ODOEOUS SUBSTANCES : 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 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 

 acuteness. 



506. The most advantageous position of this organ is evi- 

 dently at the commencement of the respiratory passages j so 

 that the air which is being re- 

 ceived into the lungs may pass 

 through it and be tested (as it 

 were) by its peculiar sensibility. 

 In all the air-breathing Verte- 

 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 (6), 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 OP THE 

 NASAL CAVITY. 



a, mouth; b, nostril; c, posterior open- 

 ing ; d, portion of the base of the 

 skull ; e, forehead ; /, h, passages be- 

 tween the spongy bones, g, i, k; I, 

 frontal sinus; m," sphenoidal sinus; 

 w.opening of Eustachian tube ; o, cur- 

 tain of the palate. 



