442 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



The nasal sacs, then, of different Vertebrates, differ 

 only in the extent of the complication of their internal 

 walls, and of the membrane which covers them. This 

 complication, is chiefly effected by the development of 



Fig. 189. Longitudinal Section through a Dog's Nose, showing the 



Spongy Bones. 



a, Region of the olfactory sense ; 6, air chamber (" sneezing region ") ; r, a bristle 

 passed through the nostril into the nasal chamber; d, a bristle from the 

 nasal chamber into the passage by which the latter communicates with the 

 mouth. (After T. J. Parker.) 



cartilages, which may become more or less completely 

 ossified, in the upper or olfactory region ; these arise 

 from the side walls of the cavity, and project into it ; 

 such bones are known as the ttirbinate Jboiies. 

 While Reptiles and Birds have one only, Mammals 

 have three (excepting the Cetacea, which have none) ; 

 these vary greatly in form and in the extent to which 

 they are developed, and, as they are covered by the 



