Maxillary sinus membrane and dental roots. Endoscope view of the mucous membrane from the lower part of the maxillary sinus, with protrusion of dental roots. The sinuses are the spaces in the facial bones of the skull, and the maxillary sinuses are found in the upper jaw bones (maxillae). Richly vascular mucous membrane lines the sinuses (blood vessels can be seen here). The back teeth are very close to the bottom of the maxillary sinus, and the roots sometimes protrude upwards into the floor of the sinus space, as seen here (white areas).