8o SURGICAL APPLIED ANATOMY. [Chap. V. 



inflammation. Most of the mastoid cells are air 

 spaces, and the majority open into the antrum. Some 

 few contain diploe. The cells vary greatly in size and 

 extent in different individuals. The facial nerve 

 passes close to the mouth of the antrum, and may 

 readily be involved in mastoid disease. Mastoid in- 

 flammation may not only lead to thrombosis of the 

 lateral sinus, but it may readily extend (through the 

 antrum roof) to the membranes of the brain, and pro- 

 duce meningitis. It may extend farther and cause 

 abscess in the neighbouring part of the brain. Such 

 abscesses are usually in the temporo-sphenoidal lobe of 

 the cerebrum, and next in frequency in the cerebellum. 

 (See page 10.) In either case the abscess would appear 

 to be due to direct extension of inflammation from 

 the bone. 



In perforating the antrum to evacuate pus care 

 must be taken not to wound the posterior auricular 

 artery. The drill is entered at a point about ^ of an inch 

 behind the orifice of the meatus and a little below the 

 level of its upper wall. The instrument is thrust into 

 the bone in a direction nearly parallel with the 

 auditory canal, i.e. inwards and a little forwards and 

 upwards. The antrum should be reached at a depth 

 not exceeding -|ths of an inch (Buck). 



In cases where the outer surface of the mastoid 

 has been spontaneously perforated, a tumour has 

 appeared on the skull that contained air, and that 

 could be increased in size by forcing air into the ear 

 through the Eustachian tube. Such tumours are 

 known as pneumatoceles, and the process that leads 

 originally to the perforation of the bone is of obscure 

 nature. In some cases it seems to have been simply 

 atrophic, and in other instances to have been due to 

 " caries sicca." 



On the anterior wall of the tympanum is the open- 

 ing of the Liu star Iii a ii tube. This tube is 1] inches 



