400 
SENSE OP SMELL. 
the roof of the cavity, are distributed; and the extent of its 
surface is increased, by its being folded over certain projec¬ 
tions from the walls of the cavity, which are termed spongy 
bones. Of these there are three in Man ( g , i, ic). Prolonga¬ 
tions of this membrane are carried also into cavities hollowed 
out in the neighbouring bones, which are termed sinuses . 
Thus we have the frontal sinuses l, situated just above the 
nose, between the eyebrows ; and the sphenoidal sinuses m, 
situated further back. There is also a large cavity hollowed 
out in the bone of the upper jaw on either side. The mem¬ 
brane which lines these is kept moist by its own secretion; 
and it is covered with vibratile cilia, the office of which seems 
to be to prevent that secretion from unduly accumulating, by 
conveying it over the surface of the membrane to the outlet. 
507. The mechanism of the sense of Smell is very simple. 
When air charged with odoriferous particles passes over the 
membrane that lines the nose, some of these particles are 
delayed by the mucous secretion that covers it, and act upon 
the delicate sensory extremities of the olfactory nerve with 
which it is thickly set. The highest part of the nasal cavity 
appears to be that in which there is the most acute sensibility 
to odours ; and hence it is that when we snuff the air, so as 
to direct it into the upper part of the nose, instead of allowing 
it to pass simply along the lower portion from the anterior to 
the posterior nares, we perceive delicate odours which would 
have otherwise escaped us. The acuteness of the sense of 
smell depends, in no small degree, upon the extent of surface 
exposed by the membrane lining the nasal cavity; and in this 
respect Man is far surpassed by many of the lower Mammalia, 
especially among the Carnivora, Kuminantia, and some Pa- 
chydermata. The extreme delicacy of this sense in Deer, 
Antelopes, &c., is well known, from its being a source of great 
difficulty to the hunter, who cannot advance near enough to 
attack them, except by stealing upon them in the direction 
contrary to that of the wind. In these animals it serves as 
the chief means by which they are warned of the proximity 
of their enemies; in the Carnivora, on the other hand, it 
serves to direct them to their prey. In general, however, it 
seems to have for its office to assist in directing animals to 
their food, and in warning them of the presence of noxious 
vapours. 
