OF SMELL. 81 



28. The nasal fossa, (Fig. 27.) are 

 two large cavities in the face, which 

 communicate externatty by the open- 

 ings of the nose or nostrils, and open 

 behind, into the pharynx, by the 

 posterior nares or nostrils. The walls 

 of these cavities, form in front, a more 

 or less prominent ridge, which con- 

 stitutes the nose, and a verticle par- 

 tition s^p irat.es one from the other. 

 Finally, they are lined by a soft and 

 very delicate membrane, called the 

 pituitary m< nthrane. 



29. The first pair of cerebral nerves which are called the olfac- 

 tory nerves, are distributed to this membrane, and transmit to the 

 brain the impressions produced by the contact of odoriferous 

 particles 



30. The air which traverses the nasal fossae in order to reach 

 the lungs, carries with it the odorous particles of substances, and 

 it is by touching the pituitary membrane that these particles pro- 

 duce the sensations of smells. The form of the nasal fossae is 

 such, that the air is carried towards their superior parts, where 

 the greatest number of the delicate filaments of the olfactory 

 nerve is distributed. 



31. It is vulgarly believed that the humors with which the 

 pituitary membrane is lubricate 1 come from the brain ; but this 

 is an error. They are secreted by this membrane itself, and the 

 slight diseases known under the name of cold in the head, rheum 

 oj the. head, are nothing else than inflammation of this membrane. 



Explanation of Fiy 27. The nasal >ssae open and seen from the inside. 

 c. the cranium sf the frontal .sinuses, c .vities hollowed out in the substance 

 of the frontal bo >e, and in communication witli the nasal fossae. no. the 

 olf.cory nerve r. its branches ramifying 1 on t'>e pituitary or schneirlerian 

 membrane. cs. cm. ci. superior, middle and inferior turbinatcd bones ; these 

 are projec in<r pl.tes of bone which ^erve to increase the extent of the surface 

 of the organ of smell ph. the swallow or pharynx t. the opening of the 

 Eustachian tube. 



28. Whit are the nasal fossae or nostrils ? What is the name of the lining 

 Membrane of the nose ? 



2!). What nerves are distributed to the pituitary membrane 1 



30. How is the sense of smells perceived 1 



31 What is the origin of the humors which cover the pituitary rnero 

 brane? What disease consists of an inflamtnlion of this membrane? 



