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HAND-BOOK OF PHYSIOLOGY. 



in structure, which, secrete a peculiar yellow substance called cerumen, 

 or ear-wax. 



(2.) Middle Ear or Tympanum. The middle ear, or tympanum 

 (3, Fig. 357), is separated by the membrana tympani from the external 

 auditory canal. It is a cavity in the temporal bone, opening through its 

 anterior and inner wall into the Eustachian tube, a cylindriform flattened 

 canal, dilated at both ends, composed partly of bone and partly of carti- 

 lage, and lined with mucous membrane, which forms a communication 

 between the tympanum and the pharynx. It opens into the cavity of 

 the pharynx just behind the posterior aperture of the nostrils. The cavity 



FIG. 357. 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, 2', meatus 

 externus; 2', membrana tympani; 3, cavity of the tympanum; 3', its opening backward into the mas- 

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

 taining the facial (uppermost) and the auditory nerves; 6, placed on the vestibule of the labyrinth 

 above the fenestra ovalis: a, apex of the petrous bone; 6, 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.) 



of the tympanum communicates posteriorly with air-cavities, the mastoid 

 cells in the mastoid process of the temporal bone; but its only opening 

 to the external air is through the Eustachian tube (4, Fig. 357). 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 tym- 

 pani. The cavity of the tympanum is lined with mucous membrane, the 

 epithelium of which is ciliated and continuous with that of the pharynx. 



