Paranasal sinuses and nose. Coloured 3D computed tomography (CT) scan of the human nose and surrounding regions in oblique view, seen from above, showing the paranasal sinuses (upper frame) and the nasal cavity (lower centre). The paranasal sinuses are spaces in the bones surrounding the eyes and nose, and the cheek bones of the upper jaw. They lighten the bones of the skull and reduce the weight of the head. They are connected to the nasal passages and are lined with mucous membranes. They can fill with fluid and secrete mucus during an infection. The paired triangular structures are the maxillary sinuses.