876 THE SENSE ORGANS AND SKIN OF THE HORSE 
and the greater part of the incus; (38) a relatively large ventral recess in the bulla 
ossea. 
The lateral, membranous wall (Paries membranacea) is formed largely by the 
thin membrana tympani, which closes the medial end of the external acoustic 
meatus, and thus forms the septum between the external and middle parts of the ear. 
The membrane is an oval dise, which slopes ventro-medially at an angle of about 
30 degrees with the lower wall of the external acoustic meatus. The circumference 
is attached in a groove (Suleus tympanicus) in the thin ring of bone (Annulus 
tympanicus) which almost completely surrounds it. The handle of the malleus 
(the outermost of the auditory ossicles) is attached to the inner surface of the 
membrane and draws the central part mward, producing a slight concavity of the 
outer surface. The periphery is thickened, forming the annulus fibrosus. The 
membrana tympani consists of three layers. The external cutaneous layer (Stratum 
cutaneum) is a prolongation of the lining of the external acoustic meatus. The 
Fig. 702.—Ricur Perrous TEMPORAL BoNE oF Horse; Fie. 703.—RicuHr Avuprirory OssicLes AND Mem- 
ANTERO-MEDIAL VIEW. BRANA TYMPANI, ENLARGED AND VIEWED FROM 
1, External acoustic meatus; 2, annulus tym- Inner Sipe AND Brevow. 
, lamine radiating from 2; 4, membrana 1, Malleus; 2, incus; 2’, 2’, short and long proe- 
panicus; 
3 
tympani; 4, malleus; 6, incus; 7, stapes; 8, fenestra esses of 2; 3, stapes; 4, tensor tympani; 4, ligament 
cochles (s. rotundum); 9, facial canal; 11, 11’, semi- attaching long process of malleus; 6, stapedius muscle; 
circular canals; 11’, vestibule; 12, cochlea. (After 7, fenestra cochlew (s. rotundum). (After Ellenberger, 
Ellenberger, in Leisering’s Atlas.) in Leisering’s Atlas.) 
middle fibrous layer or membrana propria includes two chief sets of fibers; the 
external layer (Stratum radiatum) consists of fibers which radiate from the handle 
of the malleus, while the internal layer (Stratum circulare) is composed 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 medial, labyrinthine wall of the tympanic cavity (Paries labyrinthica) 
separates it from the internal ear; it presents a number of special features. The 
promontory (Promontorium) is a distinct eminence near the center which corre- 
sponds to the first coil of the cochlea, and is marked by a faint groove for the super- 
ficial petrosal nerve. Above this is the fenestra vestibuli, a reniform opening which 
is closed by the foot-plate of the stapes and its annular ligament. The fenestra 
cochlee is situated below and behind the preceding; it is an irregularly oval 
opening and is closed by a thin membrane (Membrana tympani secundaria), 
which separates the tympanic cavity from the scala tympani of the cochlea. 
The anterior, tubal wall (Paries tubaria) is narrow, and is pierced by the slit- 
