PRACTICAL CONSIDERATIONS : THE MIDDLE EAR. 1505 



Roofing in the antrum and the passage leading into it from the attic is a thin 

 layer of bone (tegmen antri), which is particularly thin over the antrum and 

 separates these spaces from the middle fossa of the skull. Not infrequently there 

 are membranous defects in the tegmen, upon which the dura rests (Macewen). 

 Pus frequently passes through this bony plate, or its deficiencies, to the temporo- 

 sphenoidal region of the brain, which is the most frequent seat of brain abscess. 



Fractures of the base of the skull in the middle fossa may pass through the 

 tegmen, rupturing the adherent dura, and permitting cerebro-spinal fluid to pass into 

 the tympanum. If there is coincident rupture of the tympanic membrane, the fluid 

 will likely appear at the external auditory meatus, or if the membrane remains intact, 

 the fluid may pass to the pharynx through the Eustachian tube. 



Often the hearing in chronic plastic otitis media is better during a great noise 

 than when the surroundings are more quiet, because the stiffened ossicles transmit 

 additional ordinary sounds more readily after they have been loosened by the more 

 violent vibrations; or it may be because the auditory nerve, owing to the greater 

 irritation, becomes more sensitive (Urbantschitsch). 



The various relationships of the tympanum as involved in infectious disease 

 should be understood from the standpoint of etiology and from that of sequelae or 

 complications. 



Infection may reach the tympanum from (a) the naso-pharynx through the 

 Eustachian tube (scarlatina, diphtheria, pharyngitis, tonsillitis, rhinitis); or (d) the 

 mastoid antrum and cells posteriorly. It may extend from the tympanum (a) 

 upward, by perforation of the tegmen, often deficient at places, leading to external 

 pachymeningitis, or to subdural abscess ; the dura, arachnoid, and pia mater at 

 this level are fused, so that when the dura is ulcerated through, a diffuse meningeal 

 infection does not ensue, but the process tends rather to spread into the brain along 

 the perivascular lymphatic sheaths of the pial vessels, resulting in an abscess in 

 the temporal lobe (Taylor); (I?) to the internal jugular vein through venules that 

 penetrate the fundus tympani to empty into the jugular bulb, and thence to the 

 lateral sinus ; (c) to the superior petrosal sinus and the dura mater of the middle 

 fossa of the skull by the structures (veins and areolar tissue) passing through the 

 petro-squamous suture ; (d) to the facial canal either through congenital defects in 

 its walls, or through carious openings, or along the course of the stylo-mastoid 

 artery ; facial paralysis may follow, or infection may travel along the internal auditory 

 meatus and give rise to a diffuse leptomeningitis in the cerebellar fossa (Taylor); 

 (e) to the labyrinth by way of the fenestra ovalis, or through the membrana 

 tympani secondaria, which closes the fenestra rotunda opening into the scala 

 tympani ; permanent deafness may result from destruction of the labyrinth, and the 

 infection may pass along the cochlear branch of the auditory nerve and the nerve 

 itself to the cerebellar fossa ; ( /) to the ossicles causing caries and deafness ; (g-) 

 to the mastoid antrum (q.v. ). 



The Tympanic Membrane. — The tympanic membrane is oblique in its 

 lateral as well as in its vertical direction, so that the inferior wall of the auditory 

 canal is longer than the superior, and the anterior wall longer than the posterior. 

 The firm attachment of the handle of the malleus to the membrane causes it to 

 assume the shape of a hollow cone with its convexity pointing internally. The 

 innermost point of the cone is at the lower end of the handle of the malleus and 

 is called the timbo. The distance between it and the promontory on the internal 

 wall of the tympanic cavity is only about 2 mm. 



Retention of the products of inflammation within the tympanum may decrease 

 the inward bulging of the membrane or even cause it to protrude outward. When 

 the Eustachian tube is obstructed, the air then confined within the middle ear, may 

 become partly absorbed, allowing the external atmospheric pressure to increase the 

 inward bulging, and to press the base of the stapes more firmly into the fenestra 

 ovalis, giving rise to a ringing in the ears. 



If an imaginary line in the axis of the handle of the malleus is continued to the 

 lower margin of the membrane, and a second at right angles to this is carried through 

 the umbo, the membrane will be divided by the vertical line into a lesser anterior and 

 a greater posterior portion, and by the horizontal line into a greater upper and a lesser 



95 . 



