876 



THE SENSE ORGANS AND SKIN OF THE HORSE 



and the greater part of the incus; (3) 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 disc, which slopes ventro-medially at an angle of about 

 30 degrees mth the lower wall of the external acoustic meatus. The circumference 

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

 tAiupanicus) 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 inward, 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 



' A 



Fig. 702. — Right Petrous Temporal Bone of Horse; 



Antero-medial View. 

 1, External acoustic meatus; 2, annulus tym- 

 panicus; 5, laminae radiating from 2; -J, membrana 

 tj'mpani; 5, malleus; 6, incus; 7, stapes; 8, fenestra 

 cochleEe (s. rotundum); 9, facial canal; 11, 11", semi- 

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

 Ellenberger, in Leisering's Atlas.) 



Fig. 703. — Right Auditory Ossicles and Mem- 

 brana Tympani, Enlarged and Viewed from 

 Inner Side and Below. 



ly Malleus; 2, incus; 2' , 2", short and long proc- 

 esses of 2; 3, stapes; 4, tensor tympani; -5, ligament 

 attaching long process of malleus; 6, stapedius muscle; 

 7, fenestra cochleae (s. rotundum). (After Ellenberger, 

 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 malleuSj 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 J 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 

 cochleae is situated below and behind the preceding; it is an irregularly oval 

 opening and is closed by a thin membrane (Membrana tympani secundaria), 

 which s(']:>arates the tympanic cavity from the scala tympani of the cochlea. 



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



