MIDDLE EAU. 527 



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

 posed of three layers ; an ext&i-nal, or epidermic, a prolongation 

 of the skrn lining th« 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 intermal, or mucous, derived from the 

 mucous lining of the middle ^r. 



A chain of small bones, the auditory ossicles 

 (ossicula auditus) stretch across the tympanum. 

 Thev are the malleus, incus, stapes, and len- ^ ,?' tt j 



J J > r -J The malleus, a, Head; 



ticular bone, or os orbiculare. 6, Neck ; c. Handle ; 



The Malleus, or Hammer, presents a '^. ^p«x=«^ 8™<^- 

 rounded head (capituluTn), a neck, a liandle (man/iibriu'iirh), a 

 long and a short process (processus gracilis and brevis). The 

 head articulates with the incus ; the handle is received between 

 the inner and middle coats of the 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 Anyil, 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 cms is connected with 

 the posterior wall of the tympanum ; and the incus" "af" Body; b, 

 long crus bears the os orbicidare, a round Articuiar^aceforrmd- 

 bone, about the size of a grain of sand, at its Long cms; e, oa orwcn- 

 extreme point. ^®" 



The Stapes, or STnmup, possesses -a base, two crura, and a. 

 head. The base is oval, and placed upon ± 



the fenestra ovalis ; the. crura converge irom ^'Mri 



each extremity of the base to a constricted ^»» 



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



the head, which articulates with the os orbi- stapes, a, Head; 6 », 



Culare. crura ; c. Base. 



Th^ 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 



Fig. 200. 



