A sinus is a hollow air pocket behind the face. There are different pairs of sinuses, including the frontal sinuses behind the forehead, the ethmoid sinuses behind the eyes, the sphenoid sinuses behind the ethmoid sinuses, and the maxillary sinuses behind the cheeks. The sinuses are part of the same system as the nose.
The air pocket in the skull has a lining of mucus-producing tissues. These tissues also contain tiny hairs called cilia, whose job is to propel the mucus out of the sinuses, through small holes called ostia, into the nose and throat. There, the mucus gets swallowed. When this functions properly, the sinuses remain free and clear, allowing one to breathe effectively. However, when a sinus or the nose becomes inflamed, it produces the condition called rhinosinusitis.
The sinuses form a beautiful design, and can be considered the butterfly of the face!