36-4 



AN AMERICAN TEXT-BOOK OF I'HYSIOLOGY 



membrane and the ring of hone 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, eoncave 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, i- 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 

 antrum. 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. 181, 

 182) 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 

 5 is longer than its roof. The 

 membrana tympani, though 

 8 so thin as to be semi-trans- 

 parent, is composed of three 

 layers of tissue. Externally 

 it is covered by a thin plate 

 12 of skin ; internally, by mu- 

 ll cons membrane ; a nd between 

 these lies the proper sub- 



\P*~y£ 



Fig. 182.*— Tympanum of righl Bide with ossicles in place, viewed 

 in. in u itlun (after Morris) : I, body of incus ; 2, suspensory ligament 

 of malleus; :;, ligament of Incus; i, head of malleus; .".. epitym- 

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



muscle; 8, fool pie< f stirrup; 9, ■ ■ ,• 10, manubrium; 



U, tensor tympani muscle; 12, membrana tympani; i"., Eustachian 

 tuiir. 



Fig. 188.— The chain of auditory 

 OBsicles, anterior view (after TeB- 

 tut) : l, head of malleus; 'J, long 

 process of incus; 3, stapes. 



stance (membrana propia) 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 Burface of the tympanic membrane is not flat, but is 

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

 J Macewen: Pyogenic Diseases of the Brain ami Spinal ('mil, l.S'.t:;. 

 Figs. 180, L81,and 1 82 are taken by permission from Morris's Text-Book of Anatomy, Phila., 1893. 



