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The sphenoidal sinus is a small cranial cavity that, in adults, is approximately equivalent in size to a large grape. There is one sphenoidal sinus on either side of the nose. The purpose of the sphenoidal sinus is to drain mucus from the back of the nose through an opening called the ostium.
Behind and bedside the eyes lies the sphenoid bone. It's an unpaired cranial bone, meaning there is only one. It assists in connecting the bones of the skull to the bones of the face. The sphenoid bone has three pairs of bony projections and a hollow space, which houses the sphenoidal sinus.
The sphenoidal sinus is one of four pairs of paranasal sinus cavities, which are hollow air-filled spaces that surround the nasal cavity. Above the eyes, in the lower forehead region, sit the frontal sinuses. Below the eyes, on both sides of the nose, sit the maxillary sinus cavities.
Between the eyes, on either side of the nose, sit the ethmoidal sinuses. Behind the ethmoidal sinuses sit the sphenoidal sinuses. Each sinus pair is named after the cranial bones that sit closest to them. The sphenoidal sinus sits in the middle of the bottom of the skull underneath the pituitary gland. Surgeons who need to access the pituitary gland will often go through sphenoidal sinus because of its proximity to the gland, and because the bony wall separating the sinus from the nasal cavity is very thin and easy to puncture.
In most people, the sphenoidal sinuses are asymmetrical, meaning irregular, because the length, thickness, and shape of the septum, or bony wall, separating them is highly variable. Infections of the sphenoidal sinuses are not common. When infection does occur, it may present without fever, runny nose, or general sinus congestion. The most common symptom is a persistent bad headache that worsens with changes in posture like bending or stooping. A headache caused by a sphenoidal sinus infection typically occurs behind the eyes and across the forehead.
All of the paranasal sinuses are covered, on their inner walls, in a tissue called the epithelium. This epithelial lining secretes mucus, which keeps the sinus cavities moist. Every cell on the surface of the epithelium has a structure called the cilium, which can push sinus mucus across the walls.
The movement of sinus mucus is called mucociliary clearance. It's an active process that moves mucus in a specific pattern. Gravity or the position of the head has no effect on the mucociliary process.