THE SENSE OF SMELL. 495 



ing sensation on the nerves of touch ; but the sensation of odor 

 is yet separate and distinct from these, though it may be sim- 

 ultaneously perceived. The second condition of smell is a pe- 

 culiar state of the olfactory nerve, or a peculiar change pro- 

 duced in it by the stimulus or odorous substance. 



The material causes of odors are, usually, in the case of ani- 

 mals living in the air, either solids suspended in a state of ex- 

 tremely fine division in the atmosphere ; or gaseous exhalations 

 often of so subtile a nature that they can be detected by no 

 other reagent than the sense of smell itself. The matters of 

 odor must, in all cases, be dissolved in the mucus of the mu- 

 cous membrane before they can be immediately applied to, or 

 affect the olfactory nerves ; therefore a further condition neces- 

 sary for the perception of odors is, that the mucous membrane 

 of the nasal cavity be moist. When the Schneiderian mem- 

 brane is dry, the sense of smell is impaired or lost ; in the first 

 stage of catarrh, when the secretion of mucus within the nos- 

 trils is lessened, the faculty of perceiving odor is either lost, or 

 rendered very imperfect. 



In animals living in the air, it is also requisite that the 

 odorous matter should be transmitted in a current through 

 the nostrils. This is effected by an inspiratory movement, the 

 mouth being closed ; hence we have voluntary influence over 

 the sense of smell ; for by interrupting respiration we prevent 

 the perception of odors, and by repeated quick inspiration, as- 

 sisted, as in the act of sniffing, by the action of the nostrils, 

 we render the impression more intense (see p. 184). 



The human organ of smell is essentially formed by the fila- 

 ments of the olfactory nerves, distributed in minute arrange- 

 ment, in the mucous membrane covering the upper third of the 

 septum of the nose, the superior turbinated or spongy bone, 

 the upper part of the middle turbinated bone, and the upper 

 wall of the nasal cavities beneath the cribriform plates of the 

 ethmoid bones (Figs. 176 and 177). 



This olfactory region is covered by cells of cylindrical epithe- 

 lium not provided with cilia ; and interspersed with these are 

 peculiar fusiform cells with fine processes, called olfactory cells. 

 They are supposed to have some connection with the terminal 

 filaments of the olfactory nerve. The lower, or respiratory 

 part, as it is called, of the nasal fossae is lined by cyliindrical 

 ciliated epithelium, except in the region of the nostrils, where 

 it is squamous. 



In all the distribution, the branches of the olfactory nerves 

 retain much of the same soft and grayish texture which dis- 

 tinguishes their trunks (as the olfactory lobes of the brain are 

 called) within the cranium. Their individual filaments, also, 



