1496 HUMAN ANATOMY.. 



ridge indicates the position of the facial canal or aquedudus Fallopii. This 

 ridge is bordered posteriorly and superiorly by the elevation which corresponds to 

 the wall of the horizontal semicircular canal (prominentia canalis semicircularis later- 

 alis). The sinus tympani, a well-marked depression, is behind the promontory, 

 between the niche of the round window and the pyramid, below and behind the 

 oval window. It is separated from the fossulae of the two windows by bony ridges. 

 It varies in depth from 2-5 mm. , with a vertical diameter of from 2-6 mm. 



The superior wall (paries tegmentalis) is formed by a plate of bone, the teg- 

 men tympani, which forms part of the upper and anterior surface of the petrous 

 portion of the temporal bone. Posteriorly it forms the roof of the antrum tympani- 

 cum, and anteriorly contributes the roof of the canal for the tensor tympani muscle 

 and of the adjoining part of the Eustachian tube. It varies in thickness and may be 

 defective to a large extent from atrophy or arrested development. 



The inferior wall (paries jugularis), narrower than the superior, separates the 

 typanum from the jugular fossa. Its bony plate may be incomplete and may lie 

 considerably below the le\'el of the membrana tympani. 



The anterior wall (paries carotica) separates the tympanum from the carotid 

 artery and at times presents a fissure. At its upper part is the irregular trian- 

 gular opening of the Eustachian tube and above this opening lies the small canal for the 



Fig. 1254. 



Outer end of horizontal 

 part of facial canal 



Promontory 



Tensor tympani 



Stapes lying in 



oval window '~W 



Stapedius muscle — * ^ 

 Round window — ^^-^ — 



Facial canal 



Eustachian tube 



\ 

 Outer aspect of inner wall of right tympanic cavity; stapes lies within oval window. X 2j^. 



tensor tympani muscle. The canaliculus caroticus tympanicus perforates the anterior 

 wall just below the mouth of the Eustachian tube, and transmits the tympanic branch 

 of the internal carotid artery and carotico-tympanic nerves. 



The posterior wall (paries mastoidea) of the tympanum at its upper part is 

 occupied by the antrum tympanicum, which leads into numerous irregular cavities, 

 the mastoid cells. At the lower border of the antrum is a saddle-shaped notch, the 

 fossa incudis, which lodges the short process of the incus. Extending forward 

 from the posterior wall, on a level with the lower border of the oval window, projects 

 the small bony elevation, the pyramid (eminentia pyramidalis), which encloses the 

 stapedius muscle (Fig. 1254). Its apex is pierced by a small round opening for 

 the exit of the stapedius tendon. The canal within this eminence communicates 

 posteriorly with the facial canal. On a level with the eminentia pyramidalis, close 

 to the posterior margin of the drum-membrane, lies the apertura tympanica canaliculi 

 chordae tympani, the opening through which the chorda tympani nerve enters the 

 middle ear. 



THE CONTENTS OF THE TYMPANUM. 



The Auditory Ossicles. — Three small bones (ossicula auditus) form a chain 

 extending across the ujjper part of the tympanum from the tympanic membrane to 

 the labyrinth. The outermost of these, the malleus (hammer), is attached to the 

 tympanic membrane ; the innermost, the stapes (stirrup), is fixed in the oval window, 

 and between these two bones and connected with both of them, lies the third link in 

 the chain, the incus (anvil). 



