464 CLINICAL APPLIED ANATOMY. 



only be reached with safety by an intimate knowledge of the 

 anatomy of the part. The supra-meatal triangle is perhaps the 

 best guide in evacuating it. This little triangle is formed by the 

 horizontal part of the posterior portion of the temporal ridge 

 and the posterior curving margin of the auditory process, upon 

 which there is frequently a little prominence of bone called the 

 supra-meatal spine. Important structures lie in relationship 

 with this small cavity : above is the dura mater, only separated 

 by the thin tegmen antri, anteriorly is the facial nerve, inter- 

 nally the external semi-circular canal, and posteriorly the lateral 

 sinus. The cranial cavity can be avoided by directing the chisel 

 or drill horizontally inwards, the level of the floor of the cranial 

 cavity being slightly above the temporal ridge. The facial nerve 

 will be free from danger except when the chisel is directed 

 forwards, particularly when the opening between the antrum 

 and the tympanum is being enlarged. The external semi- 

 circular canal will not be injured unless the inner wall of the 

 antrum, which consists of a fairly thick layer of white compact 

 bone, is perforated. The lateral sinus will be in danger, more 

 particularly when the mastoid cells are being attacked, after the 

 antrum itself has been exposed. 



