The mastoid bone is the lower part of the temporal bone of the skull. Jutting from the lower part of the mastoid bone is a bony projection called the mastoid process.

 

The mastoid bone is honeycombed with air cells, which are connected to a large cavity called the mastoid antrum in the upper part of the mastoid. The mastoid antrum leads into the middle ear. As a result, bacterial infections of the middle ear occasionally spread through the mastoid bone to cause acute mastoiditis. Usually the infection responds to antibiotics but in severe cases surgery (notably mastoidectomy, involving removal of some or all of the air cells in the mastoid process) may be required.