84 



THE EAK. 



membrane is not quite circular but is prolonged superiorly and posteriorly into a 

 somewhat triangular extension, which lies parallel and close to the lamina spiralis 

 cochlese, and forms an angle with the principal part of the membrane ; the latter 

 looks somewhat backwards and downwards, as well as outwards. 



Another fossa is seen on the inner wall, behind the promontory and between the 

 fossula ovalis and fossula rotunda, from each of which it is separated by a bony 

 prominence, while behind it is the base of the pyramid (see below). This fossa, 

 which has been named the sinus tympiuii, is about 4 mm. in diameter and 3 mm. 



<mtrti.m niastoidcum 



surface for attachment 

 a f short process of incus 



cvchleariform process for 

 tendon of tensor tympani 



canal for tensor tynipctni 



sums posterior 



carotid canal 



grooi-e for Eusta- 

 chian tu'jc 



canal for 



tympanic 



nerve 



p rot ub. 

 stt/loidca 



Fig. 93 A. SECTION OF THE RIGHT PETROUS BONE PASSING THROUGH THE MIDDLE OF THE TYMPANIC 



CAVITY SO AS TO EXPOSE THE INNER WALL OF THAT CAVITY AND TO EXHIBIT ITS CONNEXION 



WITH NEIGHBOURING SPACES WITHIN THE BONE. (E. A. S.) About twice the natural size. 



tiyep. The ampulla of the posterior semicircular canal lies close to its floor, and it 

 is marked by one or two small apertures for vessels (Stein briigge). 



The posterior wall of the tympanum has at its upper part a large opening 

 which leads into a considerable air-space (antrum mastoideum, figs. 93, A and B) 

 behind and above the proper tympanic cavity. From this antrum numerous irregular 

 cavities, the mastoid cells, pass into the substance of the mastoid portion of the 

 temporal bone. These cells communicate for the most part freely with one another, 

 and are lined by a thin mucous membrane continuous with that of the tympanum. 

 In the foetus and new-born child the mastoid cells are not developed (corresponding 

 with the slight development of the mastoid process), but the antrum mastoideum is 

 formed at birth. The petroso-squamosal fissure passes through the antrum. The 



