ANATOMY AND PHYSIOLOGY 11 lobe, the olfactory tract, the optic chiasma and the pituitary gland lying in the hypophyseal fossa; the floor adjoins the pterygoid canal; while the medial wall is a septum separating it from its neighbour. The ostium is placed high up in the cavity of the sinus. Physiology of the Sinuses. The sinuses are lined by mucous membrane continuous with that of the nasal cavity through the ostia, but the sinus Fig. 12. Coronal section through the nasal cavities and maxillary sinuses on the plane of the ostia of the sinuses (viewed from behind). 1, Left frontal sinus; 2, Left anterior ethmoidal cell; 3, Olfactory sulcus; 4, Right anterior ethmoidal cells; 5, Right frontal sinus; 6, Ethmoidal bulla; 7, Hiatus semilunaris; 8, Infundibulum or semilunar groove; 9, Uncinate process; 10, Middle conchal cell; 11, Middle meatus; 12, Inferior concha; 13, Maxillary sinus; 14, Inferior meatus. mucosa is thinner. The lining epithelium consists of a low stratified columnar type containing cilia which move the mucus secreted from the subepithelial glands towards the ostium. The function of the sinuses is imperfectly understood although many theories have been proposed, none of which withstand critical scrutiny. It has been postulated that they assist in humidification of inspired air, that they aid vocal resonance, that they act as insulators to protect the base of the brain from cold air, and that they serve to lighten the bones of the skull.