THE SENSES. 



071 



merit, the month being closed; hence we have voluntary influence over 

 the sense of srnell; for hy interrupting respiration we prevent the per- 

 ception of odors, and by repeated quick inspiration, assisted, as in the 

 act of sniffing, by the action of the nostrils, we render the impression 

 more intense. An odorous substance in a liquid form injected into the 

 nostrils appears incapable of giving rise to the sensation of smell; thus 

 Weber could not smell the slightest odor when his nostrils were com- 

 pletely filled with water containing a large quantity of eau-de-Cologne. 



The nose is not entirely an organ for the seat of smell. In fact the 

 nasal cavities are divided into three districts called respectively (a) Regio 

 vestibularis, which is the entrance to the cavity. 

 It is lined with a mucous membrane very closely 

 resembling the skin, and contains hair (vibris- 

 see) with sebaceous glands, (b) Regio respira- 

 toria, which includes the lower meatus of the 

 nose, and all the rest of the nasal passages ex- 

 cept (c); it is covered with mucous membrane 

 covered by stratified columnar ciliated epitheli- 

 um. The mucosa is thick and consists of fibrous 

 connective tissue; it contains a certain number 

 of tubular mucous and serous glands. (c) Re- 

 gio olfactoria. This includes the anterior two- 

 thirds of the superior meatus, the middle meatus, 

 and the upper half of the septum nasi. It is of 

 a yellowish color. It consists of a thicker muc- 

 ous membrane than in (#), made up of loose are- 

 olar connective tissue covered by epithelium of 

 a special variety, resting upon a basement mem- 

 brane. The cells of the epithelium are of two 

 principal kinds: (a) columnar epithelial cells 

 whose function is to support (b) the bipolar 

 olfactory cells, (a) The epithelial cells are pris- 

 matic in shape and have upon their surfaces 

 facets into which the olfactory cells fit them- 

 selves. They are thus analogous to the cells of 

 Muller of the retina (fig. 392.). (b) The olfac- 



tory cells have an oblong or fusiform shape, which d gj2 d from the tr te eminus - 

 is mainly determined by the large nucleus. The 



thin protoplasmic body has two processes, an external and an internal. 

 The external is large and passes up to the free surface to end in a small 

 bunch of fibrils that are not vibratile. The internal process is very 

 fine, often varicose, and passes through the mucous membrane to be- 

 come continuous with the fibres of the olfactory bulb. 



The olfactory bulb must be studied in relation with the nerve- 



Fig. 392. Bipolar olfactory 

 cells from the nasal fossae of 

 the rat (full-term foetus). A, 



