SENSE OF SMELL. 551 



one or other of which headings all our tastes, properly so called, 

 would naturally fall. Though this classification has no just claim 

 to being a chemical one, it is interesting to know that each taste 

 pretty well corresponds to a distinct group of substances chemi- 

 cally allied one to the other, Thus, acids are sour, alkaloids 

 are bitter, the soluble neutral salts of the alkalies are salt, and 

 poly-atomic alcohols, as glycerin, grape-sugar, etc., are commonly 

 sweet. 



The substances most probably act on the nerve terminals as 

 chemical stimuli, because they must be in solution to be appre- 

 ciated. If solid particles be placed on the tongue they must be 

 dissolved in the mouth fluid before they can excite the taste 

 organs. 



In order to explain the appreciation of the different tastes we 

 may imagine that there are different kinds of terminals, each of 

 which is or is not influenced by various substances according as 

 they possess a special sweet, sour, bitter, or salt energy. From 

 these different terminals pass fibres bearing impulses to certain 

 central cells, each of which is capable of exciting a sweet, sour, 

 bitter, or salt sensation, as the case may be. 



SENSE OF SMELL. 



The numerous delicate nerves which pass from the olfactory 

 bulb to the mucous membrane of the upper and part of the middle 

 meatus of the nose form the special nerves of smell. When cer- 

 tain subtle particles we call odors come in contact with the ter- 

 minals of these nerves, they excite impulses which, on arriving in 

 the special centres of the brain, give rise to the impressions of 

 smell. 



Anatomically the relations of the olfactory region are well de- 

 fined. Its mucous membrane is not covered with motile cilia, as 

 is that of the rest of the nasal cavity, and it is less vascular and 

 peculiarly pigmented, looking yellow to the naked eye when com- 

 pared with the neighboring membrane. The epithelial cells are 

 elongated into peculiar cylinders, between which lie long thin rods, 

 ending on the surface in free hair-like processes. The deeper ex- 



