400 SENSE OF 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 (<jr, i, k). 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 I, 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, Euminantia, 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 j 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. 



