850 



AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



The olfactory mucous membrane, which alone is the peripheral organ for 

 smell, is seated in the upper part of the nasal chamber, away from the line 

 of the direct current of inspired air. The membrane is thick and is covered 

 by an epithelium composed of two kinds of cells, columnar and rod cells. 

 The latter are the true olfactory cells (Figs. 290, 291), with which the fibres 

 of the olfactory nerve are known to be connected. These olfactory cells, in 

 fact, are comparable to nerve-cells in that the fibres connected with them, the 

 fibres composing the olfactory nerve, are direct outgrowths from the cells 

 (Fig. 292), essentially similar in every way to the nerve-fibre processes springing 

 from nerve-cells in the nerve-centres. In this respect the olfactory cells differ 

 from the sensory cells in other organs of special sense. The membrane 



oJf.c 



FIG. 292. Diagram of the connections of cells and fibres in the olfactory bulb (Schafer, in Quain's Anat- 

 omy) : olf.c, cells of the olfactory mucous membrane ; olf.n, deepest layer of the bulb, composed of the 

 olfactory nerve-fibres which are prolonged from the olfactory cells ; gl, olfactory glomeruli, containing 

 arborization of the olfactory nerve-fibres and of the dendrons of the mitral cells ; m.c, mitral cells ; 

 a, thin axis-cylinder process passing toward the nerve-fibre layer, n.tr, of the bulb to become continuous 

 with fibres of the olfactory tract ; these axis-cylinder processes are seen to give off collaterals, some of 

 which pass again into the deeper layers of the bulb ; n', a nerve-fibre from the olfactory tract ramifying 

 in the gray matter of the bulb. 



appears to be not ciliated except near its juncture with the Schneiderian 

 membrane, where the columnar cells acquire cilia and gradually pass over 

 into the cells covering the respiratory tract. Substances exciting the sense 

 of smell exist as gases or in a fine state of division in the air inspired. 

 They reach the olfactory mucous membrane by diffusion, assisted by the 

 modified inspiratory movements of "sniffing" and "smelling," and are 

 most acutely perceived when the air containing them is warmed to the 

 body-temperature. The amount of odoriferous matter that may thus be 

 recognized is extraordinarily small ; thus, it is said that in one liter of air 

 the odor of 0.000,005 gram of musk and of 0.000,000,005 gram of oil of 

 peppermint can be perceived. 1 The odoriferous particles probably excite the 

 1 Passy : C&mptes-rendus de la Societe de Biologie, 1892, p. 84. 



