The sphenoid sinuses are normally located behind the ethmoid, although in some cases the posterior ethmoid can wrap around the sphenoid. The sphenoid sinuses lie just below the brain and almost in the center of the head. The two sphenoid sinuses are separated by another bony wall or partition, called a septum (5). The sphenoid sinuses are barely visible at birth and begin to develop between the ages of 2 and 3 years. They continue to grow throughout childhood, and fully mature by the age of 18. At their largest, they are about the size of a marble.
The sphenoid sinuses have many important structures around them.
The carotid artery, which carries the blood supply to the brain, travels along each outside wall of the sphenoid sinus. In about 15 percent of the population, this wall is absent and the artery actually sits in the sphenoid sinus. This is known as a dehiscent carotid artery. The optic nerve, which carries visual signals from the eye to the brain, travels through the roof of the sphenoid sinus. Behind the sphenoid sinuses is the cavernous sinus, the vein that drains the blood away from the brain. The pituitary gland, which is responsible for many of your hormone levels, sits above the back part of the roof of the sphenoid sinuses.