One of two cavities between the floor of the cranium and the roof of the mouth, opening to the nose anteriorly and the nasopharynx posteriorly. Its lining of highly vascularized, ciliated epithelium warms and moistens inhaled air, and traps dust and pathogens on mucus that is then swept toward the pharynx. The nasal septum (ethmoid and vomer) separates the nasal cavities.