678 TEXT-BOOK OF PHYSIOLOGY. 



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-mem- 

 branous 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 



FIG. 336. 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.} 



condition can be easily removed, but in the adult condition cannot be re- 

 moved, owing to its consolidation with the surrounding bone. This mem- 

 brane consists primarily of a layer of fibrous tissue which is covered extern- 

 ally 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 

 another at their extremities. These bones are known as the malleus, incus, 



