Sinuses are hollow, air-filled cavities that are located throughout the skull. There are several sets of sinuses in the head. The maxillary sinuses are behind the cheeks. The ethmoid sinuses lie under the inside corners of the eyes. The sphenoid sinuses are located behind the ethmoid sinuses.
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They are the most superior of the paranasal sinuses, and are triangular in shape. Drainage is via the frontonasal duct. It opens out at the hiatus semilunaris, within the middle meatus of the nasal cavity.