HANDBOOK OF PHYSIOLOGY. 



III. Smell. 



Conditions necessary. (1.) The first conditions essential to the sense 

 of smell are a special nerve and nerve-terminations in the form of special 

 cells, the changes in whose condition stimulate a special nerve-centre, 

 and are perceived in sensations of odor, for no other nervous structure 

 is capable of these sensations, even though acted on by the same causes. 

 The same substance which excites the sensation of smell in the olfac- 

 tory centre may cause another peculiar sensation through the nerves of 

 taste, and may produce an irritating and burning sensation on the nerves 

 of touch; but the sensation of odor is yet separate and distinct from 

 these, though it may be simultaneously perceived. (2.) The material 

 causes of odors are, usually, in the case of animals living in the air, 





Fig. 391. Nerves of the septum nasi. seen from the right side. %. I. the olfactory bulb: 

 1, the olfactory nerves passing through the foramina of the cribriform plate, and descending to 

 be distributed on the septum: 2, the internal or septal twig of the nasal branch of the ophthal- 

 mic nerve; 3, naso-palatine nerves. (From Sappey, after Hirschfeld and Leveill6.) 



either solids suspended in a state of extremely fine division in the atmos- 

 phere ; or gaseous exhalations often of so subtle 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 

 mucous membrane before they can be immediately applied to, or affect 

 the olfactory nerves; therefore a further condition necessary for the 

 perception of odors is, that the mucous membrane of the nasal cavity 

 be moist. When the Schneiderian membrane is dry, the sense of smell 

 is impaired or lost; in the first stage of catarrh, when the secretion of 

 mucus within the nostrils is lessened, the faculty of perceiving odor is 

 either lost, or rendered very imperfect. (3.) 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 move- 



