THE MIDDLE EAR, DRUM, OR TYMPANUM 



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the jugular fossa; the carotid canal lying in front, the mastoid cells behind, the 

 external auditory meatus externally, and the labyrinth internally. TTis lined 

 with mucous membrane, is filled with air, and communicates with the mastoid 

 cells, through the mastoid antrum, and with the nasopharynx by the Eustachian 

 tube. The tympanum is traversed by a chain of movable bones, which connect 

 the membrana tympani with the labyrinth, and serve to convey the vibrations 

 communicated to the membrana tympani across the cavity of the tympanum 

 to the internal ear. In shape it is roughly biconcave, the concave surfaces being 

 placed vertically and forming the external and internal walls. The cavity forms 

 an angle of 45 degrees with the median plane. 



The Tympanic Cavity (cavum tympani) (Figs. 839 and 840) consists of two 

 parts the atrium or tympanic cavity proper (Fig. 840), opposite the tympanic mem- 

 brane, and the attic or epitympanic recess (recessus epitympanicus) (Figs. 838 and 

 839), above the level of the upper part of the membrane; the latter contains the - 

 upper half of the malleus and the greater part of the incus. The diameter of the- 

 tympanic cavity, including the attic, measures about 15 mm. (three-fifths of an- 

 inch) vertically and transversely. From without inward it measures about 6 mm. 

 (one-quarter of an inch) above and 4 mm. (one-sixth of an inch) below; opposite 

 the centre of the tympanic membrane it is only about 2 mm. (one-twelfth of an 

 inch). It is bounded externally by the membrana tympani and meatus; internally, 

 by the outer surface of the internal ear; it communicates behind w r ith the mastoid 

 antrum and through it with the mastoid cells, and in front with the Eustachian 

 tube. 



Chorda tympani 



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FIG. 835. View of the inner wall of the tympanum (enlarged). 



The roof of the tympanum (paries tegmentalis) is broad, flattened, and formed 

 of a thin plate of bone (legmen tympani) (Fig. 839), which separates the cranial 

 and tympanic cavities. It is situated on the anterior surface of the petrous portion 

 of the temporal bone, close to its angle of junction with the squamous portion of 

 the same bone, and is prolonged backward so as to roof in the mastoid antrum: 

 it is also carried forward to cover in the canal for the Tensor tympani muscle. 

 Its outer edge corresponds with the remains of the petrosquamous suture. 



The floor (paries jugular is) (Fig. 839) is narrow, and is separated by a thin 

 plate of bone (fundus tympani) from the jugular fossa. Tf presents, near the 

 inner wall, a small aperture, the opening of the canaliculus tympanicus, for the 



^transmission of Jacobson's nerve (n. tympanicus). On the floor near the posterior 



wall there js often to be found a slight bony projection (prominentia styloideae). 



The outer wall (Fig. 834) is formed mainly by the membrana tympani, partly 



Jbv the ring of bone into which this membrane is inserted. This ring of bone is 

 incomplete at its upper part, forming a notch (incisura tympanica [Rivini]) 



