Maxillary sinus membrane. Light micrograph of a section through a mucous membrane from one of the two maxillary sinuses. The sinuses are the spaces in the facial bones of the skull, and the maxillary sinuses are found in the upper jaw bones (maxillae). The mucous membrane (mucosa) comprises columnar epithelial (surface) cells (upper frame) that include many goblet cells that secrete mucus. This outer layer is supported by a richly vascular layer (containing many blood vessels) called the lamina propria (across bottom). The mucosa serves to trap particles in inspired air, and also helps to humidify and heat inspired air.