular change (which Hartley was in all proba- 



