THE TYMPA X f -.V. 



917 



is furrowed on its surface by three small grooves which lodge branches of the 

 tympanic plexus. 



The r<>un<l>'J eminence of the aqueductus FaUopii. the prominence of the bony 

 canal in which the portio dura is contained, traverses the inner wall of the tym- 

 panum above the fenestra ovalis. and behind that opening curves nearly vertically 

 downward along the posterior wall. 



The posterior wall of the tympanum is wider above than below, and presents 

 for examination the Opening of the mastoid antrum . 



Pyramid. 



The mastoid antrum is an irregular cavity with several small apertures opening 

 into it. situated above and behind the tympanum proper; the smaller openings 

 lead into canals which communicate with large irregular cavities contained in the 

 interior of the mastoid process. These cavities vary considerably in number, 



Fenestra ovalit 

 Pyramid \ Proc. cocMearif. 

 Hiatus PaUop. 



Canal for tens. tymp. 



Canal of Eust. tube 

 Groove for Jacobton's nerve 

 Fenettra rotunda 

 Can. for chorda tympani 



FIG. 542. Antero-posterior section through the tympanum. (Gegenbaur.) 



size, and form : they are lined by mucous membrane continuous with that lining 

 the cavity of the tympanum. Just below the opening of the antrum is the 

 pyramid. The antrum really opens into an upward and backward prolongation 

 of the tympanum, known as the attic or epitympanic recess, in which are situated 

 the head of the malleus and greater part of the incus (Fig. 543). 



The pyramid is a conical eminence situated immediately behind the fenestra 

 ovalis. and in front of the vertical portion of the Fallopian eminence above 

 described : it is hollow in the interior, and contains the Stapedius muscle ; its 

 summit projects forward toward the fenestra ovalis. and presents a small aperture 

 which transmits the tendon of the muscle. The cavity in the pyramid is pro- 

 longed into a minute canal, which communicates with the aqueductus Fallopii 

 and transmits the nerve which supplies the Stapedius. 



The anterior wall of the tympanum corresponds to the carotid canal, from 

 which it is separated by a thin plate of bone. It presents the 



Canal for the Tensor tympani muscle. Orifice of the Eustachian tube. 

 The processus cochleariformis. 



The orifices of the canal for the Tensor tympani and of the Eustachian tube 



are separated from each other by a thin, delicate, horizontal plate of bone, the 



x coeUeariformit. These canals run from the tympanum, forward. 



inward, and a little downward, to the retiring angle between the squamous and 



petrous portions of the temporal bone. 



The canal for the Tensor tympani muscle is the superior and the smaller 

 of the two : it is rounded, and lies beneath the upper surface of the petrous bone, 

 close to the hiatus Fallopii (Fig. 542). 



The Eustachian tube is the channel through which the tympanum communi- 

 cates with the pharynx. Its length is from an inch and a half to two inches, and 

 its direction downward, forward, and inward. It is formed partly of bone, partly 

 of cartilage and fibrous tissue. 



