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ductus endolymphaticus and a small vein. The posterior part of the vestibule receives the five rounded apertures of the semicircular canals; its anterior part leads, by an elliptical opening, into the scala vestibuli of the cochlea. This opening is bounded inferiorly by a thin osseous plate, the lamina spiralis ossea, which springs from the floor of the vestibule immediately lateral to the recessus sphæricus, and forms, in the cochlea, the bony part of the septum between the scala tympani below and the scala vestibuli above. From the anterior part of the floor of the vestibule a narrow cleft, the fissura vestibuli, extends forwards into the bony canal of the cochlea. It is bounded internally by the lamina spiralis ossea, and externally by a second, smaller lamina, the lamina spiralis secundaria, which projects from the outer wall of the cochlea. These two lamina are continuous with each other round the posterior extremity of the fissura vestibuli.
Canales Semicirculares Ossei.-The osseous semicircular canals (Figs. 716, 717), three in number, are situated above and behind the vestibule. They are distinguished from each other by their position, and are named superior, posterior, and lateral. They open into the vestibule by five apertures, since the medial end of the superior and the upper end of the posterior join to form a common canal or crus commune. Differing slightly in length, each forms about two-thirds of a circle, one extremity of which is dilated and termed the osseous ampulla. They are somewhat compressed from side to side, and their diameter is from 1 to 1.5 mm., whilst that of the ampullæ is about 2 mm.
The superior semicircular canal, 15 to 20 mm. in length, is vertical and placed transversely to the long axis of the petrous part of the temporal bone. Its convexity is directed upwards, and its position is indicated on the anterior surface of the petrous part of the temporal bone by the arcuate eminence. Its ampulla is anterior and lateral, and opens into the vestibule immediately above that of the lateral canal. Its opposite extremity joins the non-ampullated end of the posterior canal to form the crus commune, which is about 4 mm. in length, and opens into the upper and medial part of the vestibule. The posterior semicircular canal is the longest and measures from 18 to 22 mm. Its ampulla is placed inferiorly, and opens into the lower and back part of the vestibule, where may be seen about six or eight small apertures (macula cribrosa inferior), for the transmission of the nerves to this ampulla. Its upper extremity ends in the crus commune. The lateral canal is the shortest; it measures from 12 to 15 mm., and arches nearly horizontally. Of its two extremities the lateral is ampullated, and opens into the vestibule immediately above the fenestra vestibuli and in close relationship to the ampullary end of the superior canal.
Crum Brown (Journ. Anat. and Physiol., London, vol. viii.) pointed out that the lateral canal of one ear is very nearly in the same plane as that of the other; while the superior canal of one ear is nearly parallel to the posterior canal of the other.
Cochlea. When freed from its surroundings the cochlea assumes the form of a short cone (Fig. 720); the central part of its base corresponds with the bottom of the internal acoustic meatus, whilst its apex or cupula is directed forwards and laterally, and comes into close relation with the semicanal for the tensor tympani muscle. It measures about 9 mm. across the base and about 5 mm. from base to apex, and consists of a spirally arranged tube, which forms from 2 to 22 coils around a central pillar, termed the modiolus. The length of the tube is from 28 to 30 mm., and its diameter, near the base of the cochlea, 2 mm. Its coils are distinguished by the terms basal, central, and apical; the first, or basal coil, gives rise to the promontory on the labyrinthic wall of the tympanic cavity.
The modiolus is about 3 mm. in height, and diminishes rapidly in diameter from base to apex, while its tapered extremity fails to reach the cupula by a distance of 1 mm. Its base corresponds with the area cochleæ on the fundus of the internal acoustic meatus, and exhibits the tractus spiralis foraminosus, which transmits the nerves for the basal and central coils of the cochlea and the foramen centrale, which gives passage to the nerves for the apical coil. The foramina of the tractus spiralis foraminosus traverse the modiolus, at first parallel to its long axis, but, after a
1 In the following description the cochlea is supposed to be placed on its base.
varying distance, they bend outwards to reach the attached edge of the lamina spiralis ossea, where they expand and form by their apposition a spiral canal, the canalis spiralis cochleæ, which lodges the ganglion spirale cochleæ. From this spiral canal numerous small foramina, for the transmission of vessels and nerves, pass outwards to the free edge of the lamina spiralis ossea. The lamina spiralis ossea, a thin, flat shelf of bone, winds round the modiolus like the thread of a screw, and, projecting about half-way into the cochlear tube, incompletely divides it into two passages-an upper is named the scala vestibuli; a lower, the scala tympani. The lamina spiralis ossea begins at the floor of the vestibule, near the fenestra cochleæ, and ends close to the apex of the cochlea in a sickle-shaped process, the hamulus laminæ spiralis, which assists to bound an aperture named the helicotrema. In the que
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Lamina spiralis ossea secundaria
Lamina spiralis ossea.
Recessus cochlearis of vestibule
-Canalis spiralis cochleæ
Internal acoustic meatus
Area vestibularis superior Area n. facialis
basal coil the upper surface of the lamina spiralis ossea forms almost a right angle with the modiolus, but the angle becomes more and more acute on ascending the tube. In the lower half of the basal coil a second smaller bony plate, the lamina spiralis secundaria, projects from the outer wall of the cochlea towards the lamina spiralis ossea, without, however, reaching it. If viewed from the vestibule the slit-like fissura vestibuli, already referred to (p. 844), is seen between the two laminæ. A membrane, the membrana basilaris; stretches from the free edge of the lamina spiralis ossea to the outer wall of the cochlea, and completes the septum between the scala vestibuli and scala tympani, but the two scalæ communicate with each other through the opening of the helicotrema at the apex of the cochlea. The scala tympani begins at the fenestra cochleæ, which is closed by the secondary tympanic membrane (vide p. 833). At the commencement of the scala tympani a crest, termed the crista semilunaris, stretches from the attached margin of the lamina spiralis ossea towards the orifice of the fenestra cochleæ. Close to this crest is seen the inner orifice of the aquæductus cochleæ, a canal measuring from 10 to 12 mm. in length, and opening on the under surface of the petrous part of the temporal bone medial to the fossa jugularis. Through it a communication is established between the scala tympani and the subarachnoid cavity, and through it, also, a small vein passes to join the inferior petrosal sinus. The scala vestibuli, the higher of the two passages, begins in the vestibule; its diameter in the basal coil is less than that of the scala tympani, but in the upper coils it exceeds that of the scala tympani.
Area vestibularis inferior
FIG. 719. BOTTOM OF INTERNAL ACOUSTIC
Meatus Acusticus Internus. It is convenient, at this stage, to study the fundus of the internal acoustic meatus, which has been referred to as forming the medial wall of the vestibule and the
base of the modiolus. It is divided by a transverse ridge, the crista transversa, into two parts-an upper or fossula superior and a lower or fossula inferior. The anterior part of the fossula superior is termed the area n. facialis and exhibits a single large opening, the commencement of the facial canal, for the transmission of the facial nerve. Its posterior part is named the area vestibularis superior, and is perforated by the nerves for the utricle and the ampullæ of the superior and lateral semicircular ducts. The anterior part of the fossula inferior is termed the area cochleæ, and consists of the canalis centralis and the surrounding tractus spiralis foraminosus, for the passage of the nerves to the cochlea. Behind the area cochleæ, and separated from it by a ridge, is the area vestibularis inferior, which is pierced by the nerves to the saccule; at the posterior part of the fossula inferior is the foramen singulare, which gives passage to the nerves for the ampulla of the posterior semicircular duct.
The membranous labyrinth (Fig. 720) is contained within the bony labyrinth, but does not nearly fill it. It contains a fluid termed endolymph, while the interval between it and the bony labyrinth is named the perilymphatic space, and is occupied by a fluid termed perilymph. The perilymphatic space in the vestibule is continuous behind with the perilymphatic space of the semicircular canals, and opens in front into the scala vestibuli. At the apex of the cochlea it is continuous, through the
helicotrema, with the scala tympani, which is shut off Ampulla of superior from the tympanic cavity
by the secondary tympanic
and lie along the inner surface of the outer walls of the corresponding bony tubes. The bony vestibule, on the other hand, contains two chief membranous structures, the utricle and saccule. The former receives the extremities of the ductus semicirculares, whilst the latter communicates with the ductus cochlearis. Moreover, the cavities of the utricle and saccule are indirectly connected, and thus all parts of the membranous labyrinth communicate with each other, and the endolymph is free to move from one portion to another. The vestibule contains also the ductus endolymphaticus and the commencement of the ductus cochlearis.
Ampulla of posterior duct
utriculosaccularis -Sinus inferior
FIG. 720.-DIAGRAMMATIC REPRESENTATION OF THE DIFFERENT
Utriculus. The utricle, the larger of the two sacs (Fig. 720), occupies the postero-superior portion of the vestibule. Its highest part, or recessus utriculi, lies in the recessus ellipticus and receives the ampullæ of the superior and lateral semicircular ducts. Its central part receives on its lateral aspect the non- ! ampullated end of the lateral semicircular duct, and is prolonged upwards and backwards as the sinus superior, into which the crus commune of the superior and posterior semicircular ducts open. The ampulla of the posterior semicircular duct opens into the lower and medial part, or sinus inferior. The floor and anterior wall of the recessus utriculi are thickened to form the macula acustica utriculi, to which the utricular fibres of the acoustic nerve are distributed. Whitish in colour, and of an oval or nearly rhombic shape, this macula measures 3 mm. in length and 2-3 mm. in its greatest breadth.
Sacculus. The saccule occupies the recessus sphæricus, in the lower and anterior part of the vestibule (Fig. 717). Smaller than the utricle, it is of an oval shape and measures 3 mm. in its longest, and about 2 mm. in its shortest diameter. It presents
anteriorly an oval, whitish thickening, the macula acustica sacculi. This has a breadth of about 15 mm., and to it the saccular fibres of the acoustic nerve are distributed. The superior extremity of the saccule is directed upwards and backwards, and forms the sinus utricularis sacculi, which abuts against, but does not fuse with, the wall of the utricle. From the lower part of the saccule a short canal, the ductus reuniens (Henseni), opens into the ductus cochlearis, a short distance in front of its vestibular extremity. A second small channel, the ductus endolymphaticus, is continued from the posterior part of the saccule, and, passing between the utricle and the medial wall of the vestibule, is joined by a small canal, the ductus utriculosaccularis, which arises from the medial side of the utricle. It then enters and traverses the aquæductus vestibuli and ends, under the dura mater on the posterior surface of the petrous part of the temporal bone, in a dilated blind extremity, termed the saccus endolymphaticus; this, according to Rüdinger, is perforated by minute foramina, through which the endolymph may pass into the meningeal lymphatics.
The vestibule also contains the vestibular extremity of the ductus cochlearis, which lies immediately below the saccule in the recessus cochlearis.
The walls of the utricle and saccule are composed of connective tissue which blends with the periosteal lining of the vestibule. It is modified medially to form a homogeneous membrana propria, which is covered with a layer of pavement epithelium and is thickened at the maculæ acusticæ. Towards the periphery of the maculæ the epithelium is cubical, while on them it is columnar.
The structure of the maculæ in the utricle and saccule is practically the same; two kinds of cells are found, viz., (a) supporting cells, and (b) hair cells. The supporting cells are somewhat fusiform, each containing, near its middle, a nucleus. Their branched, deep extremities are attached to the membrana propria; their free ends lie between the hair cells and form a thin inner limiting cuticle. The hair cells are flask-shaped and do not reach the membrana propria, but end in rounded extremities which lie between the supporting cells. Each contains, at its deepest part, a large nucleus, the rest of the cell being granular and pigmented. From the free end of each there projects a stiff, hair-like process, which, on the application of reagents, splits into several finer filaments. The nerve-fibres pierce the membrana propria, and ramify around the deep extremities of the hair cells (Fig. 722). A collection of small, rhombic crystals of carbonate of lime, termed otoconia, adheres to each of the macula.
propria Fibrous stratum
Lumen of semicircular duct
FIG. 721.-TRANSVERSE SECTION OF HUMAN SEMICIRCULAR
Ductus Semicirculares.-The semicircular ducts are elliptical on transverse section (Fig. 721), and are attached to the walls of the bony canals. The convex wall of each duct is fixed to the periosteal lining of the canal, whilst the opposite part is free, except that it is connected by irregular ligamentous bands, which pass through the perilymphatic space to the bony wall. Like the bony canals, each of the semicircular ducts is dilated at one extremity into a membranous ampulla,
which is especially developed towards the concavity of the tube. The membranous ampullæ nearly fill the corresponding portions of the bony tubes, but the diameter of the semicircular ducts is only about one-fourth of that of the osseous canals.
Each semicircular duct consists of three layers, viz.: (a) an outer vascular and partly pigmented fibrous stratum which fixes the duct to the bony wall;
Fibres of the ramus recessus utriculi
FIG. 722.-VERTICAL SECTION OF THE WALL OF THE RECESSUS UTRICULI WITH THE MACULA ACUSTICA AND THE BUNDLES OF NERVE FIBRES.
(b) an intermediate, transparent tunica propria, presenting a number of papilliform elevations which project towards the lumen. The fibrous layer and tunica propria are thinnest along the attached surface of the duct, and in this region also the papilliform elevations are absent; (c) an internal epithelial layer, composed of pavement cells. In the ampullæ the tunica propria is much thickened, and projects into the cavity as a transverse elevation, termed the septum transversum, which, when seen from above, is somewhat fiddle-shaped; its most prominent part is covered by acoustic epithelium forming the crista ampullaris, at each end of which is a half-moon-shaped border of small columnar cells, the planum semilunatum. The cells covering the crista ampullaris consist of supporting cells and hair cells, and are similar in their arrangement to those in the macula of the utricle and saccule; the hairs of the hair cells are, however, considerably longer, and project as far as the middle of the ampullary lumen. In fresh specimens they appear to end free, but in hardened preparations are seen to terminate in a soft, clear, dome-like structure, the cupula terminalis, which is striated, the striæ converging towards its concavity. The nerves form arborisations around the bases of the hair cells.
Cochlear nerve fibres
FIG. 723.-SECTION ACROSS THE DUCTUS COCHLEARIS (Retzius).
Ductus Cochlearis.-The ductus cochlearis (O.T. membranous cochlea or scala media) is closed at both of its extremities; the lower extremity occupies the recessus cochlearis of the vestibule and communicates with the saccule through the ductus reuniens. It forms a spirally arranged canal inside the cochlea, and at