THE MIDDLE EAR OR TYMPANUM 



615 



The Middle Ear or Tympanum. The middle ear, or tympanum, 

 3, figure 430, 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. 



The Eustachian canal establishes communication between the tympanic 

 cavity and pharynx, thus equalizing the air pressure on the sides of the 



FIG. 430. Diagrammatic View from Before of the Parts Composing the Organ of Hearing of 

 the Left Side. The temporal bone 9f the left side, with the acc9mpanying 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, i, The pinna 

 and lobe; 2, 2', meatus externus; 2', membrana tympani; 3, cavity of the tympanum; 3', its 

 opening backward into the mastoid cells; between 3 and 3', the chain of small bones; 4, Eusta- 

 chian tube; 5, meatus internus, containing 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; 

 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.) 



surface for incus; 5, head, 6, neck; 7, processus brevis; 8, manubrium; 9, body; 10, short proc- 

 ess; IT, long process; 12, processus longus; 13, head; 14, facet for incus; 15, manubrium, 16^ 

 head; 17, neck; 18, crus anterius; 19, crus posterius; 20, foot plate. 



