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TEXT-BOOK OF PHYSIOLOGY. 



mastoid cells, anteriorly with the pharynx through the Eustachian 

 tube. 



The Eustachian Tube. The passageway between the tympanic 

 cavity and the naso-pharynx is known from its discoverer as the 

 Eustachian tube. It is composed internally of bone, externally of 

 cartilage, and is lined by mucous membrane covered with ciliated 

 epithelium. Near the middle of its course the tube is contracted, 

 though expanded at either extremity (Fig. 312). It measures about 

 40 mm. in length. Its general direction from the pharyngeal orifice 

 is outward, backward, and upward at an angle of about 45 degrees. 



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FIG. 309. THE EAR. i. Pinna, or auricle. 2. Concha. 3. External auditory canal 

 4. Membrana tympani. 5. Incus. 6. Malleus. 7. Manubrium mallei. 8. Tensor 

 tympani. 9. Tympanic cavity. 10. Eustachian tube. 1 1 . Superior semicircular 

 canal. 12. Posterior semicircular canal. 13. External semicircular canal. 14. 

 Cochlea. 15. Internal auditory canal. 16. Facial nerve. 17. Large petrosal nerve. 

 18. Vestibular branch of auditory nerve. 19. Cochlear branch. (Sappey.} 



The middle ear cavity is separated from the external ear by 

 a membrane the membrana tympani and from the internal ear by 

 an osseo-membranous partition which forms a common wall for both 

 cavities. The interior of the cavity is crossed from side to side by a 

 chain of bones and lined by a mucous membrane continuous with 

 that lining the pharynx. 



The membrana tympani is a thin, translucent, nearly circular 

 membrane, measuring about 10 mm. in diameter, placed at the inner 

 termination of the external auditory canal. It is inclosed in a ring 

 of bone which in the fetal condition can be easily removed, but in 



