THE SEWSE OF .SMELL. 



409 



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

 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 



Fig. 205— Diagram of the connections of cells and fibres in the olfactory bulb (Schiifer, in Quoin's Avat- 

 omy) : otf.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; ro.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; «', a nerve-fibre from the olfactory tract ramifying 

 in the gray matter of the bulb. 



sense of smell by coming into contact with the olfactory epithelium after solu- 

 tion in the layer of moisture covering it. This epithelium is easily thrown 

 out of function, as the common loss of smell when there is a "cold in the 

 head " testifies. When the nostril is filled "with water in which an odorous 

 substance is dissolved, no sensation of smell is excited, but it is said thai if 

 normal salt-solution, which injures the living tissues less than water, be used 

 as the solvent, the odor can still be perceived. In many lower animals the 

 sense of smell has an acuteness and an importance in their economy unknown 

 in the human race. It is probable that not only do differenl races have their 

 distinctive odors, but that each individual exhales an odor peculiar to himself, 

 distinguishable by the olfactory organs of certain animals. The classification 

 1 Passy : Comptes-rendus de la Sodete de Biologic, 1892, p. 84. 



