OLFACTORY ORGANS. 



195 



There are three folds developed on the wall of each nasal cavity in 

 birds, an anterior and inferior concha vestibuli, a middle and a superior 

 fold, the middle supported by the maxillo-turbinal, the superior by the 

 naso-turbinal bones. The vestibular conch lacks olfactory epithelium 

 at all times, while it disappears from the middle one after hatching, 



FIG. 199. Olfactory region of hen in longitudinal and transverse section, after Gegen- 

 baur. c, middle concha; ch, choana; i, inferior (anterior) concha; 0, connection of air 

 cavity of head; p, septum of nose; s, superior concha. 



leaving the upper conch as the sole seat of smell hi the adult, which 

 corresponds with the limited sense of smell in these animals. Jacob- 

 son's organ is never developed in the adult, though traces of it appear 

 in the embryos. 



With the great increase of the sense of smell in the mammals the 



J 



FIG. 200. FIG. 201. 



FIG. 200. Model of the nasal cavity of a rabbit embryo, 13^ mm. head length, after 

 Peter, ch, choana; et, first ethmoturbinal; ;, organ of Jacobson; oj, opening of same; mt, 

 maxilloturbinal; nt. nasoturbinal. 



FIG. 201. Nasal cavity of Erinaceus, after Paulli, showing the foldings of the maxillo- 

 turbinals (mt} and the nasoturbinals (nf). 



nasal labyrinth undergoes a corresponding complication, and is farther 

 characterized by the great length of the naso-pharyngeal duct, and by 

 the position of the olfactory area below a part of the brain cavity. The 

 folds of the labyrinth may be supported by processes, more or less com- 

 plicated, of three bones or cartilages, the ethmo-turbinals, the naso- 



