492 



STUDIES IN ADVANCED PHYSIOLOGY. 



THE SENSE OF SMELL. 



The sense of smell is located in the olfactory region, 

 which includes the mucous membrane covering the folds of 

 the ethmoid bone, and the turbinated bones. The mucous 

 membrane in these parts is not provided with cilia, as it is 

 in the regular respiratory tract behind. The difficulty of 

 establishing definite end organs con- 

 cerned in the sense of smell is even 

 greater than in those of taste, and the 

 histology of the mucous membrane 

 reveals but slightly differentiated struc- 

 tures for this purpose. There are, how- 

 ever, in the epithelium covering this 

 mucous membrane certain more slender 

 cells placed in between the ordinary 

 epithelial cells. These more slender or 

 sensory cells are connected with fibers 

 from the olfactory nerve beneath, are 

 pointed at the upper end, which point 

 projects very slightly above the mucous 

 membrane into the nasal cavity. The 

 statement of some observers that in the 

 human nose these sensory cells have 



Fig. 156. OLFACTORY CELLS, little hairs or cilia at their end is prob- 

 (After M. schuitze.) ao ly not true. It is true, however, in 



1, from the frog; 2, human; ,-t r i j j 1 't 



a. ordinary epUhelial cells! the CaSG f birds aild amphibians. 



6, olfactory cells; c, peri- TllCSC pOSSCSS OU the ends of tllCSC 



pheral process prolonged ,11 11 i ATA-I 



in i into fine hairs ; a, their cells rather long immovable hairs . The 

 central ends connected a b se nce of such hairs in man may 



with the nerve. * 



account for his bluntness of this sense 



when compared with the intense acuteness of that of some 

 of the lower animals. The tips of these cells projecting into 

 the nasal cavity are the points where the stimuli that pro- 

 duce sensations of odor affect the nerves. It is at once ap- 

 parent how much more efficient such a stimulus would be 

 when acting upon a number of sensitive protoplastic hairs 

 projecting freely above the surface than when obliged to act 



