SENSE OF SMELL. 553 



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

 polyatomic alcohols, as glycerin, grape sugar, etc., are commonly 

 sweet. 



The substances most probaby 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 certain subtle particles we call odors come in contact 

 with the terminals 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 

 defined. 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 

 compared 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 extremities of these rod-shaped filaments expand to sur- 

 round a nucleus, and are then continued into a network of fila- 

 ments, into which prolongations of the epithelial cells also seem 

 to pass, and in which the delicate fibrils of the olfactory nerve 

 can be traced. The existence of direct communication between 

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