588 



THE HUMAN BOD T. 



The olfactory organ proper consists of the upper portions of 

 the two nasal cavities, over which the endings of the olfactory 

 nerves are spread and where the mucous membrane has a 

 brownish-yellow color. This region (regio olfactoria) covers 

 the upper and lower turbinate bones, which are expansions of 

 the ethmoid on the outer wall of the nostril chamber, the 

 opposite part of the partition between the nares, and the part 

 of the roof of the nose separating it from the cranial cavity. 

 The epithelium covering the mucous membrane contains three 

 varieties of cells (2, Fig. 173). The cells of one set are much 

 like ordinary columnar epithelium, but with long branched 

 processes attached to their deeper ends; mixed with these are 



peculiar cells, each of which has 

 a large nucleus surrounded by a 

 little protoplasm; a slender ex- 

 ternal process reaching to the sur- 

 face ; and a very slender deep one. 

 The latter cells have been sup- 

 posed to be the proper olfactory 

 end organs, and to be connected 

 with the fibres of the olfactory 

 nerve, which enter the deeper 

 strata of the epithelium and there 

 divide. In Amphibia the corre- 

 sponding cells have fine filaments 

 on their free ends. The cells of 

 the third *kind are irregular in 

 form and lie in several rows in the 

 deeper parts of the epithelium. 

 It may be that the cylindrical cells 

 if not (as is possible) directly con- 



K.i73.-ceiis from the olfactory cerned in olfaction, have import- 

 ant functions in regard to the 

 nourishment of the olfactory cells 

 outer processed, its slender central which they surround i they may 



process. 3, gray nerve-fibres of the ' ..? 



olfactory nerve, seen dividing into for example Supply them With 

 fine peripheral branches at a. -, , - , , -i 



needful material, as the pigment- 

 cells of the retina are concerned in the formation of visual 

 purple in the rods. 



Odorous substances, the stimuli of the olfactory apparatus, 

 are always gaseous and frequently act powerfully when present 

 in very small amount. We cannot, however, classify them by 



