CHAPTER XV.

THE EAR.

THE ear may be divided into three portions-the external ear, the middle ear, and the internal ear.

The external ear consists of the pinna, or auricle, and the auditory canal.

The pinna, or auricle, consists of a basis of yellow elastic cartilage, which is covered by the skin and its subcutaneous tissue. In the inner surface are found coiled tubular glands, which are modified sweat-glands, and secrete a waxy substance. These are called the ceruminous glands.

The auditory canal is a little over an inch in length, extending inward to the tympanum.

The outer half of the canal is composed of cartilage continuous with that of the pinna. The inner half is osseous in structure. The entire canal is lined with a thin stratified epithelium, containing a few hairs and ceruminous glands near the outer portion of the canal.

The middle ear includes the tympanum, the Eustachian tube, the mastoid cells, and the tympanic cavity, with the small ear bones, or ossicles.

The tympanum consists of three layers. The external layer is a thin, stratified epithelium, continuous with the lining of the auditory canal.

The middle layer is a thin membrane of white, fibrous tissue.

The internal or mucous layer consists of a single layer

EUSTACHIAN TUBE.

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of cuboid epithelial cells, continuous with the lining of the tympanic cavity.

ous.

Blood-vessels, lymphatics, and nerves are very numer

The Eustachian tube consists of three portions. The inner, opening into the tympanic cavity, has bony walls; the middle portion, yellow, elastic, cartilaginous walls; and the portion opening into the pharynx is fibrous.

Fig. 63.-Longitudinal section of cochlea, showing the relation of the scale, the ganglion spirale, etc. (after Gray): s. v., Scala vestibuli; s. t., scala tympani; s. m., scala media; . s., ligamentum spirale; g. s., ganglion spirale.

The entire tube is lined by stratified, ciliated, columnar epithelium.

The tympanic cavity and mastoid cells are lined by their periosteum, upon which is a thin layer of fibrous tissue supporting the epithelium. This and the mastoid cells and the promontory are flat, and but a single layer in thickness. Over the other portions of the cavity the epithelium is columnar-ciliated.

The ossicles have the usual structure of compact bone, being covered at their articulations with hyaline cartilage.

The internal ear, or labyrinth, comprises the vestibule, semicircular canals, the cochlea, and the membranous labyrinth.

The vestibule and semicircular canals are lined by a

Fig. 64. Section of a single turn of cat's cochlea (after Piersol): SV, SM, ST, Scala vestibuli, media, and tympani; a, a, a, osseous tissue projecting as spiral lamina (b); c, basilar membrane attached to spiral ligament (d) on outer wall; e, concave surface lined by flat cells (q), interrupted by spinal prominence (f) containing blood-vessels; g, striæ vascularis; h, Reissner's membrane covered by epithelium (1) of cochlear duct and by endothelium (u) of scala vestibuli; i, limbus from which extends membrana tectoria (k) overhanging Corti's organ; m, tunnel of Corti; r, s, inner and outer hair cells; p, cells of Claudius; n, spiral ganglion; o, nerve bundles; v, blood-vessels.

very thin fibroserous membrane, which secretes a thin fluid the perilymph.

The perilymph fills the space between the walls and the membranous labyrinth.

The membranous labyrinth has three portions—the saccule, the utricle, and that portion in the semicircular canals. All, however, have a similar structure consisting of three layers. The outer is of fibro-elastic tissue, the middle layer a delicate basement membrane, and the inner a single layer of squamous epithelium.

At certain areas marking the entrance of the auditory nerve-fibers the membrane is much thickened, the epithelium covering these areas becoming columnar, until at the end of the nerve-fibers the neuro-epithelium is reached. These spots are called the maculæ cribrosæ in the saccules and utricles, and the crista acusticæ in the semicircular canals. The neuro-epithelium is of two kinds-fiber cells and hair cells.

The fiber cells are long, slightly hourglass-shaped elements, the bottom expansion being slightly larger than the upper. They have good-sized oval nuclei situated in the lower portion. These cells serve as supporting elements.

The hair cells are more cylindric elements, situated between the fiber cells. They have a large spheric nucleus, and their free surface is covered by long, delicate filaments, called the auditory hairs. The hair cells are the special endings of the auditory nerve.

Upon the surface of the hair cells a gelatinous substance is spread, in which are embedded numerous slightly elongated crystals of calcium carbonate, called the otoliths, or ear stones.

The center of the membranous labyrinth is filled with a clear fluid, which is called the endolymph.

THE COCHLEA.

The cochlea consists of a small tube coiled spirally two and a half times around a central pillar (the modiolus).

The modiolus contains the fibers of the cochlear branches of the auditory nerve-fibers from which they pass into the spiral tube.

The spiral tube contains a long spiral lamina, which extends as a shelf from the modiolus to its center.

From the free edge of this bony lamina a triangular duct (the membranous cochlea or ductus cochlearis) extends, its base being attached to the periphery of the bony tube.

This arrangement constitutes virtually three tubes; that above the bony lamina is called the scala vestibuli, that within the ductus cochlearis is called the scala media, that below is called the scala tympani.

The membrane forming the boundary between the scala vestibuli and the scala media is called the membrane of Reissner, or membrana vestibularis. That between the scala tympani and the scala media has received the name of the membranous spiral lamina.

The scale vestibuli and the tympani are lined by the periosteum, which is considerably thickened at the periphery or point of attachment of the ductus cochlearis, and is called in this location the ligamentum spirale.

The membrane of Reissner consists of three layers : that bounding the scala is of epithelium; the middle layer is of thin fibrous tissue; that layer lining the scala media is a single layer of squamous epithelium.

The membranous spiral lamina also has three layers the tympanic lamella, the basilar membrane, and the epithelium.

The tympanic lamella forms part of the lining of the scala tympani, and consists of a dense layer of fibrous tissue, the outer surface of which is covered by fusiform cells of undeveloped mesoblastic nature.

The basilar membrane forms the supporting por