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Before 5 Years. The sphenoid sinuses begins to form at age 2, and are air-filled and visible on X-rays by age 5. The sphenoid sinuses sit behind the eyes and are located deeper than the other sinuses, behind the ethmoid sinuses. The right and left sphenoid are next to each other, only separated by the septum, a thin plate of bone.
This development places the floor of the sinus well below the floor of the nasal cavity. The shape of the sinus is a pyramid, with the base along the nasal wall and the apex pointing laterally toward the zygoma. The natural ostium of the maxillary sinus is located in the superior portion of the medial wall.