570 



THE HUMAN BODY. 



nucleus surrounded by a little protoplasm; a slender exter- 

 nal process reaching to the surface; and a very fine deep one. 

 The latter cells have been supposed to be the proper "olfac- 

 tory end organs, and to be connected with the fibres of the 

 olfactory nerve, which enter the deeper strata of the epithe- 

 lium and there divide; but it is doubtful whether both 

 kinds of cells are not so connected. 



Odorous substances, the 

 stimuli of the olfactory appa- 

 ratus, are always gaseous and 

 frequently act powerfully 

 when present in very small 

 amount. We cannot, how- 

 ever, classify them by the sen- 

 sations they arouse, or arrange 

 them in series; and smells are 

 but minor sensory factors in 

 our mental life. We commonly 

 refer them to external objects 

 since we find that the sensa- 

 tion is intensified by " sniff- 

 ing" air powerfully into the 

 nose, and ceases when the 

 nostrils are closed. Their 

 peripheral localization is, how- 

 ever, imperfect, for we con- 

 found many smells with tastes 

 (see below); nor can we judge 

 well of the direction of an 

 odorous body through the 

 olfactory sensations which it 

 arouses. 



The nose possesses also nerves of common sensation, 

 which are stimulated by such substances as ammonia 

 vapor. 



Taste. The organ of taste is the mucous membrane on 

 the dorsum of the tongue and possibly other parts of the 

 boundary of the mouth cavity. The nerves concerned are 

 the glossopharyngeals, distributed over the hind part of the 



FIG. 154. Cells from the olfactory 

 epithelium. 1, from the frog; 2, 

 from man: a, columnar cell, with its 

 branched deep process; 6, so-called 

 olfactory cell; c, its narrow outer 

 process; d, its slender central pro- 

 fess. 3, gray nerve-fibres of the ol- 

 factory nerve, seen dividing into 

 fine peripheral branches at a. 



