ODOROUS SUBSTANCES :—STRUCTURE OF NOSE. 399 
ems necessary that the odoriferous particles should come into 
actual contact with the membrane on which the nerve of smell 
is spread out. In this respect, the sense of Smell agrees with 
the senses of taste and touch ; whilst it differs from those of 
sight and hearing, which take cognisance of changes that are pro- 
duced by vibrations or undulations in the surrounding medium. 
It is, moreover, desirable that these odoriferous particles should 
be conveyed by the air, and not be diffused through fluid ; 
for though it is necessary to the perfection of the sense of 
smell that the olfactory membrane should be kept moist, too 
great a quantity of fluid upon its surface deadens its peculiar 
sensibility,—as we find to be the case when we are suffering 
under an ordinary “cold.” Hence it is only in air-breathing 
animals, that the sense of Smell can possess any considerable 
_ 506. The most advantageous position of this organ is evi- 
dently at the commencement of the respiratory passages ; so 
that the air which is being re- 
eeived into the lungs may pass 
through ‘it and be tested (as it 
were) by its peculiar sensibility. 
Tm all the air-breathing Verte- 
we find a pair of cavi- 
ties, the nasal fossee (fig. 200), 
which are situated between the 
th and the orbits. They 
two orifices,—the ante- 
m & i e é 
rior nares, or nostrils (5), usually 5 2 
opening upon the front of the Fig. 200.—Verticat SECTION OF THE 
face,—and the posterior nares, Nasat Cavity. 
which open into the upper part % mouth; 4, nostril; c, posterior open- 
of the pharynx (c). The two skull b e. forehead; J. possages be. 
ities are separated from each {Wee the spongy bones, 9, 4, B: 
e thee frontal sinus; m, sphenoidal sinus; 
other by a vertical partition, m,opening of Eustachian tube; 0, cur- 
ich passes backwards and ee 
by a delicate mucous membrane, whereon the Olfactory 
es, which enter through a multitude of minute orifices in 
