216 THE HEAD AND NECK 



downwards behind it on to the mastoid process. In the infant, 

 prior to the development of the mastoid process, the facial 

 nerve runs serious risks of injury if the incision is carried too 

 far forwards, because its point of exit from the skull, the stylo- 

 mastoid foramen, is situated on the infero-lateral aspect (p. 210). 

 The incision is carried down to the bone, and the periosteum and 

 soft structures of the flap are dissected forwards until the 

 osseous part of the external acoustic meatus is visible, and the 

 suprameatal spine and the posterior root of -the zygomatic pro- 

 cess of the temporal bone are recognised. If the posterior edge 

 of the incision is elevated, an emissary vein is exposed emerging 

 from the mastoid foramen, through which it communicates with 

 the transverse sinus. When it is found thrombosed, the sinus 

 will be similarly affected. Pus may sometimes be seen emerging 

 from the mastoid foramen, and its presence indicates that pus 

 will be found in the transverse sinus and is suggestive of a sub- 

 dural abscess. 



In chiselling through the bone to reach the antrum, care must 

 be taken not to do so too high, lest the middle cranial fossa be 

 opened, or too far posteriorly, lest the transverse (lateral) sinus 

 be injured. The site selected lies below the posterior root of 

 the zygomatic process and immediately behind the supra- 

 meatal spine. In the child, however, these bony prominences 

 are not developed, and the bone lying just behind the postero- 

 superior quadrant of the external acoustic meatus is the best 

 guide (Stiles). The chisel is directed medially, downwards and 

 forwards, i.e. parallel to the external acoustic meatus. It is 

 advisable to bevel down the edges as the opening is deepened, 

 and if the transverse sinus lies further forwards than normally, 

 its bluish wall will be observed and can be protected. As a rule 

 the sinus lies half an inch behind the external acoustic meatus, 

 but its position is subject to variation (p. 225). 



As soon as a cavity is reached, a probe is inserted and directed 

 forwards in search of the aditus. The sensation produced as 

 the probe slips into the passage is unmistakable, and if it is not 

 experienced, the cavity, which is only an air-cell, is deepened 

 until the antrum is opened. It may be necessary to remove 

 the lateral wall of the aditus in order to throw the two cavities 

 tympanic cavity and antrum into one. Before this is carried 

 out, a Stacke's protector is passed into the aditus from the 

 antrum, to preserve the facial nerve and the lateral semicircular 

 canal (p. 213) from injury should the chisel accidentally slip 



