554 



MANUAL OF PHYSIOLOGY. 



the nerves and the rod-shaped filaments and the epithelial cells 

 is satisfactorily established. 



The odorous particles must be in the form of gases, in order to 

 be carried by the air into the olfactory region, and the air must 

 be kept in motion, by sniffing it in and out of the nasal cavity, 

 in order to excite the nerve terminals, which are not influenced 

 by the odors of air absolutely at rest, though it be in contact 

 with the mucous membrane of the olfactory tract. 



FIG. 214. 



Section through the mucous membrane of the nasal fossa in the level of the olfactory 

 region. , Epithelial cells and bundles of nerves; b, glands separated from each other 

 by bundles of nerves, c. (Cadiat.) 



The extreme delicacy of appreciation of odors by the olfactory 

 nerve terminals is very remarkable. Even in human beings, 

 whose sense of smell is but poorly developed when compared with 

 that of animals, an amount of odorous substance can be perceived 

 which the finest chemical tests fail to appreciate. Thus, Valentin 

 has estimated that the two-millionths of a milligram of musk is 

 sufficient to excite the specific energy of a man's olfactory appa- 

 ratus. 



