208 THE HUMAN BODY. 



or even completely destroys, the sense of smell. Smelling 

 is ordinarily involuntary, but it may be rendered more 

 active by the exertion of the will. The inspirations are 

 then stronger and more frequent, in order that the odour 

 which we wish to perceive or enjoy may be carried in greater 

 quantity toward the nasal arch. But if, on the contrary, we 

 wish to avoid a disagreeable odour, a sudden expiration takes 

 place from the nose, and we breathe instinctively through 

 the mouth, and the soft palate closes the olfactory cavities 

 behind. It is in this way that we are able to diminish the 

 disagreeable impression arising from the odour in drinking 

 sulphureous waters. 



Whether odours reach the seat of smell by the nose, or 

 through the posterior opening of the nasal fossae, the result 

 is the same; it is by this means that we perceive the aroma 

 of the food when eating with the mouth shut; but under 

 these latter conditions the persistence of the impressions 

 very soon blunts the sensibility. A man fasting immediately 

 perceives it, if a man with whom he is speaking has taken 

 the smallest quantity of alcohol, even though it was only a 

 glass of red wine; after eating we distinguish much less easily 

 in others the odour of the aliments of which we ourselves 

 have partaken, and the odoriferous principles of which have 

 already saturated the olfactory membrane. 



The sinuses of the bones of the skull and of the face, 

 which are in communication with the nasal fossae, take no part 

 in the perception of odours. It has been thought that they 

 may contribute by their secretions to moisten the pituitary 

 membrane, or serve as receptacles for the air, which is after- 

 ward carried from their cavities to the organ of smell. 



Duration of olfactory impressions. When we have inspired 

 a strong and penetrating odour, the sensation is prolonged 

 for a certain time, sometimes for several hours. It is pro- 

 bable that in this case the impression is not single, but is in- 

 cessantly renewed by the odoriferous particles with which 

 the mucus of the pituitary membrane is impregnated, or 

 which is confined in the air in the sinuses. Sometimes also 

 the odour has penetrated the garments, or is attached to the 

 skin or the hair, and from thence continues its impressions. 



