256 SENSATION AND THE SENSIFEROUS OEGANS. [LECT. 



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 with- 

 out 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 it therefore lies behind the apparent 

 limits of that feature which, in ordinary language, is 

 called the nose. From the side walls of the upper 

 and back part of these arched chambers, certain deli- 

 cate 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 



