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VERTEBRATE SKELETON 



frontal lobes and consequent overriding of nasal by cranial cavity, it 

 becomes nearly horizontal (elephant, Suids, several Edentates and 

 Primates). Its horizontal position in Echidna is due to the large 

 olfactory lobes. 



One centre of ossification is in the posterior part of the septum 

 and forms the mesethmoid (perpendicular plate of man). The 

 anterior end of the septum, extending to the tip of the snout, usually 

 persists as cartilage. In pigs and a few other forms a prenasal 

 (telethmoid) bone may form in the tip of the nose. There is another 

 centre in each lateral wall, from which ossification extends to the 

 lamina cribrosa, the separate bone thus formed on either side (ecteth- 

 moid) soon unites with the hinder part of the mesethmoid, a part of 



(A) (B) iC) 



Fig. 201. — Nasal region of mammals; .4, Nasal cavity of Erinaceus: B. section of 

 nasal cavity of new-born dog; C, Schema of mammalian turbinals (PauUi). c. ecto- 

 turbinals; /, lateral wall of capsule; ml, maxillo-turbinal; n, entoturbinals; yit, naso- 

 turbinals; s, septum; I-IV, entoturbinals; 1-5, entoturbinals; el, ethmoturbinal. 



which (crista galli) may project into the cranial cavity. With the 

 ossification of the cribrosa the primitive olfactory foramina become 

 divided into the several or many openings through which branches 

 of the first nerve pass. 



The lateral wall of each capsule extends ventrally to the level 

 of the base of the septum, with which it connects behind, forming the 

 terminal (transverse) lamella, the lower closure of the olfactory (not 

 respiratory) tract. The ventral wall of each capsule forms the 

 lamina papyracea, and its lower margin bends medially, becomes 

 variously complicated, and ossifies separately, forming the max- 

 illo-turbinal bone (lower concha, figure 201, A, B) which, with a later 

 reduction of the lower lateral wall, becomes attached to the maxilla. 

 Dorsally and farther back each lateral wall bears a series of out- 



