Step 4. Identify the antrum

4.1 Continue removing cells evenly in this area up to the limits defined by the facial nerve and middle fossa inscriptions. Concentrate on defining the cortical bone over the sigmoid sinus with sweeps that pass from the middle fossa to the tip of the mastoid process.

4.2 Continue in this way into the depth of bone while defining the feature of sinodural angle. If there is a Korner septum the lateral surface of the lateral sinus will appear to form a barrier nearly as far as the ear canal. Remove cells over this plane till a gap is found near the ear canal or floor of the middle fossa. Remove the septum when the areas of cortical bone over the middle fossa and lining the mastoid process have been defined.