If the abscess arises from middle-ear disease, it is customary to first open the mastoid antrum (see chapter on ear) and then by removing the bone above to explore the surface of the petrous portion of the temporal bone. To explore the temporal. lobe an opening may be made 2.5 cm. (1 in.) above the external auditory meatus and a needle passed inward, forward, and a little downward.

To reach the cerebellum, the trephine should be applied 5 to 7 cm. (2 to 2 3/4 in.) behind the external meatus and well below the superior curved line. The bone at this point is apt to be thin and care is to be exercised not to wound the membranes. The place of trephining in abscesses from other causes is to be decided by the localizing symptoms.