THE SENSE OF HEARING. 



363 



%, 





Yd 



the bottom and anterior edge of the concha to the membrana tympani, or 



tympanic membrane. Starting 

 from the bottom of the concha, 

 the general direction of the audi- 

 tory canal is first obliquely up- 

 ward and backward for about 

 half an inch, and then inward 

 and forward. Therefore, to look 

 into the ear or to introduce the 

 aural speculum the canal must be 

 .straightened by pulling the pinna 

 upward and backward. The 

 canal-wall is cartilaginous and 

 movable for about half an inch 

 from the exterior, but is osseous 

 for the rest of its extent ; it is 

 lined by a reflexion of thin skin, 

 on whose surface, in the cartilag- 

 inous part of the canal, open the 

 ducts of numerous sebaceous and 

 ceruminons glands. 



Tympanum. — The middle ear, 

 or tympanum (Figs. 179, 180), is 

 shut off from the auditory canal 

 by the tympanic membrane. It 



is an air-holding cavity of irregular shape in the petrous bone, and it is broader 



behind and above than it is below and 



in front. Posteriorly it is in open com- 

 munication with the complex system of 



air-cavities in the mastoid bone known 



as the mastoid antrum and the mastoid 



cells. Anteriorly it is continuous with the 



pharynx through the Eustachian tube. 



The inner wall slants somewhat outward 



from top to bottom, and it is formed 



chiefly by part of the bony envelope of 



the internal ear. The surface of this wall 



is pierced by two apertures, the fenestra 



ovalis, or oval window, and the fenestra 



rotunda, or round window, leading into 



the cavity of the bony labyrinth; in life 



each fenestra is covered by a thin sheet 



of membrane, and the foot of the stapes 



is fastened by a ligamentous fringe in the 



oval window. The outer wall of the middle ear is made up of the tympanic 



Fig. 180.— Tympanum of left ear, with ossicles in situ 

 (after Morris) : 1, suspensory ligament of malleus ; 2, head 

 of malleus ; 3, epitympanic region ; 4, external ligament 

 of malleus ; 5, processus longus of incus ; 6, base of stapes ; 

 7, processus brevis of malleus; 8, head of stapes; 9, os 

 orbiculare; 10, manubrium ; 11, Eustachian tube; 12, exter- 

 nal auditory meatus; 13, membrana tympani; 14, lower 

 part of tympanum. 



Fig. 181. Otoscopic view of left lmnihrana 

 tympani (Morris): 1, membrana flaccida; 2,2', 

 folds bounding the former ; 3, reflection from 

 processus brevis of malleus ; i. processus lon- 

 gus ni' incus (occasionally seen); i, mem- 

 brana tympani; 6, umbo and end of manu- 

 brium ; 7, pyramid <>f Light. 



