SKULL — MAMMALS 



191 



growths on the medial side, these reaching back to the cribrosa. 

 These ossify as ethmoturbinals, each supporting a part of the olfac- 

 tory membrane. These conchae or turbinals constitute the nasal 

 labyrinth, the fibres of the olfactory nerve being distributed to the 

 various folds. 



The ethmoturbinal plates are usually arranged in two series (fig. 201, C), 

 the ectotxirbinals which extend farthest towards the septum, and the shorter 

 entotiirbinals between the first, entoturbinals not developing in Ornithorhynchus 

 and being degenerate in Primates. The anterior ethmoturbinal sometimes 

 becomes attached to the nasal bone, and then is a nasoturbinal. In a few cases 

 {Echidna, Dasypodidae, Cholcepus) other turbinals occur on the perpendicular 

 plate. In general the labyrinth is reduced in vegetable feeders and in Cetacea 

 the olfactory nerve is lost, the cribrosa is imperforate and the labyrinth is gone. 



Each nasal cavity begins with a naris at or near the tip of the 

 head (whales excepted, fig. 225; the two nares are united in dol- 

 phins). Each naris leads to the nasal cavity, the length of which 

 depends on that of the snout. Each 

 contains the turbinals, largely in the 

 upper (olfactory) part of the chamber, 

 the lower part being the respiratory tract 



which is continued back of the nasal / >«J!lWil^lL >^>L,. 

 region as a naso-pharyngeal duct, ter- ^^^^^ 



minating in a choana at the hinder / P^^^w ^1^ 



border of vomer and bounded laterally | i, m pf 



by the descending pterygoid. 



In young stages the choanae are far for- 

 wards (fig. 202), about in the same position as 

 in primitive Sauropsida and Amphibia. Then 

 palatal processes of premaxilla, maxilla and 

 palatine extend to the middle line, forming 

 the hard palate which comes between the yig. 202.- — Roof of mouth of 

 primitive choanas and the mouth, so that the Echidna embryo (Seydel, '99) 

 former now open into the nasopharvngeal showing primitive choan^ ./.. 

 ^ f . e> g^j^i^ palatal folds which meet later 



ducts which are bounded below by the hard cutting off secondary nasal pas- 

 palate and open behind by the definitive sages from roof of mouth, et, egg- 



, „,..., , tooth: 7, opening of Jacobson's 



choanae. The primitive choanae may be organ; /./, palatal folds. 

 recognized at about the level of the junction 



of the palatal processes of maxillae and premaxillae. The nasal cavity also con- 

 nects with the mouth by one or a pair of foramina incisiva (fig. 203, Ji) in the 

 premaxillary region; these convey nasopalatine canals between the mouth and 

 the organ of Jacobson (vomeronasal organ), the canals being closed in bats. 



