SENSE OF SMELL. 



409 



and its effects. Smells cannot be divided into classes, and, 

 setting aside the arbitrary and special appellation of agree- 

 able and disagreeable odors^ we can distinguish them only by 

 the names of the bodies to which they belong. 



The sense of smell resides in the nasal chambers (Fig. 107), 

 but only a small part of this cavity serves for the purpose, 

 the remainder either giving rise to the resonance of the voice 

 (especially the annexed cavities: maxillary, frontal sinus, etc.), 

 or preparing the air inhaled by imparting to it the degree 

 of heat and moisture necessary to the respiratory mucous 

 membrane, as we learned when studying that mucous tissue 

 (p. 286). These parts are formed by three turbinated bones, 

 one above the other, which enclose rather narrow passages 

 (Fig. 109), the whole being lined 

 with an extremely soft, thick, vas- 

 cular mucous^ containing rich ve- 

 nous plexus, and coated with a co- 

 lumnar epithelium having vibratile 

 cilia ; these latter are also found 

 in the remainder of the conducting 

 tube of the respiratory tree, of 

 which this part of the nasal cham- 

 bers forms the beginning. In this 

 mucous (Schneiderian membrane) 

 are found numerous glands, which 

 help to keep moist the surface 

 which would otherwise be dried by 

 the movement of the air. 



The sense of smell itself appears 

 to be intended as a guard to the purity of the air inhaled : 

 for most substances which are capable of vitiating the air, 

 have some odor, and are naturally under the control of the 

 sense of smell. 



The sense of smell resides only in the highest portion of 

 the nasal chambers, in those parts through which the olfac- 

 tory nerve, the nerve of special sensibility, extends, while the 

 lower parts receive only the branches of the trifacial nerve 

 or the nerves of general sensibility (see Cranial Nerves, pp. 

 35-38). In this region (called the olfactory region, of a 



* 1, Inferior turbinated bone. 2, Middle turbinated bone. 3, Superior tur- 

 binated bone. 



A, Thickness of the mucous membrane and soft parts (which are very vas- 

 cular) with which it is lined. B, Skeleton (bones or cartilages). C, Partition, 

 showing the same parts (mucous membrane and skeleton). 



Fig. 108. Diagram of transverse 

 section of the nasal chambers.* 



