THE SENSE OF HEARING. 



809 



membrane and the ring of bone into which this membrane is inserted. 

 The roof is formed by a thin plate of bone, the tegmen, which separates it 

 from the cranial cavity, and the narrow floor, concave upward, is just above 

 the jugular fossa. The cavity is lined by mucous membrane continuous with 

 that of the Eustachian tube and the pharynx, and the membrane, like that 

 of the Eustachian tube, is ciliated except over the surfaces of the ossicles and 

 the tympanic membrane. Suppurative inflammation of the middle ear may 

 not only involve the mastoid cells, but may also cause absorption of the thin 

 plate of bone forming the roof of the tympanic cavity and the mastoid 

 antrtim. In this and in other ways inflammation may extend from the tym- 

 panic to the cranial cavity, making otitis media, or inflammation of the middle 

 ear, the commonest source of pyogenic affections of the brain. 1 



Tympanic Membrane, or Drum-skin. The membrana tympani (Figs. 268, 

 269) is a somewhat oval disk whose longer axis is directed from behind and above 



downward and forward, and 

 whose length is about nine 

 millimeters. The membrane 

 is inserted obliquely to the 

 axis of the auditory canal, 

 so that the floor of the canal 

 is longer than its roof. The 

 membrana tympani, though 

 so thin as to be semi-trans- 

 parent, is composed of three 

 layers of tissue. Externally 

 it is covered by a thin plate 

 of skin ; internally, by mu- 

 cous membrane ; and between 

 these lies the proper sub- 



FIG. 269 2 . Tympanum of right side with ossicles in place, viewed 

 from within (after Morris) : 1, body of incus ; 2, suspensory ligament 

 of malleus ; 3, ligament of incus ; 4, head of malleus ; 5, epitym- 

 panic cavity ; 6, chorda tympani nerve ; 7, tendon of tensor tympani 

 muscle ; 8, foot-piece of stirrup ; 9, os orbiculare ; 10, manubrium ; 

 11, tensor tympani muscle; 12, membrana tympani; 13, Eustachian 

 tube. 



FIG. 270. The chain of auditory 

 ossicles, anterior view (after Tes- 

 tut) : 1, head of malleus ; 2, long 

 process of incus ; 3, stapes. 



stance (membrana prop-ia) of the membrane, made up chiefly of fibrous tissue. 

 The greater number of the fibres of the membrana propria radiate from near 

 the centre to the periphery of the membrane ; but there are also circular fibres 

 of elastic tissue which are most numerous in a ring near the attached margin 

 of the membrane. The surface of the tympanic membrane is not flat, but is 

 funnel-shaped, with the apex of the funnel pointing inward. Moreover, lines 

 ^acewen: Pyogenic Diseases of the Brain and Spinal Cord, 1893. 

 * Figs. 267, 268, and 269 are taken by permission from Morris's Text-Book of Anatomy, Phila., 1893. 



