ORGANS OF PERCEPTION. 907 
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 mawillury, from the bones in which they 
are imbedded. They are covered with a continuation of 
the olfactory membrane. It is thinner im 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, itis 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 
