THE SENSE OF HEARING 



729 



membrane covered with ciliated epithelium. Near the middle of its course 

 the tube is contracted, though expanded at either extremity (Fig. 314). It 

 measures about 40 mm. in length. Its general direction from the pharyn- 

 geal orifice is outward, backward, and upward at an angle of about 45 

 degrees. 



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 Membrani Tympani. The membrana tympani, is a thin, translucent 

 nearly circular membrane, measuring about 10 mm. in diameter, placed at 



10 



FIG 314. 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. n. Superior semicircular canal. 12. Posterior semi- 

 circular 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 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 the adult 

 condition cannot be removed, owing to its consolidation with the surrounding 

 bone. This membrane consists primarily of a layer of fibrous tissue which 

 is covered externally by a thin layer of skin continuous with that lining the 

 auditory canal, and internally by a thin mucous membrane. The tympanic 

 membrane is placed obliquely at the bottom of the auditory canal, inclining 

 from above and behind downward and forward at an angle of about forty- 

 five degrees. The external surface of this membrane presents a funnel- 

 shaped depression, the sides of which are slightly convex. 



The Ear-bones. Running across the tympanic cavity and forming an 

 irregular line of joined levers is a chain of bones, which articulate one with 



