698 ORGANS OF SENSE. 



is furnished with hairs and sebaceous glands. In the thick subcutaneous tissue 

 of the cartilaginous part of the meatus are numerous ceruminous glands, which 

 secrete the ear-wax: their ducts open on the surface of the skin. 



The arteries supplying the meatus are branches from the posterior auricular, 

 internal maxillary, and temporal. 



The nerves are chiefly derived from, the auriculo-temporal branch of the infe- 

 rior maxillary nerve. 



MIDDLE EAR, OR TYMPANUM. 



The Middle Bar, or Tympanum, is an irregular cavity, compressed from, 

 without inwards, and situated within the petrous bone. It is placed above the 

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

 meatus auditorius externally, and the labyrinth internally. It is filled with air, 

 and communicates with the pharynx by the Eustachian tube. The tympanum 

 is traversed by a chain of movable bones, which connect the membrana tym- 

 pani with the labyrinth, and serve to convey the vibrations communicated to 

 the membrana tympani across the cavity of the tympanum to the internal ear. 



The cavity of the tympanum measures about five lines from before backwards, 

 three lines in the vertical direction, and between two and three in the trans- 

 verse, being a little broader behind and above than it is below and in front. 

 It is bounded externally by the rnembrani tympani and meatus; internally, by 

 the outer surface of the internal ear; and communicates, behind, with the 

 mastoid cells; and, in front, with the Eustachian tube and canal for the Tensor 

 Tympani. Its roof and floor are formed by thin osseous laminae, which connect 

 the squamous and petrous portions of the temporal bone. 



The roof is broad, flattened, and formed of a thin plate of bone, which sepa- 

 rates the cranial and tympanic cavities. 



The^oor is narrow, and corresponds to the jugular fossa, which lies beneath. 



The outer wall is formed by the membrana tympani, a small portion of bone 

 being seen above and below this membrane. It presents three small apertures, 

 the iter chordas posterius, the Glaserian fissure, and the iter chordae anterius. 



The aperture of the iter chordse posterius is behind the aperture for the mem- 

 brana tympani, close to its margin, on a level with its centre; it leads into a 

 minute canal, which descends in front of the aqua3ductus Fallopii, and termi- 

 nates in that canal near the stylo-mastoid foramen. Through it the chorda 

 tyrnpani nerve enters the tympanum. 



The Glaserian fissure opens just above and in front of the orifice of the mem- 

 brana tympani ; in this situation it is a mere slit, about a line in length. It 

 gives passage to the long process of the malleus, the Laxator Tympani muscle, 

 and some tympanic vessels. 



The aperture of the iter chordse anterius is seen just above the preceding fissure ; 

 it leads into a canal (canal of Huguier), which runs parallel with the Glaserian 

 fissure. Through it the chorda tympani nerve leaves the tympanum. 



The internal wall of the tympanum (Fig. 379) is vertical in direction, and looks 

 directly outwards. It presents for examination the following parts: 



Fenestra ovalis. Ridge of the Aquaeductus Fallopii. 



Fenestra rotunda. Pyramid. 



Promontory. Opening for the Stapedius. 



The fenestra ovalis is a reniform opening, leading from the tympanum into 

 the vestibule; its long diameter is directed horizontally, and its convex border 

 is upwards. The opening in the recent state is closed by the lining membrane 

 common to both cavities, and is occupied by the base of the stapes. This 

 membrane is placed opposite the membrana tympani, and is connected with it 

 by the ossicula auditus. 



The fenestra rotunda is an oval aperture, placed at the bottom of a funnel- 

 shaped depression, leading into the cochlea. It is situated below and rather 



