ORGANS OF PERCEPTION. 207 



septum and windings of the ethmoidal bone. Besides the 

 olfactory nerve, which is considered as essential to the sen- 

 sation of smell, the nose is likewise supplied with a division 

 of the ophthalmic branch of the fifth pair, which is princi- 

 pally distributed on the inferior part of the membrane, and 

 termed the nasal nerve. 



Connected with the cavities of the nostrils which we 

 have already mentioned, there are numerous cells termed 

 Sinuses, in some of the adjoining bones, which communi- 

 cate by contracted apertures. They are termed frontal, 

 sphenoidal, and maxillary, from the bones in which they 

 are imbedded. They are covered with a continuation of 

 the olfactory membrane. It is thinner in these cells than 

 in the cavity of the nose, is not supplied with any branch 

 of the olfactory nerve, although filaments of the nasal nerve 

 may be traced into it. 



In Fishes, the nose does not communicate with the mouth. 

 The nasal cavity on each side is simple, the olfactory mem- 

 brane is expanded on the walls, and kept moist by a secret- 

 ed mucus. One or two openings externally lead to this ca- 

 vity. 



2. Functions of the organ of Smell. In order to com- 

 prehend the functions of the nostrils, it is necessary to make 

 a few remarks on Odours. 



Many bodies allow excessively small particles to escape 

 from their surface, and diffuse themselves through the at- 

 mosphere. These particles, which are termed Odours, are 

 always emanating from certain bodies, producing a rapid 

 decrease of weight in some, as ether, while in others, as 

 musk, no sensible diminution takes place. Such particles 

 are not given off by some bodies when in a particular state 

 of combination, as ammonia, when united with the stronger 

 acids, while other bodies require to be combined, previous 



