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he apex of the latter its upper extremity, the lagena, or cæcum capulare, is xed to the cupula and partly bounds the helicotrema. As already stated, the membrana basilaris extends from the free edge of the lamina spiralis ossea to the uter wall of the cochlea. A second, more delicate membrane, the membrana estibularis (O.T. membrane of Reissner), stretches from the thickened periosteum overing the upper surface of the lamina spiralis ossea to the outer cochlear wall, ome little distance above the external attachment of the membrana basilaris. A anal is thus enclosed between the underlying scala tympani and the overlying cala vestibuli, and constitutes the ductus cochlearis. Triangular on transverse ection, the duct possesses a roof, an outer wall, and a floor, and is lined throughout with epithelium and filled with endolymph. On its floor the epithelium is greatly modified, and there the endings of the cochlear nerve are found.
The roof or vestibular wall of the ductus cochlearis is formed by the memrana vestibularis, a delicate, nearly homogeneous membrane, covered on each surface by a layer of epithelium. Its entire thickness is about 3 μ.
The outer wall of the ductus cochlearis (Fig. 724) consists of the periosteal ining of the bony cochlea, which, however, is thickened and greatly modified to form the ligamentum spirale cochleæ. Occupying the whole outer wall, this ligament projects inwards inferiorly as a triangular prominence, the crista basilaris, to which the outer edge of the membrana basilaris is attached. In the upper part of the ligamentum spirale the periosteum is of a reddish-yellow colour, and contains, immediately under its epithelial lining, numerous small blood-vessels and capillary loops, forming the stria vascularis. The lower limit of this stria is bounded by a prominence, the prominentia spiralis, in which is seen a vessel, the vas prominens, and between this prominence and the crista basilaris is a concavity, the sulcus spiralis externus. The height of the outer wall diminishes towards the apex of the cochlea.
Outer attachment of the membrana vestibularis
Bony wall of cochlea -Ligamentum spirale
The floor or tympanal wall of the ductus cochlearis is formed by the periosteum covering that portion of the lamina spiralis ossea which is situated to the outer side of the membrana vestibularis, and by the membrana basilaris, which stretches from the free edge of the lamina spiralis ossea to the crista basilaris. On the inner part of the membrana basilaris the complicated structure termed the organon spirale (O.T. organ of Corti) is situated. The lamina spiralis ossea consists of two plates of bone, between which are placed. the canals for the branches of the cochlear nerve. On the upper plate the periosteum is thickened and modified to form the limbus laminæ spiralis, the outer extremity of which forms a C-shaped concavity, the sulcus spiralis internus. The portions of the limbus which project above and below this concavity are termed respectively the labium vestibulare and labium tympanicum. The latter is perforated by about 4000 small apertures, the foramina nervosa, for the transmission
Cells of Claudius
FIG. 724.-TRANSVERSE SECTION THROUGH OUTER
of the cochlear nerves, and is continuous with the membrana basilaris. The upper surface of the labium vestibulare presents a number of furrows crossing each other nearly at right angles, and intersecting a series of elevations which, at the free margin of the labium, form a row of tooth-like structures, about 7000 in number, the auditory teeth of Huschke. Covering the limbus is a layer of apparently squamous epithelium; the deeper protoplasmic portions of the cells, however, with their contained nuclei, lie in the intervals between the elevations and auditory teeth. This layer of epithelium is continuous above with that covering the under surface of the membrana vestibularis and below with that which lines the sulcus spiralis internus.
Membrana Basilaris.-The inner part of this membrane is thin, and supports the organon spirale; it is named the zona arcuata, and reaches as far as the footplate of the outer rod of Corti. Its outer part, extending from the foot-plate of the outer rod of Corti to the crista basilaris, is thicker and distinctly striated, and is termed the zona pectinata. The substantia propria of the membrane is almost homogeneous, but exhibits, in its deeper part, numerous fibres. These fibres are most distinct in the zona pectinata, and number, according to Retzius, about 24,000. Covering the under surface of the membrana basilaris is a layer of connective tissue, containing, in its inner part, small blood-vessels; one of these is larger than the others and lies below the tunnel of Corti, and is named the vas spirale. The width of the membrana basilaris increases from 210 μ in the basal coil to 360 μ in the apical coil.
Organon Spirale (O.T. Organ of Corti) (Fig. 725).-Placed upon the inner portion of the membrana basilaris, the organon spirale consists of an epithelial eminence which extends along the entire length of the ductus cochlearis, and comprises the following structures, viz.: (1) Corti's rods or pillars, (2) hair cells (inner and outer), (3) supporting cells of Deiters, (4) the cells of Hensen and Claudius, (5) the lamina reticularis, and (6) the membrana tectoria.
The rods of Corti form two rows, inner and outer, of stiff, pillar-like structures, and each rod presents a base or foot-plate, an intermediate elongated portion, and an upper
FIG. 725.-TRANSVERSE SECTION OF THE ORGANON SPIRALE FROM THE CENTRAL COIL
end or head. The bases of the two rows are planted on the membrana basilaris some little distance apart. The intermediate portions of the rods incline towards each other and the heads come into contact, so that, between the two rows above and the membrana basilaris below, a triangular tunnel, the tunnel of Corti, is enclosed; this tunnel increases both in height and width on passing towards the apex of the cochlea. The inner rods number nearly 6000, and the head of each resembles somewhat the proximal end of the ulna, presenting externally a deep concavity for the reception of a corresponding convexity on the head of the outer rod. The part of the head which overhangs this concavity is prolonged outwards, under the name of the head-plate, and overlaps the head of the
outer rod. The expanded bases of the inner rods are situated on the innermost portion of the membrana basilaris, immediately to the outer side of the foramina nervosa of the labium tympanicum. The intermediate parts of the inner rods are sinuously curved, and form with the membrana basilaris, an angle of about 60°. The outer rods number about 4000, and are longer than the inner, especially in the upper part of the cochlea. They are more inclined towards the membrana basilaris, and form with it an angle of about 40°. The head of each is convex internally, to fit the concavity on the head of the inner rod, and is prolonged outwards as a plate, the phalangeal process, which becomes connected with the lamina reticularis; in the head is an oval body which has an affinity for certain reagents. The main part of each rod consists of a nearly homogeneous material, which is finely striated. At the bases of the rods, on the side next Corti's tunnel, is a nucleated mass of protoplasm which reaches as far as the heads of the rods, and covers also the greater part of the tunnel floor; this protoplasm may be regarded as the undifferentiated part of the cell from which the rod was developed. Slit-like intervals, for the transmission of nerves, exist between the intermediate portions of adjacent rods.
Hair Cells.-These, like Corti's rods, form two sets, inner and outer. The former consists of a single row lying immediately internal to the inner rods-the latter of three, or, it may be, four rows placed to the outer side of the external rods. The inner hair cells are about 3500 in number; the diameter of each is greater than that of an inner rod, and so each inner hair cell is supported by more than one rod. Somewhat oval in shape, their free extremities are surmounted by about twenty fine hair-like processes, arranged in the form of a crescent, with its concavity directed inwards. The deep end of the cell contains a large nucleus and is rounded; it reaches only about half-way down the rod, and is in contact with the arborisations of the nerve terminations. To the inner side of this row of hair cells are two or three rows of elongated columnar cells, which act as supporting cells, and are continuous with the low columnar cells lining the sulcus spiralis internus. The outer hair cells number about 12,000, and form three rows in the basal coil and four rows in the upper two coils, although in the higher coils the rows are not so regularly arranged. The rounded free ends of the hair cells support some twenty hairlets arranged in the form of a crescent, opening inwards. Their deep extremities reach about half-way to the membrana basilaris, and are in contact with the nerve arborisations.
Alternating with the rows of the outer hair cells are the rows of Deiters' supporting cells, the lower extremities of which are expanded on the membrana basilaris, whilst their upper ends are tapered; the nucleus is placed near the middle of each cell, and, in addition, each cell contains a bright, thread-like structure called the supporting fibre. This fibre is attached by a club-shaped base to the membrana basilaris, and expands, at the free end of the cell, to form a phalangeal process of the membrana reticularis.
The cells of Hensen, or outer supporting cells, consist of about half a dozen rows, immediately outside Deiters' cells, and form a well-marked elevation on the floor of the ductus cochlearis. Their deep extremities are narrow and attached to the membrana basilaris, while their free ends are expanded; each cell contains a distinct nucleus and some pigment granules. The columnar cells, situated externally to the cells of Hensen, cover the outer part of the zona pectinata, and are named the cells of Claudius. A space, the space of Nuel, exists between the outer rods of Corti and the neighbouring row of hair cells; it communicates internally with Corti's tunnel, and extends outwards between the outer hair cells as far as Hensen's cells.
The lamina reticularis is a thin cuticular structure which lies over the organon spirale, and extends from the heads of the outer rods as far as Hensen's cells, where it ends in a row of quadrilateral areas which form its outer border. It consists of two or three rows of structures, named phalanges, which are elongated cuticular plates resembling in shape the digital phalanges. The innermost row is formed by the phalangeal processes of the heads of the outer row of Corti's rods; the succeeding row, or rows, represent the expanded upper ends of Deiters' supporting cells. The number of rows of phalanges, therefore, varies with the number of rows of outer hair cells and the alternating cells of Deiters. The free ends of the hair cells occupy the somewhat circular apertures between the constricted middle portions of the phalanges.
The membrana tectoria (Fig. 725) is an elastic membrane overlying the sulcus spiralis internus and the organon spirale. Attached, by its inner end, to the limbus lamina spiralis, near the lower edge of the membrana vestibularis, it reaches outwards as far as the outer row of hair cells. Its inner portion is thin and overlies the auditory teeth of Huschke. Its outer part is thickened, but becomes attenuated near its external border, which, according to Retzius, is attached to the outer row of Deiters' cells. Its lower edge
presents a firm, homogeneous border, and opposite the inner row of hair cells contains a clear, spirally arranged band, named Hensen's stripe.
Nervus Acusticus (Fig. 726). The acoustic nerve divides within the internal acoustic meatus into an anterior or cochlear and a posterior or vestibular nerve.
Spiral fibres Ganglion spirale
Nerve-fibres which pass out
FIG. 727.-PART OF COCHLEAR NERVE, highly magnified (Henle).
FIG. 726.-MEMBRANOUS LABYRINTH OF A FIVE MONTHS' FETUS,
Lateral semicircular duct
N. Cochleæ. The cochlear nerve is distributed to the hair cells of the organon spirale, the branches for the basal and middle coils entering the foramina in the tractus spiralis foraminosus, those for the apical coil running in the canalis centralis of the modiolus. Extending through the bony canals of the modiolus, the nervefibres radiate outwards between the lamella of the lamina spiralis ossea. Contained in the spiral canal of the modiolus, near the attached margin of the lamina, is a ganglion of bipolar nerve-cells which winds spirally round the modiolus, and is named the ganglion spirale (O.T. ganglion of Corti) (Fig. 727); the fibres of the nerve arise from the cells of this ganglion. Beyond the ganglion spirale the nerve-fibres extend outwards, at first in bundles, and then in a more or less continuous sheet, from the outer edge of which they are again collected into bundles, which pass through through the foramina nervosa of the labium tympanicum. Beyond this they appear
as naked axis-cylinders, and, turning in a spiral manner (inner or first spiral fasciculus), send fibrille towards the inner row of hair cells. Other fibrils run between the inner rods and form a second spiral fasciculus in Corti's tunnel, from which fibrils extend outwards across the tunnel, and, passing between the outer rods, enter Nuel's space. They form a spiral fasciculus on the inner aspect of each row of Deiters' cells, and fibrillæ pass from these fasciculi towards the bases of the outer hair cells.
The cochlear nerve gives off a vestibular branch, the terminal filaments of
which go through the foramina in the recessus cochlearis and are distributed to the hair cells of the vestibular part of the ductus cochlearis. On this vestibular branch, close to its origin from the cochlear nerve, is a minute ganglion (Boettcher).
N. Vestibuli. The vestibular nerve is distributed to the utricle, the saccule, and the ampullæ of the semicircular ducts. It divides into three branches, superior, inferior, and posterior, and each of these splits into filaments which pass through foramina in the fundus of the internal acoustic meatus. The filaments from the superior branch go through the foramina in the area vestibularis superior and supply the macula of the utricle and the crista ampullares of the superior and lateral semicircular ducts; those from the inferior branch run through the foramina in the area vestibularis inferior to the macula of the saccule. The posterior branch passes through the foramen singulare, and its filaments, six to eight in number, are distributed to the crista ampullaris of the posterior semicircular duct.
Ganglion Vestibulare. On the trunk of the vestibular nerve, within the internal acoustic meatus, is a ganglion, the vestibular ganglion, of bipolar nerve cells; the fibres of the nerve arise from the cells of this ganglion. Sometimes the vestibular nerve divides on the proximal side of the ganglion and the latter is then split into three parts, one on each of the three branches of the nerve.
Vessels of the Internal Ear.-The internal auditory artery, a branch of the basilar, enters the internal acoustic meatus and divides into vestibular and cochlear branches. The vestibular branch supplies the soft tissues in the vestibule and semicircular canals, each canal receiving two arteries, which, starting from opposite extremities of the canal, anastomose on the summit of the arch. The cochlear branch divides into numerous twigs, which enter the foramina in the tractus spiralis foraminosus, and run outwards in the lamina spiralis ossea to reach the soft structures; the largest of these arteries runs in the canalis centralis. The stylo-mastoid artery also supplies some minute branches to the cochlea. Siebenmann describes the internal auditory artery as dividing into three branches, viz.: (1) anterior vestibular, (2) cochlear proper, and (3) vestibulocochlear. The veins from the cochlea and vestibule unite, at the bottom of the meatus, with the veins from the semicircular canals to form the internal auditory vein, which may open either into the posterior part of the inferior petrosal sinus or into the transverse sinus. Small veins also pass through the aquæductus cochleæ and aquæductus vestibuli, the former opening into the inferior petrosal sinus or into the internal jugular vein, the latter into the superior petrosal sinus.
DEVELOPMENT OF LABYRINTH.
The epithelial lining of the labyrinth is derived from an invagination of the cephalic ectoderm, termed
subjacent meso- FIG. 728.-SECTIONS THROUGH THE REGION OF THE HIND BRAIN OF FETAL RABBITS (to illustrate the development of the labyrinthine epithelium).
derm. The vesicle soon becomes pear-shaped; and its dorsal tapering part rapidly
lengthens into a recess, the recessus labyrinthi, which later forms the ductus and saccus
In A the ectoderm is invaginated to form the auditory pit; in B the auditory pit is closed and detached from the ectoderm, forming the otic vesicle; while C shows a further stage in the development of the vesicle.