A STUDY OF THE SENSES 



297 



The Sense Organs of Smell. The whole interior of the 

 nose is lined with mucous membrane. This membrane, 

 however, differs greatly in its 

 structure in the upper or ol- 

 factory region of each cavity 

 and in the lower or respiratory 

 region. In the latter one finds 

 ciliated cells much like those 

 which line the air passages 

 from the throat to the lungs. 

 In the mucous membrane of 

 the upper or olfactory region 

 there are at least two distinct 

 types of cells, neither of which 

 have cilia. Some are cylindri- 

 cal, with branching processes 

 near the base. These cells, 

 like the outer cells of the taste 

 buds, surround and support 

 the rod-shaped olfactory cells 

 which lie between them. 

 The latter have a large nucleus 

 near the middle, and from this 

 region of the cell extend two 

 slender processes; one runs outward between the cylindri- 

 cal cells and projects on the surface of the mucous membrane 

 as the so-called sensory hairs, the other seems to be continu- 

 ous with branches of the olfactory nerve (see Fig. 134). 



Sensations of Smell. During ordinary respiration we are 

 not often conscious of sensations of smell. When, how- 

 ever, we perceive an odor, we can increase its effect by 

 sniffing. In this way larger quantities of air are drawn up 

 into the olfactory regions of the nose, and so the olfac- 

 tory cells are stimulated more intensely. The olfactory 

 sense in man is so keen that three one-hundred-millionths 

 doooooooo) ^ a g ra i n of musk can be smelled. 



S/i. PL 



FIG. 133. Cross Section of Nasal 

 Chambers. 



An = cavities in upper jaw bone. 

 Cr = perforations in ethmoid bone 

 through which pass fibers of 

 olfactory nerve. 

 IT = lower turbinate bone. 

 MT= middle turbinate bone. 

 ST= upper turbinate bone. 

 PI = hard palate. 

 Sp = partition between nasal 

 chambers. 



