FORAMINA AND PROTUBERANCES OF THE TYMPANUM. 459 



continues so to the end of the petrous bone ; this contains the 

 nternal muscle of the malleus. 



The Mastoid Cells. 



In the upper and back part of the cavity of the tympanum, 

 nearly opposite to the orifice of the Eustachian tube, is an 

 opening which communicates with the cells of the mastoid pro- 

 cess. These cells do not appear different from those in other 

 bones, and are lined with a membrane apparently similar. 

 Their size is proportioned to that of the mastoid process, and 

 consequently they do not exist in the fcEtus. 



— The mastoid cells or sinuses, communicate freely with one 

 another, and with the cavity of the tympanum, at the posterior 

 superior part of the latter. The opening into the cavity of the 

 tympanum is irregular and lodges the short leg of the incus. 

 The mastoid cells, are analogous in structure to the frontal and 

 sphenoidal sinuses, which differ entirely from the common 

 cellular tissue or diploic cavities of bones. The diplce is 

 placed between the two tables of the bones, whilst the sinuses 

 have two tables on their exterior, forming vacuities with the 

 usual compact walls and diploic structure on either side. — 



Foramina and Protuberances of the Tympanum. 



That part of the surface of the cavity of the tympanum 

 which is opposite to the membrana tympani, is very irregular, 

 but it contributes to the formation of several parts which are 

 very important in the structure of the ear. 



When the lining membrane is removed, the aperture called 

 Foramen Ovale (^fenestra ovalis,) appears in a conspicuous, 

 situation, rather above the middle of this surface. It would 

 open directly into the Vestibule, or middle chamber of the 

 Labyrinth ; but it is closed by the base of the stapes. 



Above the foramen ovale is a prominence of the surface, in 

 which passes the canal for the Fortio Dura of the seventh pair 

 of nerves. 



