SENSE OF SMELL. 843 



olfactory cells form communication with this net-work, but, also, processes 

 may be traced into it from the so-called cylindrical cells, and, this being 

 the fact, it would seem unwarrantable now to draw a sharp distinction 

 between the functions of these two classes of cells (Figs. 363 and 364). 



In the herbivora the convolutions of the inferior turbinated plate 

 are, as a rule, simpler than in the carnivora, but yet more complex than 

 in man. In the ruminants and solipedes the inferior turbinated bone 

 divides into two plates which are rolled around each other in opposite 

 directions. In most of the carnivora it is also similarly convoluted, but 

 the divisions are much more frequent and the spaces between the 

 different leaves narrow. In the case of the inferior turbinated bone the 

 higher degree of complication does not point, as in the case of the 

 superior turbinated bones, to a higher degree of development of the 

 sense of smell, but in animals where this condition is present it is to be 

 regarded as a means of mechanical protection against the entrance of 

 foreign bodies into the nose. 



The different cavities in connection with the nasal chambers, such 

 as the frontal, sphenoidal, and maxillary sinuses, have no connection 

 with the sense of smell, but are to be regarded simply as extensions of 

 the respiratory parts of the nasal chambers; for their mucous membrane 

 is identical with that lining this portion of the nose and contains no 

 terminal filaments of the olfactory nerve. This also applies to the case 

 of the ethmoidal cells ; all these cavities, therefore, are simply concerned 

 in warming the inspired air. 



Jacobson's organ, on the other hand, is to be regarded as an acces- 

 sory organ of olfaction. It is present in all mammals, and consists of 

 two narrow tubes protected by cartilage and placed in the lower and 

 anterior part of the nasal septum. Each tube is closed behind, but an- 

 terior^ opens into the nasal chamber by a furrow, the naso-palatine 

 canal. The wall next the middle line is connected with the olfactory 

 epithelium, which is in direct communication with the terminal filaments 

 of the olfactory nerve. The outer wall is covered with columnar, ciliated 

 epithelium. 



The nose in mammals is generally but slightly detached from the 

 bones of the face. In solipedes and ruminants the nares, which are pos- 

 sessed of a considerable degree of mobility, project but slightly, while in 

 various members of the hog tribe the nose is prolonged anteriorly, 

 forming the snout or muzzle. In the elephant and in the tapir this pro- 

 longation acquires its maximum development. In the cetaceans and 

 other aquatic mammals in which the olfactory nerve is absent, as in 

 the dolphin, or where it is only faintly developed, the nasal chambers 

 lose all significance as organs of olfaction and simply fulfill a respiratory 

 function. 



