204 THE HUMAN BODY. 



plate (cribrum, a sieve, being perforated with numerous holes) 

 of the ethmoid bone. They open posteriorly just above the 

 throat. A partition formed by the perpendicular plate of the 

 ethmoid, the vomer (a ploughshare), and a cartilage separates 

 the nasal fossae on the median line ; the prolongation of this 

 cartilage separates the nasal cavity into two parts, as we have 

 already seen. On the external walls of the nasal fossae there 

 are bony folds, which are called upper, middle, and lower 

 spongy bones, and are separated from each other by corre- 

 sponding passages. The nasal fossae communicate with 

 numerous sinuses in the substance of the bones of the face 

 and skull. 



The whole internal surface of the olfactory apparatus is 

 lined with a mucous membrane called the pituitary mem- 

 brane, this is the immediate organ of smell. This membrane 

 dips into the numerous inequalities of the spongy bones and 

 the passages, thus presenting a larger surface to olfactory 

 impressions. The olfactory nerve is ramified in the pituitary 

 membrane. It penetrates the nasal fossae through the cribri- 

 form plate of the ethmoid, but is distributed over the upper 

 portion only. In the lower portion of the fossae the pituitary 

 membrane receives only nervous filaments from the fifth 

 pair, a circumstance to be noted in reference to the mechan- 

 ism and seat of smell. 



Odours. The philosopher calculates the velocity and in- 

 tensity of light, he can analyze it, he knows from what sub- 

 stance a given colour emanates, and if this substance exists 

 in the star, the rays of which he is observing; he demon- 

 strates in the vibrations of bodies the origin of the sonorous 

 waves, and sees in light as in sound, not particles of matter 

 traversing space, but a movement excited in the surrounding 

 media. Some learned men have thought that odours also 

 result from a vibratory movement transmitted to the ambient 

 air by the molecules of odoriferous substances, but Fourcroy 

 demonstrated the origin of odoriferous emanations in the 

 volatility of the immediate materials of vegetables; and 

 odours are now generally considered as bodies existing by 

 themselves, and not as a purely physical result comparable 

 to sonorous or luminous waves; they are extremely minute 



