OLFACTORY ORGANS 



309 



side of the nasal cavity; a little higher it is ventral, while in the high- 

 est urodeles it has rotated to the lateral side. It may be noted that 

 some of the amphibia have smooth muscles to close the external nares. 

 Aside from the varying position of the choanae the changes from 

 amphibia to reptiles in the olfactory organs are comparatively sUght. 



Fig. 221. — Longitudinal section of nasal region of alligator, after Gegenbaur. c, 

 concha; ms, maxillary sinus; n, naris; />, pseudoconcha. 



The olfactory region becomes more distinct from the respiratory tract 

 and the latter shows a tendency to be differentiated into an anterior 

 atrixim or vestibule, a middle area connected with the olfactory region, 

 and a posterior naso-pharyngeal duct between the basis cranii and 

 the roof of the mouth. This latter duct varies in length accordingly 



Fig. 222. — Section through nasal region of Lacerta, after Schimkewitsch. Bone 

 black, cartilage dotted, ds, dental shelf; j, Jacobson's glands; j, Jacobson's organ; n, 

 main cavity of nose; oj, opening of Jacobson's organ; t, tooth germ. 



as the choanae are anterior or posterior in position, the extreme being 

 reached in the crocodiles, where by ingrowth of palatines and ptery- 

 goids, the internal nares are carried back nearly to the hinder end .of 

 the skull. A single concha, supported by bone, is developed in the 

 lateral wall of the reptilian nose. It is weak in the turtles (fig. 220), 

 but is larger elsewhere, and in the crocodiles (fig. 221) it becomes 

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