ODOROUS SUBSTANCES I—STRUCTURE OF NOSE. 399 
seems 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 df 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 
acuteness. 
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- 
. . ° vn. k l e l 
ceived into the lungs may pass 
through it and be tested (as it 
were) by its peculiar sensibility. 
In all the air-breathing Verle- 
brata we find a pair of cavi¬ 
ties, the nasal fossae (fig. 200), 
which are situated between the 
mouth and the orbits. They 
possess two orifices,—the ante¬ 
rior nares , or nostrils (b), usually 
opening upon the front of the 
face,—and the posterior nares , 
which open into the upper part 
of the pharynx (c). The two 
cavities are separated from each 
other by a vertical partition, 
which passes backwards and 
forwards on the middle line ; their sides are formed by the 
various bones of the face, and by the cartilages of the nose ; 
their extent is very considerable, especially in animals that 
have a prolonged muzzle. The interior of these cavities is 
lined by a delicate mucous membrane, whereon the Olfactory 
nerves, which enter through a multitude of minute orifices in 
Fig. 200.— Vertical Section of the 
Nasal Cavity. 
a, mouth; &, nostril; c, posterior open¬ 
ing ; d, portion of the base of the 
skull; e, forehead; /, h, passages be¬ 
tween the spongy bones, g , i, Jc; l, 
frontal sinus ; m, sphenoidal sinus; 
n ,opening of Eustachian tube; o, cur¬ 
tain of the palate. 
