SENSATION AND THE SENSIFEROUS ORGANS. 263 



Let us consider the olfactory sense organ more near- 

 ly. 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 communication 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 there- 

 fore 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 delicate plates of bone project, and 

 these, as well as a considerable part of the partition be- 

 tween the two chambers, are covered by a fine, soft, 

 moist membrane. It is to this " Schneiderian," or olfac- 

 tory, membrane that odorous bodies must obtain direct 

 access, if they are to give rise to their appropriate sensa- 

 tions ; 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 mem- 

 brane, 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 sub- 



