STRUCTURE OF SENSIFEROUS ORGANS 301 



the " tenuia simulacra rerum," or the " intentional 

 species " cast off by 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 par- 

 tition, 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 therefore lies behind the apparent 

 limits of that feature which, in ordinary lan- 

 guage, is called the nose. From the side walls 

 of the upper and back part of these arched cham- 

 bers, 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 



