n :  the vaulted chamber that lies between the floor of the cranium and the roof of the mouth of higher vertebrates extending from the external nares to the pharynx, being enclosed by bone or cartilage and usu. incompletely divided into lateral halves by the septum of the nose, and having its walls lined with mucous membrane that is rich in venous plexuses and ciliated in the lower part which forms the beginning of the respiratory passage and warms and filters the inhaled air and that is modified as sensory epithelium in the upper olfactory part .
 
 
 
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