THE MIDDLE EAR 



753 



and the greater part of the incus; (3) a relatively large ventral recess in the 

 bulla ossea. 



The external wall (Paries membranacea) is formed largely by the thin mem- 

 brana tympani, which closes the inner end of the external auditory canal, and thus 

 forms tlu> s('i)tum between the external and middle parts of the ear. The membrane 

 is an oval disc, which slopes obliquely downward and inward at an angle of about 

 30 degrees with the lower Avail of the external auditory canal. The circumference 

 is attached in a groove (Sulcus tympanicus) in the thin ring of bone (Annulus 

 tympanicus) which almost completely surrounds it. The handle of the malleus 

 (the out(Tmost of the auditory ossicles) is attached to the inner surface of the 

 membrane and draws the central part inward, producing a slight concavity of the 

 outer surface. The periphery is thickened, forming the annulus fil)rosus. The 

 niembrana tympani consists of three layers. The external cutaneous layer 

 (Stratum cutancum) is a prolongation of the lining of the external auditory 

 meatus. The middle fibrous layer or membrana propria includes two chief sets 



Fig. 565. — Right Petrous Temporal Bone of Horse, 

 .\ntero-internal View. 

 /, External auditory meatus; 2, annulus tym- 

 panicus; S, laminie radiating from 2; 4. membrana 

 tympani; 5. malleus; 6, incus; 7, stapes; S, fenestra 

 cochlece (s.rotundum); 9, facial canal; 77, 11", semicir- 

 cular canals; 11', vestibule; 12 cochlea. (After 

 EUenberger, in Leisering's Atlas ) 



Fig. 566. — Right Auditory Ossicles .\nd Mem- 

 bran.^ Tympani, Enlarged and Viewed from 

 Inner Side and Below. 



;, Malleus; 2, incus; 2', 2", short and long 

 processes of 2; 3, stapes; 4, tensor tympani; 5, 

 ligament attaching long process of malleus; 6, stape- 

 dius mu.scle; 7, fenestra cochleae (s. rotundum). 

 (After EUenberger, in Leisering's Atlas.) 



of fibers; the outer stratum (Stratum radiatum) consists of fibers which radiate 

 from the handle of the malleus, while the inner stratum (Stratum circulare) is com- 

 posed of circular fibers which are best developed peripherally. There are also 

 branched or dendritic fibers in part of the membrane. The internal mucous layer 

 (Stratum mucosum) is a part of the general mucous membrane which lines the 

 tympanic cavity 



The internal wall of the tympanic cavity (Paries labyrinthica) separates it 

 from the internal ear; it presents a numlDcr of special features. The promontory 

 (Promontorium) is a distinct eminence near the center which corresponds to the 

 first coil of the cochlea, and is marked by a faint groove for the superficial petrosal 

 nerve. Above this is the fenestra vestibuli (s. ovalis), a reniform opening w^hich is 

 closed by the foot-plate of the stapes and its annular ligament. The fenestra 

 cochleae (s. rotundum) is situated below and behind the preceding; it is an irregu- 

 larly oval opening and is closed by a thin membrane (Membrana tympani secund- 

 aria), which separates the tympanic cavity from the scala tympani of the cochlea. 



The anterior wall (Paries tubaria) is narrow, and is pierced by the slit-like 

 48 



