NERVOUS SYSTEM 871 



entiated anterior region known as the atrium or vestibule, a middle 

 region connected with the original region, and a posterior region called 

 the naso-pharyngeal duct. The naso-pharyngeal duct may vary in length 

 according to whether the choanae are anterior or posterior in position. 

 The crocodiles show an extreme elongation due to the palatines and 

 pterygoids growing inwardly, which causes the internal nares to be 

 pushed to the hinder end of the skull. There is a single concha, which 

 is supported by bone in the lateral wall of the nose of reptiles. This is 

 rather weak, however, in turtles and in the crocodiles. It is divided in 

 front, while a "pseudobranch" develops above and behind the true 

 concha. It is only in the snakes and lizards that Jacobson's organ occurs 

 (Fig. 339), and that only as a simple pocket ventral and medial to the 

 nasal cavity near the nasal septum. 



In birds there is an anterior and inferior concha vestibuli, and also 

 a middle and a superior fold, formed by three folds on the wall of each 

 nasal cavity. There is no olfactory epithelium formed on the vestibular 

 conch; it is present on the middle conch immediately before hatching, 

 and disappears immediately after hatching, so that the upper conch is 

 the sole seat of smell in the adult. Jacobson's organ is only found in 

 embryos. 



In mammals much greater complication sets in. The naso-pharyn- 

 geal duct becomes elongated, while the olfactory area lies directly below 

 a portion of the brain cavity. The interior arrangement of the bones 

 formed within the nose forms what is called the nasal labyrinth. The 

 ethmo-turbinals, the naso-turbinals, and the maxillo-turbinals are the 

 supporting bones or cartilage of the folds of the labyrinth. This arrange- 

 ment permits a great expansion of sensory surface, while the supporting 

 structures keep the folds from touching each other. 



In those animals whose sense of smell is very low, not only the folds, 

 but the bones themselves, may be likewise reduced. 



The maxillo-turbinals and the naso-turbinals arise from the lateral 

 wall of the nasal cavity. The ethmo-turbinals are outgrowths fronr the 

 ethmoid bone, growing out from the upper hinder part of the septum 

 and extending to the lateral wall. This causes the ethmo-turbinals to 

 insinuate themselves between the hinder ends of the other two. Any 

 of these turbinals may divide in turn. The subdivision of the ethmo- 

 turbinals may be of varying heights so that to form the ecto and ento- 

 turbinals the naso-turbinals may disappear in the adult. The epithelium 

 of the maxillo-turbinals is not sensory, so that it is assumed that this 

 portion of epithelium serves only to warm and moisten the air in its 

 passage to the lungs. The various forms which the ethmo-turbinals 

 assume in mammals may be seen from Figure 488. Peter gives the fol- 

 lowing table of homologies of the nasal labyrinth in the amniotes : 

 I. Concha of the anterior epithelium: concha vestibuli (birds). 



