THIO TYMPANUM. 



85 



masfcoid cells are very variable in development : when large they approach close to 

 the free surface of the bone, and may come into proximity with the lateral sinus, 

 when small there is a considerable thickness of bone between them and the surface. 

 Sometimes they are undeveloped, and their place is occupied by spongy bony 

 substance (diploe). At the lower margin of the orifice of the antrum is a depressed 

 surface for the attachment of the lower ligament of the incus. 



-20 



- 



V 



Fig. 93 B. PART OF THE SECTION SHOWN IN FIG. 93 A, MORE MAGNIFIED, SHOWING THE VARIOUS 



NAMED PARTS IN AND CONNECTED WITH THE INNER WALL OF THE TYMPANUM. (E. A. S. ) 



1, tympanic crest ; 2, entrance to raastoicl cells ; 3, mastoicl antrum ; 4, prominence due to external 

 semicircular canal ; 5, prominence over aqueduct of Fallopius (a bristle, 13, 13, has been passed through 

 this canal) ; 6, fenestra ovalis ; 7, pyramid ; 8, posterior sinus ; 9, sinus tympani ; 10, styloid protu- 

 berance ; ll,subiculum promontorii ; 12, fenestra rotunda ; 14, bristle gassed through the canal for the 

 tympanic nerve ; 15, jugular fossa ; 16, tympanic cells ; 17, canal for Eustachian tube ; 18, promontory ; 

 19, 20, 21, grooves for nerves on promontory emerging from the end of the canal for the tympanic 

 nerve, 19 being for the carotico-tympanic, 20 for the small deep petrosal, and 21 for the small super- 

 ficial petrosal ; 22, trochlear end of the canal for the tensor tympani muscle ; 23, bridge of bone uniting 

 the promontory with the pyramid. 



Below the orifice of the antrum and behind the fenestra ovalis is a small 

 conical eminence (1 m. to r~> m. high), called the pyramid, or eminentui papillaris 

 (fig. 90, pij ; fig. 93, 7). Its apex is pierced by a foramen, through which the 

 tendon of the stapedius muscle emerges. The muscle is contained within a canal 



