888 



THE TYMPANIC MEMBRANE 



FIG. 315. The External Auditory Canal, and 

 the Tympanum: M, cavities in the tem- 

 poral bone; PC, cartilaginous portion of 

 the canal; Po, osseous portion of the canal; 

 L, membranous portion between them; 

 F, glenoid cavity for the condyle of the 

 lower jaw (Urbantschitsch). 



are elevators of the helix; the transverse and oblique muscles of the auricle are 



dilators of the depressions in the auricle; the tragicus and antitragicus are con- 

 strictors of the canal; they correspond 

 to analogous muscles in animals. In 

 animals, however, the auricle and its 

 muscular activity have a decided in- 

 fluence upon hearing. The muscles, in 

 the first place, direct the openings of 

 the auricles toward or away from the 

 source of the sound (pricking up the 

 ears). The internal muscles, moreover, 

 contract or dilate the cavity of the 

 auricle. In many diving animals, valve- 

 like appendages close the canal. The 

 human auricle may be most appropriately 

 considered as a perfectly formed but 

 functionally degenerate organ. 



The external auditory canal measures 

 from 3 to 3.25 cm. in length, from 8 

 to 9 mm. in height, and from 6 to 8 

 mm. in breadth at the meatus; it is 

 the conductor of the sound-waves to 

 the tympanic membrane. As it has 

 a slightly spiral curve (in order to 

 look into the canal, the auricle should 

 be drawn upward), almost all of the 

 sound-waves strike first against its wall, 

 and are reflected thence to the tympanic 

 membrane. Occlusion of the auditory 

 canal, especially by masses of inspissated 



cerumen (secreted by the ceruminous glands, which are similar to sw r eat- 



glands) , interferes, naturally, with the hearing. 



THE TYMPANIC MEMBRANE. 



The tympanic membrane (Fig. 316) is an unyielding, and almost inexpansible, 

 elastic membrane, with a thickened border, set in a special bony groove, and 

 stretched rather loosely. It is about o.i mm. thick, 50 sq. mm. in area (in small 

 animals not much smaller), elliptical in shape (its larger diameter is from 9^5 

 to 10 mm., its smaller 8 mm.), and it is placed obliquely at the inner extrem- 

 ity of the external auditory canal at an angle of 40 from above down- 

 ward and inward. Both membranes converge anteriorly so that, if prolonged, 

 they would meet at an angle of from 130 to 135. The oblique position allows 

 the membrane to present a greater surface than if it were placed vertically, and 

 thus many more waves of sound can fall vertically upon it. The membrane is 

 not evenly stretched, but is drawn inward just below the center (umbo) by the 

 handle of the malleus, which is attached to it; while the short process of the mal- 

 leus projects forward somewhat at the upper edge of the membrane (Figs. 314 

 and 315). 



The tympanic membrane consists of three layers: (i) The membrana propria 

 is a fibrous membrane, composed of radial fibers on its outer surface, and of circular 

 fibers on its inner surface. (2) The surface facing the external auditory canal 

 has a thin covering of cuticle. (3) The side facing the tympanic cavity has a 

 delicate mucosa with a single layer of squamous epithelium. Numerous nerves 

 and lymph- vessels, as well as internal and external blood-vessels are found in 

 the membrane. 



The tympanic membrane takes up the sound-waves that enter the 

 auditory canal, and is set into vibration by them, in correspondence with 

 the number and amplitude of the movements of the sound-waves in the 

 air. Politzer connected the ossicles attached to the tympanic mem- 

 brane of a duck with a recording apparatus, and was able to register the 

 vibrations of the membrane produced by sounding any tone. On 

 account of its small dimensions the tympanic membrane moves to and 



