876 



ORGANS OF SPECIAL SENSE 



tunda below. The fenestra ovalis leads into the vestilmle of the internal ear (fig. 

 498a), and in the recent state it lodges the base of the stapes. The fenestra rotunda 

 is closed by the secondary membrane of the tympanum, which separates the 

 scala tympani of the cochlea from the tympanic cavity. Above the fenestra ovalis 

 there is a rounded ridge of l)one, indicating the position of the afjueduct of Fallo- 

 pius, Avhich contains the facial nerve. 



The posterior wall presents the hollow pyramid which lodges the stapedius 

 muscle and the openings which lead into the mastoid cells; these vary much in 

 size, and receive a mucous lining from the cavity. 



Anteriorly the tympanic cavity is continued int(j the Eustachian tube, and 

 it is separated from the carotid canal by a thin plate of bone. 



The Eustachian tube is about one and a half inches (37 mm.) in length, 

 passes from the tympanum downwards, forwards, and inwards. It is partly 

 bony and partly cartilaginous. 



The osseous portion (fig. 498a) is about half an inch (13 mm.) in length. It 

 commences just below the orifice which transmits the tendon of the tensor tympani 

 muscle, and, gradually contracting, ends at the isthmus by a jagged and oblique 

 orifice iDetween the petrous and squamous portions of the temporal bone. To it is 

 attached the cartilaginous portion of the tube, which is about an inch in length 



Fig. 498a. — Horizontal Sectiox of Left Temporal Boxe, showing the Various 



Parts of the Ear. 



EXTERNAL MEATUS 



MASTOID AIR CELLS 



TYMPANIC RING TYMPANUM 



FLOOR OF TENSOR TYMPANI CANAL 

 EUSTACHIAN TUBE 

 I COCHLEA 



CAROTID CANAL 



INTERNAL MEATUS 

 AQUEDUCTUS FALLOPII 

 POINTER PASSING THROUGH FENESTRA OVALIS 



(24 mm.). This is composed of an elongated triangular plate, so folded upon itself 

 as to leave an interval below which is completed by fibrous tissue. At first the 

 canal is narrow^ but it becomes much expanded towards its trunq)et-shaped 

 termination. Its orifice, which is somewhat oval, opens into the ui)per part 

 and side of the pharynx, on a level with and just behind the inferior meatus of 

 the nose. 



The mucous membrane possesses a ciliated lining. It is thin near the tym- 

 panic cavity, but it is much more vascular, thicker, and possesses mucous glands 

 and adenoid tissue near its pharvngeal orifice. 



Sections of the cartilaginous' portion of the tuljc show that the cartilagmous 

 plate is curled round like a hook Avith the bend above and the shank mternal. 

 The membranous portion is thin whore it is attached to the tip of the hook, but 

 that it becomes much thicker below. A small space near the bony portion of 

 the tube, immediately below the hook, remains permanently open, but elsewhere 

 the walls are in contact, except when separated by the action -of the tensor palati 

 muscle, or when forcibly driven asunder, as in the act of sneezing, etc. 



A number of small sesamoid fibro-cartilages, some of wdiich are visible to the 

 naked eve, are buried in the submucous tissue. 



