1132 



THE ORGANS OF SPECIAL SENSE 



membrane, nearly oval in form, somewhat broader above than below, and directed 

 very obliquely downward and inward, so as to form an angle of about 55 degrees 

 with the floor of the meatus (Fig. 834). The antero-inferior portion is, therefore, 

 placed at the greatest distance from the external orifice of the meatus. In a new- 

 born child the membrana tympani is almost horizontal. The greatest diameter 

 of the membrane tympani is from 9 to 10 mm. ; its least diameter is from 8 to 9 

 mm. The greater part of its circumference (limbus membranae tympana?) is 

 thickened to form an annular ring (annulus fibrocartilagineus), which is fixed in 

 a groove, the sulcus tympanicus, at the inner extremity of the external meatus. 

 This sulcus is deficient superiorly at the incisure or notch of Rivinus (incisura 

 tympanica [Rivini]) (Fig. 837). From the pxtrpmifies of the notch (spinae tym- 

 panicae) two folds, the anterior and posterior tympanomalleolar folds, are prolonged 



JUNCTION BETWEEN MAS 

 TOID ANTRUM AND 

 EPITYMPANIC RECESS 

 I TEGMEN 

 T TV M PAN I 



[ EPITYMPANIC 

 T RECESS 



/PROMINENCE OF EXTERNAL 

 SEMICIRCULAR CANAL 



PROMINENCE OF AQUEDUCT 

 OF FALLOPIUS 



TENDON OF 



STAPEDIUS MUSCLE 

 PLICA 

 STAPEDIUS 



PROCESSUS 



COCH LEARI FORM IS 



TENSOR TYMPANI 



MUSCLE (cutthrougt 



WALL OF 

 LABYRINTH 



POSTERIOR 

 SINUS 



PYRAMIDAL 

 EMINENCE 



TYMPANIC 



SINUS 



FOSSULA OF 

 FENESTRA ROTUNDA 



TYMPANIC 

 PLEXUS 



FIG. 839. The medial wall and part of the posterior and anterior walls of the right tympanic cavity, lateral 



view. (Spalteholz.) 



to the short process of the malleus (Fig. 837). The small, somewhat triangular 

 part of the membrane situated above these folds is lax and thin, and is jiamed 

 the membrana flaccida of Shrapnell (Figs. 837 and 838); in it a small orifice is some- 

 times seen. The larger lower portion of the tympanic membrane is stretched 

 tightly, and is called the tense portion or pars tensa (Figs. 837 and 838). 



The handle of the malleus is firmly attached to the inner aspect of the mem 

 brana tympani as far as its centre (Fig. 838), which it draws inward toward the 

 tympanic cavity. The most depressed part of the concavity is called the umbo 

 (umbo membranae tympanae) (Fig. 837). On the outer surface of the drum 

 membrane a light stripe (stria malleolaris] is seen. It runs from in front and 

 above downward and backward, and is produced by the handle of the malleus, 

 showing through the membrane (Fig. 837). 



Structure. The tympanic membrane is composed of three layers an external (cuticular), 

 a middle (fibroufi), and an internal (mucous). The cuticular layer (stratum cutaneum) is de- 

 rived from the integument lining the meatus. The fibrous or middle layer (membrana propria) 



