CH. LIII.] 



THE MIDDLE EAR. 



715 



The cavity 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 

 Eurftachian tube (4, fig. 536). The walls of the tympanum are 

 osseous, except where apertures in them are closed with mem- 

 brane, as at the fenestra rotunda, and fenestra ovalis, and at the 

 outer part where the bone is replaced by the membrana tympani. 

 The cavit} 7 of the tympanum is lined with mucous membrane, 

 the epithelium of which is ciliated and continuous through the 

 Kustachian tube with that of the pharynx. It contains a chain 

 of small bones which extends from the membrana tympani to the 

 fencstra ovulis. 



Fig 1 . 537. The hammer- 

 bone or malleus, seen 

 from the front, i, the 

 head ; 2, neck ; 3, 

 short process ; 4, 

 handle. (Schwalbe.) 



Fig. 5.58. -The incus, or anvil-bone, 

 i, body ; 2, ridged articulation 

 for the malleus ; 4, processus 

 brevis, with 5, rough articular 

 surface for ligament of incus ; 

 6, processus ma gnus, with articu- 

 lating surface for stapes ; 7, nu- 

 trient foramen. (Schwalbe.) 



-1 



Fig. 539. The stapes, or 

 stirrup-bone, i, base ; 

 2 and 3, arch ; 4, head 

 of bone, which articu- 

 lates with orbicular 

 process of the incus ; 

 5, constricted part of 

 neck ; 6, one of the 

 crura. (Schwalbe.) 



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 tough and tense fibres, some running radially, some circu- 

 larly ; 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. 



The ossicles are three in number ; named malleus, incus, and 

 stapes. The malleus, or hammer-bone, has a long slightly- 

 curved process, called its handle, which is inserted between 

 the layers of the membrana tympani ; the line of attachment 

 is vertical, including the whole length of the handle, and 

 extending from the upper border to the centre of the membrane. 

 The head of the malleus is irregularly rounded ; its neck, or the 

 line of boundary between the head and the handle, supports two 



