CHAP, iv.] 



HEARING. 



183 



and the mesial side is much broken up by irregularities. The 

 cavity of the drum is not completely circumscribed by a circular 



E.t 



rat 



m.e 



FIG. 165. DIAGRAM TO ILLUSTRATE THE RELATIONS OP AUDITORY PASSAGE, 

 TYMPANUM AND EUSTACHIAN TUBE. (After Schwalbe.) 



The figure represents a section not quite horizontal, being inclined downwards in 



front ; the right-hand edge of the page may be taken to represent the median 



plane of the head. 

 m e. external meatus, T. the tympanic cavity. E.t. the Eustachian tube. A. is 



placed in the antrum mastoideum. m t. indicates the attachment of the 



tympanic membrane. 

 a. b. the axis of the external meatus, c.b.d. that of the Eustachian tube. dd'. shews 



the curved axis of the antrum. 



wall like that of a drum proper. In about the lower half of the 

 cavity (Figs. 166, 167) the wall though irregular is complete ; but 

 the upper half of the cavity is continuous along an axis oblique 

 to the axis of the meatus (Fig. 165) with two more or less tubular 

 cavities, one stretching upwards and backwards (Fig. 165, dd. f ], 

 the other downwards and forwards (Figs. 165, 166, 167 E.t.). The 

 latter, distinctly tubular, is the Eustachian tube leading from 



fo 



T.T 



FIG. 166. DIAGRAM OF THE MEDIAN WALL OF THE TYMPANUM OF THE LEFT EAR. 



Magnified twice. (After Schwalbe.) 



1 The tympanic, 2. the epitympanic region; below the reference figure is seen 

 the gentle prominence due to the ampullae. A, the antrum mastoideum, the 

 line ee marking its limits. E. t. the Eustachian tube. T. T. the groove for the 

 tensor tympani. /. o the depression of the fenestra ovalis, the fenestra itself 

 being shaded. /. r the depression leading to the fenestra rotunda ; above, and 

 obliquely to the left of this, lies the projection caused by the base of the cochlea. 

 St. the prominence for the stapedius, with the orifice for the exit of the tendon 

 VII, the course of the facial nerve. The tympanum proper lies within the letters 

 a. b. d. e. 



