Chap, v.) THE EAR. 79 



still more readily evacuated by operation. Across the 

 roof of the antrum runs the petro-squatnous suture. 

 About the second year the mastoid process becomes 

 visible. As the bone increases its growth mainly 

 involves its external parts, so that as years pass on the 

 antrum becomes more and more deeply placed. In a 

 child aged nine this cell is about 1 cm. from the 

 surface (Symington). At this period no other air 

 cells exist, but at puberty an extensive series of such 

 cells develop. 



In the adult (to follow the account of Buck) these 

 mastoid cells extend upwards to within half an inch of 

 the temporo-parietul suture. In this situation they are 

 very close both to the pericranium and to the dura 

 mater. Anteriorly they extend forwards over the 

 external meatus. Posteriorly they cease abruptly at 

 the masto-occipital suture, although in rare cases they 

 are continued beyond that suture into the occipital 

 bone. Very close to the hinder cells is the lateral 

 sinus. 



About the centre of the mastoid spaces is a single 

 cavity, tJte antrum. It is about the size of a pea. It 

 opens in front into the tympanum, its floor occupying 

 a higher level than the floor of that cavity. Air cells 

 surround the antrum on all sides (except towards its 

 roof), and open into its cavity. The roof of the 

 untriun is only separated from the cranial cavity by a 

 thin layer of bone about 1 mm. in thickness. The 

 distance of the posterior end of the antrum from the 

 lateral sinus is from 3 to 6 mm., wbile its outer wall is 

 from i to ^ths of an inch from the external surface of 

 the mastoid process. Emissary veins pass from the 

 mastoid spaces to the surface of the bone, and it is no 

 doubt along these vessels that inflammat'.on often 

 spreads to the soft parts behind the ear. Since the 

 same veins join directly or indirectly the lateral sinus, 

 thrombosis of that sinus may readily follow mastoid 



