CH. LIV.] 



ANATOMY OF THE EAR 



739 



external air is through the Eustachian tube (4, fig. 539). The walls 

 of the tympanum are osseous, except where apertures in them are 

 closed with membrane, as at the fenestra rotunda, and fenestra ovalis, 

 and at the outer part where the bone is replaced by the membrana 

 tympani. The cavity of the tympanum is lined with mucous mem- 

 brane, the epithelium of which is ciliated and continuous through 

 the Eustachian tube with that of the pharynx. In some parts, how- 

 ever, viz., over the roof, promontory, ossicles, and membrana tympani, 



FIG. 539. Diagrammatic view from before of the parts composing the organ of hearing of the left side. 

 The temporal bone of the left side, with the accompanying soft parts, has been detached from the 

 head, and a section has been carried through it transversely, so as to remove the front of the 

 meatus externus, half the tympanic membrane, the upper and anterior wall of the tympanum and 

 Eustachian tube. The meatus internus has also been opened, and the bony labyrinth exposed by 

 the removal of the surrounding parts of the petrous bone. 1 , The pinna and lobe ; 2, meatus 

 externus ; 2', membrana tympani ; 3, cavity of the tympanum ; 3', its opening backwards into the 

 mastoid cells ; between 3 and 3', the chain of small bones ; 4, Eustachian tube ; 5, meatus internus, 

 containing the facial (uppermost) and the auditory nerves ; 6, placed on the vestibule of the laby- 

 rinth above the fenestra ovalis ; a, apex of the petrous bone ; b, internal carotid artery : c, styloid 

 process ; d, facial nerve issuing from the stylo- mastoid foramen ; e, mastoid process ; /, squamous 

 part of the bone covered by integument, etc. (Arnold.) 



the epithelium is of the pavement variety and is destitute of cilia. 

 A chain of small bones extends from the membrana tympani to the 

 fenestra ovalis. 



The membrana tympani is placed in a slanting direction at the 

 bottom of the external auditory canal, its plane being at an angle 

 of about 45 with the lower wall of the canal. It is formed of fibres, 

 some running radially, some circularly ; its margin is set in a bony 

 groove ; its outer surface is covered with a continuation of the 

 cutaneous lining of the auditory canal, its inner surface with the 

 mucous membrane of the tympanum. 



