MIDDLE EAR. 527 



groove round the inner end of the external meatus, and is com-, 

 posed of three layers ; an external, or epidermic, a prolongation 

 of the skin lining the meatus ; a middle, or fibrous, the fibres 

 being in two sets, one radiating from the centre to the circum- 

 ference, the other scattered and indistinct, except near the 

 periphery, where they form a dense ring ; and 

 an internal, or mucous, derived from the 

 mucous lining of the middle ear. 



A chain of small bones, the auditory ossicles 

 (ossiculaauditiis) stretch across the tympanum. 

 They are the malleus, incus, stapes, and len- ^"''- ■^^^• 



, . , The malleus, a, Head ; 



ticular bone, or os orbiculare. h, Neck; c, Haudie; 



The Malleus, or Hammer, presents a '?> Processus gracilis, 

 rounded head (cajntulum), a neck, a handle (manubrium), a 

 long and a short process {proceisus gracilis and hrevis). The 

 head articulates with the incus ; the handle is received between 

 the inner and middle coats of tlie membrana tympani ; the long 

 process, in man, extends to the Glaserian fissure ; and the short 

 process, very obtuse, lies in contact with the drum. 



The Incus, or Anvil, which in shape somewhat resembles the 

 bicuspid tooth of man, has a body and two 

 crura or processes. The body is concave on 

 its surface, and articulates with the head of 

 the malleus ; the short crus is connected with 

 the posterior wall of the tympanum ; and the jupug ^aBody; ;,, 

 lono^ crus bears the os orhiculare, a round Articular surface for mai- 

 bone, about the size oi a gram oi sand, at its Lougcrus; e, os urWcu- 

 extreme point. ■'''^"' 



The Stapes, or Stirrup, possesses a base, two crura, and a 

 head. The base is oval, and placed upon 

 the fenestra ovalis ; the crura converge from 

 each extremity of the base to a constricted 

 portion termed the neck, on which is placed ^^^^ ^^^ 



the head, which articulates Avith the os orbi- stapes, a, Head; 6 ?j, 

 culare. ^'"'''■^ • '' ^^'■ 



The inner wall of the tympanum is very uneven, and presents 

 several eminences and openings. The oval opening, near its 

 upper part, is the fenestra ovalis, which leads into the cavity 

 of the vestibule. Above this fenestra, between it and the 

 roof, is a ridge formed by the aqueduct of Fallopius, and 

 beneath is the round opening of the fenestra rotunda. The 



