The paranasal sinuses are a collection of hollow cavities connected to the nasal cavity by a small canal. They are not visible from the surface of the skull, but are hollowed-out regions encased in bone and lined with a mucous membrane.

Five skull bones contain sinuses:

  • The frontal bone contains the frontal sinuses
  • The ethmoid bone contains the ethmoidal sinuses, also known as ethmoidal cells
  • The sphenoid bone contains the sphenoidal sinuses
  • The right and left maxillary bones contain a maxillary sinus

The primary functions of the paranasal sinuses include:

  • Reduce the weight of the skull
  • Effect the sound of the voice by acting as resonating chambers

Figure 1: An anterior view and lateral cross-section showing the paranasal sinuses.