1. Organ of smell. 1ST CRANIAL NERVE. ORGAN OF VISION. 2ND CRANIAL NERVE
2. eyeball. AUXILIARY ORGANS OF EYE: eyelids, Lacrimal apparatus. mUSCLES MOVING THE EYEBALL. 3RD, 4TH,6TH CRANIAL NERVES.
3. Organs of hearing: external ear. Middle ear. Tympanic cavity. Auditory tube. Internal ear. 8th cranial nerve.
Lesson No 20
1. Organ of smell. 1ST CRANIAL NERVE
Olfactory receptors placed in olfactory region of nasal cavity (in superior nasal meatus). Receptors (1st neuron) associated with epithelial supporting cells. The peripheral process of olfactory cells carry the olfactory cilia and the central process form 15-20 olfactory nerves (1st cranial nerve), which pass through the foramens in cribriform plate and reach the olfactory bulb.
Nerves of septum of nose. Right side.
The axons of 2d neurons runs through the olfactory tract terminate in olfactory triangle and anterior perforating substance, where the bodies of the 3d neurons lie.
Plan of olfactory neurons.
Axons of the 3d neurons get the uncus and other part of limbic system, which is cortical olfactory analyser.
INNERVATION OF THE NASAL CAVITY
Olfaction is mediated via the olfactory nerves. General sensation (touch, pain and temperature) from the nasal mucosa is carried by branches of the ophthalmic and maxillary divisions of the trigeminal nerves (Fig. 32.9). Trigeminal fibres close to, and within, the epithelial layer are sensitive to noxious chemicals, e.g. ammonia and sulphur dioxide. Autonomic fibres innervate mucous glands and control cyclical and reactive vasomotor activity.
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Fig. 32.9 The
innervation of the nasal cavity. A, Lateral wall of the left nasal cavity. B,
Medial wall of the left nasal cavity. |
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Trigeminal innervation
The anterior ethmoidal branch of the nasociliary nerve leaves the cranial cavity through a small slit near the crista galli and enters the roof of the nasal cavity where it runs in a groove on the inner surface of the nasal bone, supplying the roof of the nasal cavity. It gives off a lateral internal branch to supply the anterior part of the lateral wall and a medial internal branch to the anterior and upper parts of the septum, before emerging at the inferior margin of the nasal bone as the external nasal nerve to supply the skin of the external nose to the nasal tip. The infraorbital nerve supplies the nasal vestibule. The anterior superior alveolar nerve supplies part of the septum, the floor near the anterior nasal spine and the anterior part of the lateral wall as high as the opening of the maxillary sinus; the lateral posterior superior nasal and the posterior inferior nasal branches of the greater palatine nerve together supply the posterior three-quarters of the lateral wall, roof and floor; the medial posterior superior nasal nerves and the nasopalatine nerve supply the inferior part of the nasal septum; and branches from the nerve of the pterygoid canal supply the upper and posterior part of the roof and septum.
Autonomic innervation
Sympathetic postganglionic vasomotor fibres are distributed to the nasal blood vessels. Postganglionic parasympathetic fibres derived from the pterygopalatine ganglion provide the secretomotor supply to the nasal mucous glands, and are distributed via branches of the maxillary nerves
Olfactory nerves
Olfactory nerves are bundles of very small axons derived from olfactory receptor neurons in the olfactory mucosa. The axons are unmyelinated, and in varying stages of maturity, reflecting the constant turnover of olfactory neurones that takes place in the olfactory epithelium. Bundles of axons surrounded by olfactory ensheathing cells form a plexiform network in the subepithelial lamina propria of the mucosa. The bundles unite into as many as 20 branches that cross the cribriform plate in lateral and medial groups, and enter the overlying olfactory bulb where they end in glomeruli. Each branch is ensheathed by dura mater and pia-arachnoid as it passes through the cribriform plate. The dura subsequently becomes continuous with the nasal periosteum, and the pia-arachnoid merges with the connective tissue sheaths surrounding the nerve bundles, an arrangement that may favour the spread of infection into the cranial cavity from the nasal cavity.
In severe injuries involving the anterior cranial fossa, the olfactory bulb may be separated from the olfactory nerves or the nerves may be torn, producing anosmia, i.e. loss of olfaction. Fractures may involve the meninges, so that cerebrospinal fluid may leak into the nose resulting in cerebrospinal rhinorrhoea. Such injuries open up avenues for intracranial infection from the nasal cavity.
The organization of the olfactory system reflects its phylogenetically ancient lineage: afferent olfactory pathways proceed directly to the cerebral cortex, bypassing the thalamus, and its terminal fields are primitive cortical areas that are considered to be parts of the limbic system. Details of the relationship between the olfactory pathways and the limbic system are shown in Fig. 23.4.
The olfactory nerves arise from olfactory receptor neurones in the olfactory mucosa (see Ch. 32). The axons collect into numerous small bundles ensheathed by a population of unique glia and surrounded by layers of meninges, and enter the anterior cranial fossa by passing through the foramina in the cribriform plate of the ethmoid bone. They attach to the inferior surface of the olfactory bulb, which is situated at the anterior end of the olfactory sulcus on the orbital surface of the frontal lobe, and terminate in the bulb. Olfactory receptor neurones are continually replaced throughout life by differentiation of stem cells in the olfactory mucosa. The olfactory bulb is continuous posteriorly with the olfactory tract, through which the output of the bulb passes directly to the olfactory cortex.
There is a clear laminar structure in the olfactory bulb (Fig. 23.14). From the surface inwards the laminae are the olfactory nerve layer, glomerular layer, external plexiform layer, mitral cell layer, internal plexiform layer and granule cell layer. The olfactory nerve layer consists of the unmyelinated axons of the olfactory neurones. The continuous turnover of receptor cells means that axons in this layer are at different stages of growth, maturity or degeneration. The glomerular layer consists of a thin sheet of glomeruli where the incoming olfactory axons divide and synapse on terminal dendrites of secondary olfactory neurones, i.e. mitral, tufted and periglomerular cells. The external plexiform layer contains the principal and secondary dendrites of mitral and tufted cells. The mitral cell layer is a thin sheet composed of the cell bodies of mitral cells, each of which sends a single principal dendrite to a glomerulus, secondary dendrites to the external plexiform layer, and a single axon to the olfactory tract. It also contains a few granule cell bodies. The internal plexiform layer contains axons, recurrent and deep collaterals of mitral and tufted cells, and granule cell bodies. The granule cell layer contains the majority of the granule cells and their superficial and deep processes, together with numerous centripetal and centrifugal nerve fibres which pass through the layer.
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Fig. 23.14 Organization of the olfactory bulb. |
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The principal neurones in the olfactory bulb are the mitral and tufted cells: their axons form its output via the olfactory tract. These cells are morphologically similar and most use an excitatory amino acid, probably glutamate or aspartate, as their neurotransmitter. The mitral cell spans the layers of the bulb, and receives the sensory input superficially at its glomerular tuft. The axons of mitral and tufted cells appear to be parallel output pathways from the olfactory bulb.
The main types of interneurones in the olfactory bulb are the periglomerular cells and granule cells. The majority of periglomerular cells are dopaminergic (cell group A15), but some are GABAergic: their axons are distributed laterally and terminate within extraglomerular regions. Granule cells are similar in size to periglomerular cells. Their most characteristic feature is the absence of an axon, and they therefore resemble amacrine cells in the retina. Granule cells have two principal spine-bearing dendrites which pass radially in the bulb: they appear to be GABAergic. The granule cell is likely to be a powerful inhibitory influence on the output neurones of the olfactory bulb.
Centrifugal inputs to the olfactory bulb arise from a variety of central sites. Neurones of the anterior olfactory nucleus and collaterals of pyramidal neurones in the olfactory cortex project to the granule cells of the olfactory bulb. Cholinergic neurones in the horizontal limb nucleus of the diagonal band of Broca, part of the basal forebrain cholinergic system, project to the granule cell layer and also to the glomerular layer. Other afferents to the granule cell layer and the glomeruli arise from the pontine locus coeruleus and the mesencephalic raphe nucleus.
The granule cell layer of the bulb is extended into the olfactory tract as scattered medium-sized multipolar neurones that constitute the anterior olfactory nucleus. Many centripetal axons from mitral and tufted cells relay in, or give collaterals to, the anterior olfactory nucleus; the axons from the nucleus continue with the remaining direct fibres from the bulb into the olfactory striae.
As the olfactory tract approaches the anterior perforated substance it flattens and splays out as the olfactory trigone. Fibres of the tract continue from the caudal angles of the trigone as diverging medial and lateral olfactory striae, which border the anterior perforated substance. The lateral olfactory stria follows the anterolateral margin of the anterior perforated substance to the limen insulae, where it bends posteromedially to merge with an elevated region, the gyrus semilunaris, at the rostral margin of the uncus in the temporal lobe. The lateral olfactory gyrus forms a tenuous grey layer covering the lateral olfactory stria: it merges laterally with the gyrus ambiens, part of the limen insulae. The lateral olfactory gyrus and gyrus ambiens form the prepiriform region of the cortex, passing caudally into the entorhinal area of the parahippocampal gyrus. The prepiriform and periamygdaloid regions and the entorhinal area (area 28) together make up the piriform cortex. The medial olfactory stria passes medially along the rostral boundary of the anterior perforated substance towards the medial continuation of the diagonal band of Broca. Together, they curve up on the medial aspect of the hemisphere, anterior to the attachment of the lamina terminalis.
The olfactory cortex receives a direct input from the olfactory bulb which arrives via the olfactory tract without relay in the thalamus. The largest cortical olfactory area is the piriform cortex. The anterior olfactory nucleus, olfactory tubercle, regions of the entorhinal and insular cortex and amygdala also receive direct projections from the olfactory bulb.
The entorhinal cortex (Brodmann's area 28) is the most posterior part of the piriform cortex, and is divided into medial and lateral areas (areas 28a and 28b). The lateral parts receive fibres mainly from the olfactory bulb, and also from the piriform and periamygdaloid cortices.
Projections from the piriform olfactory cortex are widespread, and include the neocortex (especially the orbitofrontal cortex), thalamus (especially the medial dorsal thalamic nucleus), hypothalamus, amygdala and hippocampal formation.
NASAL AND OLFACTORY MUCOSAE
Nasal mucosa
The lining of the anterior part of the nasal cavity and vestibule is continuous with the skin, and consists of keratinized stratified squamous epithelium overlying a connective tissue lamina propria. Inferiorly the skin bears coarse hairs (vibrissae) which curve towards the naris and help to arrest the passage of particles in inspired air. In males, after middle age, these hairs increase considerably in size. Further posteriorly, at the limen nasi, this grades into a mucosa which is lined initially by non-keratinizing stratified squamous epithelium, and then by pseudostratified ciliated (respiratory) epithelium rich in goblet cells. Respiratory epithelium forms most of the surface of the nasal cavity, i.e. it covers the conchae, meati, septum, floor and roof, except superiorly in the olfactory cleft, where the olfactory epithelium is present. It is adherent to the periosteum or perichondrium of the neighbouring skeletal structures. In some areas, cells of the respiratory epithelium may be low columnar or cuboidal, and the proportion of ciliated to non-ciliated cells is variable.
There are numerous seromucous glands within the lamina propria of the nasal mucosa. Their secretions make the surface sticky so that it traps particles in the inspired air. The mucous film is continually moved by ciliary action (the mucociliary escalator or rejection current) posteriorly into the nasopharynx at a rate of 6 mm per minute. Palatal movements transfer the mucus and its entrapped particles to the oropharynx for swallowing, but some also enters the nasal vestibule anteriorly. The secretions of the nasal mucosa contain the bacteriocides lysozyme, β-defensin and lactoferrin, and also secretory immunoglobulins (IgA). The mucosa is continuous with the nasopharyngeal mucosa through the posterior nasal apertures, the conjunctiva through the nasolacrimal duct and lacrimal canaliculi, and the mucosa of the sphenoidal, ethmoidal, frontal and maxillary sinuses through their openings into the meati.
The mucosa is thickest and most vascular over the conchae, especially at their extremities, and also on the anterior and posterior parts of the nasal septum and between the conchae. The mucosa is very thin in the meati, on the nasal floor and in the paranasal sinuses. Its thickness reduces the volume of the nasal cavity and its apertures significantly. The lamina propria contains cavernous vascular tissue with large venous sinusoids.
Olfactory mucosa
Olfactory mucosa (Fig. 32.6) covers approximately 5 cm2 of the posterior upper parts of the lateral nasal wall, including the upper part of the vertical portion of the middle concha (where it is interspersed with respiratory epithelium in a checkerboard fashion) and the opposite part of the nasal septum, the superior concha, the sphenoethmoidal recess, the upper part of the perpendicular plate of the ethmoid and the portion of the roof of the nose that arches between the septum and lateral wall, including the underside of the cribriform plate (constituting the olfactory cleft or groove). It consists of a yellowish brown pigmented pseudostratified epithelium, containing olfactory receptor neurones, sustentacular cells and two classes of basal cell, lying on a subepithelial lamina propria containing subepithelial olfactory glands (of Bowman) and bundles of axons derived from the olfactory receptor neurones which course through the mucosa on their way to the cribriform plate. The glands secrete a predominantly serous fluid through ducts which open onto the epithelial surface. These secretions form a thin fluid layer in which sensory cilia and the microvilli of the sustentacular cells are embedded.
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Fig. 32.6 A, The chief cytological features of the
olfactory epithelium. Receptor cells (neurones) (R) are situated among
columnar sustentacular cells. The axons of the receptor cells emerge from the
epithelium in bundles enclosed by ensheathing glial cells (G). Rounded
globose basal cells (B) and flattened horizontal basal cells (not shown) lie
on the basal lamina and the subepithelial glands (of Bowman) (S) open on to
the surface via their intraepithelial ducts (I). At the surface are cilia of
the receptor cells and microvilli of the supporting cells. B, C and D are longitudinal
sections through human olfactory epithelium: B, Ciliated olfactory receptor
neurones with characteristic expanded ends (N) project into the nasal lumen.
M, microvillar cell; S, supporting or sustentacular cells containing electron
dense material; B, basal cells resting upon the basal lamina. The edge of a
Bowman's gland (BG) lies deeper in the lamina propria. C, Higher power view
of the expanded end of an olfactory receptor neurone. The electron dense,
osmiophilic material within the adjacent supporting cell (S) is thought to
contribute to the pigmentation of the olfactory epithelium. C, olfactory
cilium; B, basal body with projecting ‘feet'. D, Section of human olfactory
epithelium immunostained with anti-OMP (olfactory marker protein): an immunopositive
olfactory receptor neurone lies between two unstained supporting cells (S).
V, microvilli. |
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Olfactory receptor neurones
Olfactory receptor neurones are bipolar. Their cell bodies and nuclei are located in the middle zone of the olfactory epithelium. Each neurone has a single unbranched apical dendrite, 2 μm diameter, which extends to the epithelial surface, and a basally directed unmyelinated axon, 0.2 μm diameter, which passes in the opposite direction, penetrates the basal lamina and enters the lamina propria. The tips of the dendrites project into the overlying secretory fluid and are expanded into characteristic endings (knobs) (Fig. 32.6B). Groups of up to 20 cilia radiate from the circumference of each ending and extend for long distances parallel to the epithelial surface. Internally, the short proximal part of each cilium has the ‘9 + 2′ pattern of microtubules typical of motile cilia, while the longer distal trailing end contains only the central pair of microtubules. The olfactory cilia lack dynein arms and are thought to be non-motile; their primary purpose is to increase the surface area of sensory receptor membrane available for the efficient detection of odorant molecules transferred across the mucous layer by odorant binding proteins. Mature olfactory neurones express olfactory marker protein (OMP), an abundant cytoplasmic protein involved in olfactory signal transduction (Fig. 32.6D). Each olfactory receptor neurone expresses receptors for a single (or very few) odorant molecules. In humans, over 1000 genes code for functional odorant receptors; the number of functional genes is much higher in macrosmotic animals (Buck & Axel 1991). Although neurones with the same receptor specificity are randomly distributed within anatomical zones of the epithelium, their axons all converge on the same glomerulus in the olfactory bulb. Specific odours activate a unique spectrum of receptor neurones which in turn activate restricted groups of glomeruli and their second order neurones.
The axons form small intraepithelial fascicles among the processes of sustentacular and basal cells. The fascicles penetrate the basal lamina, and are immediately surrounded by olfactory ensheathing cells. Groups of up to 50 such fascicles join to form larger olfactory nerve rootlets which pass through the cribriform plate of the ethmoid bone, wrapped in meningeal sheaths. They immediately enter the overlying olfactory bulbs, where they synapse in glomeruli with mitral cells and, to a lesser extent, with smaller tufted cells.
Microvillar cells
Microvillar cells occupy a superficial position in the olfactory epithelium. They are flask-shaped and electron-lucent, and the apical end of each cell gives rise to a tuft of microvilli that project into the mucus layer lining the nasal cavity (Fig. 32.6B). Cell counts in longitudinal sections reveal that microvillar cells occur with a density that is approximately one tenth of the density of ciliated olfactory neurones: their function and origin has yet to be determined.
Sustentacular cells
Sustentacular, or supporting, cells are columnar cells that separate and partially ensheathe the olfactory receptor neurones. Their large nuclei form a layer superficial to the neuronal nuclei within the epithelium. The cells are capped by numerous long, irregular, microvilli which lie in the secretory fluid layer that covers the surface of the epithelium, intermingled with the trailing ends of the cilia on the olfactory receptor endings. Their expanded bases contain numerous lamellated dense bodies, which are the remnants of secondary lysosomes, and which contribute significantly to the pigmentation of the olfactory area (Fig. 32.6B,C). The granules gradually accumulate with age, and because these cells are long-lived, the intensity of pigmentation also increases with age. Neighbouring sustentacular cells are linked by desmosomes close to the epithelial surface, an arrangement that helps to stabilize the epithelium mechanically. Sustentacular cells and olfactory receptor neurones are linked by tight junctions at the level of the epithelial surface.
Basal cells
There are horizontal and globose basal cells. Horizontal basal cells are flattened against the basal lamina. Their nuclei are condensed and their darkly staining cytoplasm contains numerous intermediate filaments of the cytokeratin family, inserted into desmosomes between the basal cells and surrounding sustentacular cells. Globose cells are rounded or elliptical in shape, and have pale, euchromatic nuclei, and pale cytoplasm. They form a distinct zone that is slightly internal to the basal surface of the epithelium and characterized by mitotic figures: globose basal cells are the immediate source of new olfactory receptor neurones.
Olfactory ensheathing cells
Olfactory ensheathing cells share properties with astrocytes and non-myelinating Schwann cells, but also possess distinctive features that indicate they are a separate class of glia. Developmentally they are derived from the olfactory placode rather than the neural crest. They ensheath olfactory axons in a unique manner throughout their entire course, and accompany them into the olfactory bulb, where they contribute to the glia limitans. In recent years, olfactory ensheathing cells have been the focus of intense experimental scrutiny in the search for a source of transplantable glia capable of supporting neuronal regeneration within the CNS, possibly in the treatment of paraplegia.
Olfactory glands
Olfactory (Bowman's) glands are branched tubuloalveolar structures that lie beneath the olfactory epithelium and secrete their products onto the epithelial surface through narrow, vertical ducts. Their secretions, which include defensive substances, lysozyme, lactoferrin, IgA and sulphated proteoglycans, together with odorant-binding proteins which increase the efficiency of odour detection, bathe the dendritic endings and cilia of the olfactory receptors. The fluid acts as a solvent for odorant molecules, allowing their diffusion to the sensory receptors.
Turnover of olfactory receptor neurones
Olfactory receptor neurones are lost and replaced throughout life. Individual receptor cells have a variable lifespan, thought to average 1–3 months. Stem cells situated near the base of the epithelium undergo periodic mitotic division throughout life, giving rise to new olfactory receptor neurones which then grow a dendrite to the olfactory surface and an axon to the olfactory bulb. The cell bodies of these new receptor neurones gradually move apically until they reach the region just below the supporting cell nuclei. When they degenerate, dead neurones are either shed from the epithelium or are phagocytosed by sustentacular cells. The rate of receptor cell loss and replacement increases after exposure to damaging stimuli, but declines slowly with age, a phenomenon that presumably contributes to diminishing olfactory sensory function in old age. Biopsy specimens from normosmic adults have revealed that patchy replacement of olfactory with respiratory epithelium occurs even in young healthy adults (Paik et al 1992, Holbrook et al 2005)
Theme 2. ORGAN OF VISION. 2ND CRANIAL NERVE
RETINA
The retina is a thin sheet of cells, ranging from less than 100 μm at its edge, to a maximum around 300 μm at the foveal rim. It lines the inner posterior surface of the eyeball, sandwiched between the choroid externally and the vitreous body internally, and terminates anteriorly at the ora serrata (Fig. 40.1).
When viewed with an ophthalmoscope to show the fundus oculi, the most prominent feature is the blood vessels emanating from and entering the optic disc (Fig. 40.20). Centred temporal and inferior to the disc lies the ‘central retina’ or macula (approximate diameter 5–6 mm), the middle of which is composed of the fovea and foveola, and easily identified with an ophthalmoscope as an avascular area with a yellow tinge (Fig. 40.20). The lack of blood vessels at the foveola is even more apparent in a fluorescein angiogram (Fig. 40.21). The peripheral retina lies outside the central retina.
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Fig. 40.20 Fundus photograph of the right eye of a 26-year-old Caucasian male. The central retinal vessels are seen emanating from the optic disc. Retinal arteries are lighter in colour and narrower than the veins. The avascular centre of the macular region can be seen temporal to the disc. The 6.25D of myopia of the subject is consistent with the peripapillary atrophy temporal to the optic nerve head and the visibility of the choroidal vessels. |
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Fig. 40.21 Fluorescein angiogram showing the macular region of a right eye. The main macular vessels are approaching from the right. The subject was an elderly person with considerable macular pigmentation, which masks fluorescence from the choroidal circulation. |
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MICROSTRUCTURE
The retina is composed of a variety of epithelial, neural and glial cell types whose distribution conventionally divides it into 10 layers (Fig. 40.22). These are usually apparent in conventional histological sections (Fig. 40.23), but can also be seen in vivo using techniques such as optical coherence tomography, which uses backscattered light to visualize layers by differences in their optical scattering properties (Fig. 40.24). Embryologically, the retina is derived from the two layers of the invaginated optic vesicle. The outer layer becomes a layer of cuboidal pigment cells which separates the choroidal lamina vitrea from the neural retina, and therefore forms the outermost layer of the retina, the retinal pigment epithelium (RPE – layer 1). The other nine strata of the retina develop from the inner layer of the optic vesicle and form the neural retina.
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Fig. 40.22 Neural cells whose cell bodies and interconnections account for the layered appearance of the retina in histological section (compare with Fig. 40.23). Also shown are the two principal types of neuroglial cell in the retina (although microglia are also present they are not shown). |
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Fig. 40.24 High
definition optical coherence tomography (OCT) in vivo image of the human
retina. The image has approximately 2 μm axial
resolution, is 8 mm long and consists of 10,000 axial scans. NFL, Nerve fibre
layer. GCL, Ganglion cell layer. IPL, Inner plexiform layer. INL, Inner
nuclear layer. OPL, Outer plexiform layer. ONL, Outer nuclear layer. ELM,
External limiting membrane. IS/OS, Boundary between the photoreceptor inner
and outer segments. RPE/CH, Retinal pigment epithelium and choriocapillaris.
The image is expanded in the vertical direction to permit better visualization
of retinal layers. |
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The outermost layer of the neural retina contains the light sensitive parts of the photoreceptors, which convert the optical image into neural activity. From the photoreceptors, neural activity flows radially to bipolar and ganglion cells, and laterally via horizontal cells in the outer retina and amacrine cells in the inner retina. Photoreceptors synaptically contact each other and bipolar and horizontal cells in the outer plexiform layer (OPL – layer 5), while bipolar, amacrine and ganglion cells synapse in the inner plexiform layer (IPL – layer 7). The axons of ganglion cells run towards the optic disc in the nerve fibre layer (NFL – layer 9), where they leave the retina as the optic nerve, which transmits the retinal output to the visual areas of the brain where visual processing is completed. Although most neural activity flows from the photoreceptors towards the brain, some information flow occurs in the opposite direction via centripetal fibres in the optic nerve and interplexiform cells in the retina which connect the inner and outer plexiform layers.
The classic ten layered appearance of the retina is absent in the optic nerve head, the fovea and foveola, and the ora serrata. At the optic nerve head, the axons of the retinal ganglion cells leave the retina to form the optic nerve and all the other neural cell types of the retina are missing. At the fovea and foveola, the inner five layers of the retina are ‘pushed aside'. At the ora serrata, where the retina borders the ciliary body (Fig. 40.9), the retinal pigment epithelium merges with the outer pigmented epithelium of the ciliary body, while the neural retina borders the inner unpigmented ciliary epithelium: the retina is thinnest at this point. The normal layered arrangement of the neural retina approaching the ora serrata is frequently disrupted by cysts in older individuals (Fig. 40.25).
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Fig. 40.25 Junction between the retina and ciliary body (ora serrata). The retinal pigment epithelium is continuous with the outer pigmented epithelium of the ciliary body, while the neural retina abuts the inner unpigmented epithelium of the ciliary body. The layered appearance that is apparent elsewhere in the neural retina is disrupted adjacent to the ora serrata by cystic degeneration. |
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Cells of the retina
Retinal pigment epithelium
The retinal pigment epithelium, RPE, is composed of approximately cuboidal cells that form a single continuous layer extending from the periphery of the optic disc to the ora serrata, where it continues as the outer ciliary epithelium. The cells are flat in radial section and hexagonal or pentagonal in surface view, and number 4–6 million in the human retina. Their cytoplasm contains numerous melanosomes. Apically (towards the rods and cones), the cells bear long (5–7 μm) microvilli which contact, or project between, the outer segments of rods and cones. The tips of rod outer segments are deeply inserted into invaginations in the apical membrane of the RPE. The different embryological origins of the RPE and neural retina mean that the attachments between these two layers are unsupported by junctional complexes; the neural retina and RPE are therefore easily parted in the clinical condition of retinal detachment arising from trauma or disease.
RPE cells play a major role in the turnover of rod and cone photoreceptive components. Their cytoplasm contains the phagocytosed tips of rods and cones undergoing lysosomal destruction. The final products of this process are lipofuscin granules, which accumulate in these cells with age.
Light reaching the outer retina but missing the photoreceptors is absorbed by the RPE, which, like melanin elsewhere in the eye, prevents such stray light degrading image quality. The zone of tight junctions between adjacent cells also allows the epithelium to function as an important blood-retinal barrier between the retina and the vascular system of the choroid. The RPE is required for the regeneration of bleached visual pigment and may have antioxidant properties. It also secretes a variety of growth factors necessary for the integrity of the choriocapillaris endothelium and the photoreceptors, and produces a number of immunosuppressive factors. A failure of any of the diverse functions of the RPE could result in compromised retinal function and eventual blindness (Strauss 2005).
Rods and cones are the ‘image forming’ photoreceptors of the outer retina and function at low (scotopic) and higher (photopic) light levels respectively. Both are long, radially orientated structures with a similar organization, although details differ (Fig. 40.26). From the choroidal end inwards, the cells consist of outer and inner segments connected by a thin connecting cilium (together making up layer 2 of the retina), a cell body containing the nucleus, and a synaptic terminal (either a more complex pedicle for cones or a simpler rod spherule) where they make synaptic connections with adjacent bipolar and horizontal cells and with other cone or rod cells within the OPL.
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Fig. 40.26 The major features of a retinal rod cell (A) and a retinal cone cell (B). The relative size of the pigment epithelial cells has been exaggerated for illustrative purposes. |
OPTIC NERVE HEAD
The axons of more than a million retinal ganglion cells converge at the optic nerve head (ONH) and leave the eye by penetrating the sclera to form the optic nerve (ON). The ONH represents that part of the optic nerve lying within the bulb of the eye. Since all retinal neural elements, apart from ganglion cell axons, are absent from this region it is insensitive to light and forms the ‘blind spot'.
Histologically the ONH can be divided into three zones (Fig. 40.32). These are prelaminar (the anterior part terminating at the vitreous); laminar (formed by the lamina cribrosa); postlaminar (continuous with the retrobulbar optic nerve). The surface view of the ONH, usually seen with an ophthalmoscope, is referred to as the optic disc.
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Fig. 40.32 The optic nerve head, showing the distribution of
collagenous tissue (grey) and neuroglial nuclei (solid blue circles). 1a,
retinal internal limiting membrane; 1b, inner limiting membrane of Elschnig;
2, central meniscus of Kuhnt; 3, spur of collagenous tissue separating the
anterior lamina cribrosa (6) from the choroid; 4, border tissue of Jacoby; 5,
intermediary tissue of Kuhnt; 7, posterior lamina cribrosa; Sep, connective
tissue septa from pia mater; Gl. M, astroglial membrane; Gl. C, astrocytes
and oligodendrocytes among the fibres in their fascicles; Du, Ar, Pia: dura,
arachnoid and pia mater, respectively. The dotted lines represent the borders
of the lamina cribrosa. |
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Prelaminar zone
The inner surface of the ONH is covered by an astroglial membrane (of Elschnig) that is continuous with the ILM of the retina. At the centre of the disc, the layer of astrocytes thickens into a central meniscus (of Kuhnt). Retinal ganglion cells turn into the ONH accompanied by astrocytes, which gradually increase in number posteriorly, eventually forming a sieve-like structure, the glial lamina cribrosa, through which the nerve fibres pass as separate fasciculi. At the perimeter of the ONH, a collar of astrocytes several cells thick (the intermediary tissue of Kuhnt) separates the ON from the terminating outer layers of the retina. This layer continues posteriorly and forms a barrier between the ONH and the choroid (the border tissue of Jacoby) (Fig. 40.32).
The lamina cribrosa is composed of discrete trabeculae of collagenous and elastic connective tissue which extend from the sclera to form a meshwork through which the optic nerve fascicles and central retinal vessels pass. Each trabecula has a lining of astrocytes which are continuous with those of the glial lamina cribrosa.
Postlaminar zone
The ON thickens in the postlaminar zone as its axons become myelinated. The reflected sclera, and the dura mater with which it is continuous, invests the nerve together with the other two meningeal sheaths, the arachnoid and pia mater. Fine fibrous septa penetrate the optic nerve from the pia mater dividing it into 300–400 fascicles, giving pial blood vessels access to the nerve.
Optic disc
As it is visible by ophthalmoscopy (Fig. 40.20), the disc is a region of much clinical importance. Oedema of the disc (papilloedema) may be the first sign of raised intracranial pressure, which is transmitted into the subarachnoid space around the ON. The disc is also sensitive to the raised intra-ocular pressure which occurs in glaucoma and shows characteristic structural changes due to retinal ganglion cell loss.
The optic disc is superomedial to the posterior pole of the eye, and so lies away from the visual axis. It is round or oval, usually approximately 1.6 mm in transverse diameter and 1.8 mm in vertical diameter, and its appearance is very variable. In light-skinned subjects, the general retinal hue is a bright terracotta-red, with which the pale pink of the disc contrasts sharply; its central part is usually even paler and may be light grey. These differences are due in part to the degree of vascularization of the two regions, which is much less at the optic disc, and also to the total absence of choroidal or retinal pigment cells. In subjects with strongly melanized skins, both retina and disc are darker. The optic disc rarely projects sufficiently to justify the term papilla, although it is usually a little elevated on its lateral side, where the papillomacular nerve fibres turn into the optic nerve (Fig. 40.28). There is usually a slight depression where the retinal vessels traverse its centre.
Vascular supply
The blood supply to the three regions of the ONH differs. The prelaminar region is supplied mainly by branches of the central retinal artery (Fig. 40.31). Branches from the short posterior ciliary arteries form an often incomplete circle within the sclera around the ONH (circle of Zinn/Haller); centripetal branches from this structure supply the laminar region of the ONH. The short posterior ciliary arteries may also give off centripetal branches directly to supply the lamina, and branches that pass anteriorly to augment the prelaminar blood supply (Fig. 40.31). In the postlaminar region, arteries from the prepapillary choroid and circle of Zinn pass retrogradely as pial vessels, providing centripetal branches that supply the optic nerve. More posteriorly, the optic nerve receives pial arterioles directly from the posterior ciliary arteries. The central retinal artery may also contribute some centrifugal branches in this region.
The central retinal vein drains the ONH at all levels; other drainage pathways are minor
VISUAL PATHWAY
The visual pathway includes the interneurones of the retina, retinal ganglion cells whose axons project via the optic nerve, chiasma, and optic tract to the lateral geniculate nucleus (LGN) and neurones within the LGN which project via the optic radiation to the primary visual cortex (Fig. 40.33).
The retina can be divided by a horizontal line bisecting the fovea. Axons arising from the nasal half of this line within each retina cross in the chiasma to enter the contralateral optic tract. Fibres from the temporal hemiretinas do not cross in the chiasma. Upper and lower temporal fibres pass laterally in the chiasma and shift respectively to medial and inferolateral positions in the ipsilateral optic tract. They are joined by the crossed fibres, the upper and lower nasal quadrants sharing the same positions as their uncrossed counterparts. Thus, each tract carries a binocular representation of the contralateral half fields as shown in Figure 40.33. It is important to remember that visual space is optically inverted by the crystalline lens when relating the spatial location of neurones within the visual pathway to corresponding visual field locations.
The LGN contains cells arranged in six laminae. Each layer receives input from either crossed or uncrossed projections from the retina. The contralateral nasal retina projects to laminae 1, 4, and 6, whereas the ipsilateral temporal retina projects to layers 2, 3 and 5. Layers 1 and 2 contain magnocellular cells, the remaining layers are parvocellular. There is a point-to-point retinotopic arrangement between corresponding points in each hemiretina so that the contralateral visual field is mapped within each lateral geniculate nucleus.
Axons from the LGN run in the retrolenticular part of the internal capsule and form the optic radiation. This curves dorsomedially to the primary visual cortex, located around and within the depths of the calcarine sulcus in the occipital lobe (also known as the striate cortex, Brodmann area 17, or V1; see Ch. 23). The visual cortex also has a strict retinotopic organization. Fibres representing the lower half of the visual field sweep superiorly to reach the visual cortex above the calcarine sulcus, while those representing the upper half of the visual field curve inferiorly into the temporal lobe (Meyer's loop) before reaching the visual cortex below the calcarine sulcus. The periphery of the retina is represented anteriorly within the visual cortex, and the macula is represented towards the posterior pole, occupying a disproportionately large area that reflects the high number of foveal retinal ganglion cells that subserve the enhanced acuity of this region.
The primary visual cortex is connected to pre-striate and other cortical regions where further processing of visual stimuli occurs (see Ch. 23).
Almost all of the RGC axons (90%) terminate on neurones in the LGN. Extra-geniculate axons (10%) leave the optic tract before the LGN: they may leave the optic chiasma dorsally and project to the suprachiasmatic nucleus of the hypothalamus, others branch off the optic tract at the superior brachium and project to the superior colliculus, pretectal areas, and inferior pulvinar.
The basis for clinical assessment of damage to the visual pathway is an understanding of the retinotopic projections within the pathway. Moreover, plotting visual field loss frequently reveals the approximate location of the causative lesion and sometimes its nature (Fig. 40.33). Since retinal lesions can be visualized with an ophthalmoscope, field testing might appear to be redundant for such defects, but visual field measurement is still helpful in assessing the extent of the damage and may be the key factor in confirming a diagnosis. Glaucoma serves as an example. Field defects in glaucoma, occurring as a consequence of damage to the nerve fibre bundles at the optic nerve head, may be detectable ophthalmoscopically, but confirmation of the diagnosis frequently depends on field assessment. Early defects consist of one or more areas of paracentral focal field loss, progressing to arcuate scotomas. The shape of the defect corresponds to the anatomical arrangement of ganglion cell axons (Fig. 40.28).
So far as the location of lesions central to the retina is concerned, deficits in the vision of one eye are usually attributable to optic nerve lesions. Lesions of the optic chiasma, involving crossing nerve fibres, produce a bilateral field loss as exemplified by a pituitary adenoma (Fig. 40.33). The tumour expands upwards from the pituitary fossa, compressing the inferior midline of the chiasma, and eventually produces bitemporal hemianopia, starting with an early loss in the upper temporal quadrants. Field defects in the rare instances of optic tract lesions are distinctive. The tract contains contralateral nasal and ipsilateral temporal retinal projections and damage will cause a homonymous contralateral loss of field with substantial incongruity (dissimilar defects in the two fields). Incongruity probably results from a delay in achieving coincidence between retinal topographical projections of the two inputs of the visual pathway, as contiguous projections adjust their location, gradually achieving coincidence. It also probably reflects the reorganization of fibres which occurs normally in the optic tracts, as some fibres leave the tract in the superior brachium and others progress to the lateral geniculate nucleus. Incongruity is most marked in defects of the optic tract, less obvious in optic radiation defects, and is usually absent in cortically induced field defects, thus providing an additional clue in assessing location of the cause.
Lesions of the optic radiations are usually unilateral, and commonly vascular in origin. Field defects therefore develop abruptly, in contrast to the slow progression of defects associated with tumours, and the resulting hemifield loss follows the general rule that visual field defects central to the chiasma are on the opposite side to the lesion. Little or no incongruity is seen in visual cortical lesions, but they commonly display the phenomenon of macular sparing, the central 5–10° field being retained in an otherwise hemianopic defect.
Eye consists of eyeball, auxiliary eye organs and the optic nerve. Eyeball has nucleus and wall. Auxiliary eye apparatus includes eyelids, muscles of eyeball, lacrimal apparatus, orbital fasciae, vessels and nerves.
Eyeball wall consists of three coats: fibrous (external), vascular (middle) and internal (retina).
The crystalline lens, hardened and divided.
Internal coat of eyeball – ‘retina’. There are external pigmental layer and internal nervous layer in visual part of the retina. According to function they distinguish posterior larger visual part of retina, which contains rods and cones, and lesser blind part of retina. There are neither rods nor cones in blind part.
The sclera and choroid are derived from the mesoderm surrounding the optic cup.
Teme 3. eyeball. AUXILIARY ORGANS OF EYE: eyelids, Lacrimal apparatus. Extraocular mUSCLES
3RD, 4TH,6TH CRANIAL NERVES
The lacrimal apparatus. Right side.
There is ciliary muscle
in ciliary body, its contraction provides eye accommodation.
The refracting media are three, viz.:
Aqueous humor.
Vitreous body.
Crystalline lens.
The Aqueous Humor (humor aqueus).—The aqueous humor fills the anterior and posterior chambers of the eyeball. It is small in quantity, has an alkaline reaction, and consists mainly of water, less than one-fiftieth of its weight being solid matter, chiefly chloride of sodium.
The
Vitreous Body (corpus vitreum).—The vitreous body forms about
four-fifths of the bulb of the eye. It fills the concavity of the retina, and
is hollowed in front, forming a deep concavity, the hyaloid fossa, for
the reception of the lens. It is transparent, of the consistence of thin jelly,
and is composed of an albuminous fluid enclosed in a delicate transparent
membrane, the hyaloid membrane. It has been supposed, by
The hyaloid
membrane envelopes the vitreous body. The portion in front of the ora
serrata is thickened by the accession of radial fibers and is termed the zonula
ciliaris (zonule of Zinn). Here it presents a series of radially
arranged furrows, in which the ciliary processes are accommodated and to which
they adhere, as is shown by the fact that when they are removed some of their
pigment remains attached to the zonula. The zonula ciliaris splits into two
layers, one of which is thin and lines the hyaloid fossa; the other is named
the suspensory ligament of the lens: it is thicker, and passes over the
ciliary body to be attached to the capsule of the lens a short distance in
front of its equator. Scattered and delicate fibers are also attached to the
region of the equator itself. This ligament retains the lens in position, and
is relaxed by the contraction of the meridional fibers of the Ciliaris muscle,
so that the lens is allowed to become more convex. Behind the suspensory
ligament there is a sacculated canal, the spatia zonularis (
No bloodvessels penetrate the vitreous body, so that its nutrition must be carried on by vessels of the retina and ciliary processes, situated upon its exterior.
The Crystalline Lens (lens crystallina).—The crystalline lens, enclosed in its capsule, is situated immediately behind the iris, in front of the vitreous body, and encircled by the ciliary processes, which slightly overlap its margin.
The capsule of the lens (capsula lentis) is a transparent, structureless membrane which closely surrounds the lens, and is thicker in front than behind. It is brittle but highly elastic, and when ruptured the edges roll up with the outer surface innermost. It rests, behind, in the hyaloid fossa in the forepart of the vitreous body; in front, it is in contact with the free border of the iris, but recedes from it at the circumference, thus forming the posterior chamber of the eye; it is retained in its position chiefly by the suspensory ligament of the lens, already described.
The lens is a transparent, biconvex body, the convexity of its anterior being less than that of its posterior surface. The central points of these surfaces are termed respectively the anterior and posterior poles; a line connecting the poles constitutes the axis of the lens, while the marginal circumference is termed the equator.
Structure.—The lens is made up of soft cortical substance and a firm, central part, the nucleus (884). Faint lines (radii lentis) radiate from the poles to the equator. In the adult there may be six or more of these lines, but in the fetus they are only three in number and diverge from each other at angles of 120° (885); on the anterior surface one line ascends vertically and the other two diverge downward; on the posterior surface one ray descends vertically and the other two diverge upward. They correspond with the free edges of an equal number of septa composed of an amorphous substance, which dip into the substance of the lens. When the lens has been hardened it is seen to consist of a series of concentrically arranged laminæ, each of which is interrupted at the septa referred to. Each lamina is built up of a number of hexagonal, ribbon-like lens fibers, the edges of which are more or less serrated—the serrations fitting between those of neighboring fibers, while the ends of the fibers come into apposition at the septa. The fibers run in a curved manner from the septa on the anterior surface to those on the posterior surface. No fibers pass from pole to pole; they are arranged in such a way that those which begin near the pole on one surface of the lens end near the peripheral extremity of the plane on the other, and vice versa. The fibers of the outer layers of the lens are nucleated, and together form a nuclear layer, most distinct toward the equator. The anterior surface of the lens is covered by a layer of transparent, columnar, nucleated epithelium. At the equator the cells become elongated, and their gradual transition into lens fibers can be traced (887).
In the fetus, the lens is nearly spherical, and has a slightly reddish tint; it is soft and breaks down readily on the slightest pressure. A small branch from the arteria centralis retinæ runs forward, as already mentioned, through the vitreous body to the posterior part of the capsule of the lens, where its branches radiate and form a plexiform network, which covers the posterior surface of the capsule, and they are continuous around the margin of the capsule with the vessels of the pupillary membrane, and with those of the iris. In the adult, the lens is colorless, transparent, firm in texture, and devoid of vessels. In old age it becomes flattened on both surfaces, slightly opaque, of an amber tint, and increased in density (886).
Vessels and Nerves.—The arteries of the bulb of the eye are the long, short, and anterior ciliary arteries, and the arteria centralis retinæ. They have already been described (see p. 571).
The ciliary veins are seen on the outer surface of the choroid, and are named, from their arrangement, the venæ vorticosæ; they converge to four or five equidistant trunks which pierce the sclera midway between the sclero-corneal junction and the porus opticus. Another set of veins accompanies the anterior ciliary arteries. All of these veins open into the ophthalmic veins.
The ciliary nerves are derived from the nasociliary nerve and from the ciliary ganglion.
Additional eye structures include: extrinsic muscles of eyeball, eyebrows, eyelids, conjuctiva, lacrimal apparatus.
Extraocular Musculature
Levator palpebrae superioris
•Origin: inferior aspect of the lesser wing of sphenoid (adjacent to the common annular tendon) •Insertion:
1.medial and lateral walls of the orbit 2.superior tarsus
•Action: elevates the eyelid •Blood: branches of ophthalmic artery •Nerve: oculomotor nerve (III cranial)
Lateral rectus
•Origin:
1.common annular tendon (which comes off the body and lesser wing of sphenoid) 2.margins of the optic canal
•Insert: posterior to the sclerocorneal junction (each muscle inserting along its own directional axis) •Action: abducts eye •Blood: branches of ophthalmic artery •Nerve: abducens nerve (VI cranial)
Medial rectus
•Origin:
1.common annular tendon (which comes off the body and lesser wing of sphenoid) 2.margins of the optic canal
•Insert: posterior to the sclerocorneal junction (each muscle inserting along its own directional axis) •Action: adducts eye •Blood: branches of ophthalmic artery •Nerve: oculomotor nerve (III cranial)
Superior rectus
•Origin:
1.common annular tendon (which comes off the body and lesser wing of sphenoid) 2.margins of the optic canal
•Insert: posterior to the sclerocorneal junction (each muscle inserting along its own directional axis) •Action:
1.elevates 2.medially rotates 3.adducts the eye
•Blood: branches of ophthalmic artery •Nerve: oculomotor nerve (III cranial)
Superior rectus
•Origin:
1.common annular tendon (which comes off the body and lesser wing of sphenoid) 2.margins of the optic canal
•Insert: posterior to the sclerocorneal junction (each muscle inserting along its own directional axis) •Action:
1.elevates 2.medially rotates 3.adducts the eye
•Blood: branches of ophthalmic artery •Nerve: oculomotor nerve (III cranial)
Inferior rectus
•Origin:
1.common annular tendon (which comes off the body and lesser wing of sphenoid) 2.margins of the optic canal
•Insert: posterior to the sclerocorneal junction (each muscle inserting along its own directional axis) •Action:
1.depress 2.laterally rotates 3.adducts the eye
•Blood: branches of ophthalmic artery •Nerve: oculomotor nerve (III cranial)
Superior oblique
•Origin: body of sphenoid •Insert: upper lateral quadrant of the posterior half of the sclera (via the trochlea, as a pulley) •Action:
1.depress 2.medially rotates 3.abducts the eye
•Blood: branches of ophthalmic artery •Nerve: trochlear nerve (IV cranial)
Inferior oblique
•Origin: orbital surface of maxilla •Insert: lower lateral quadrant of the posterior half of the sclera (via the suspensory ligament, as a pulley) •Action:
1.elevates 2.laterally rotates 3.abducts the eye
•Blood: branches of ophthalmic artery •Nerve: oculomotor nerve (III cranial)
Extrinsic ocular muscles subdivide into recti (straight) and oblique
Muscle
Rotation of eyeball
Cranial nerve innervation
Lateral rectus
Medial rectus
Superior rectus
Inferior rectus
Inferior oblique
Superior oblique
Lateral
Medial
Superior and medial
Inferior and medial
Superior and lateral
Inferior and lateral
Abducens (VI)
Oculomotor (III) - inferior branch
Oculomotor (III) - superior branch
Oculomotor (III) - inferior branch
Oculomotor (III) - inferior branch
Trochlear (IV)
They originate in common annular tendom and insert into sclera and can rotate eyeball on frontal and vertical axis. As result pupilla moves up, down laterally and medially. Superior oblique muscle psses through a pulleylike cartilagenous loop, the trochlea, before attaching to the eyeball. Levator palpebrae superioris muscle elevates upper eyelid.
Periosteum of orbit is ‘periorbita’ forming the cover of bones and passes through the optic canal into dura mater encephali. Eyeball is enveloped by vagina of eyeball (Tenon`s capsule). Adiposal body of orbit localised between vagina of eyeball and periorbita, which formes elastic pillow for eyeball.
Superior and inferior eyelids cover and protect eyeball. Front surface of eyelids is covered by skin. Posterior surface of the eyelids and anterior free surface of eyeball are covered by thin conjuctiva. Last forms superior and inferior sac of conjuctiva.
Tarsal plates, composed of dense regular connective tissue, are important in maintaining the shape of the eyelids. Yeybrows (supercilium) consist of short, sick hair positioned transversally above both eyes along the superior orbital ridges of the scull.
Lacrimal apparatus consists of lacrimal gland lying in superolateral portion of the orbit and a series of lacrimal ducts that drain the secretion into the nasal cavity. The excretory ductuli of lacrimal gland (10-15) open into conjuctival sac of upper yeylid (superior rivus). With each blink of the eyelids, tears passe medially and downward and drains into lake and two small openings, called lacrimal puncta on both sides of the lacrimal caruncle. From here, tears drains through the lacrimal canaliculus into the lacrimal sac and continious through the nasolacrimal duct to the inferior meatus of the nasal cavity.
Visual tract
Optic nerve is a part of visual analyser. Three neurons of visual tract are located in retina: 1 - fotoreceptors rods and cones, 2 – bipolar cells and 3 – ganglionic (multipolar) cells. Axons of third neurons form the II Optic nerve, which passes through the optic canal and get the cranial cavity. Medial fibres of the optic nerve pass to the opposite side and form the optic chiasma. Lateral fibres part do not cross each other and keep their own side. Then fibres of the optic nerve form optic tract wich get the subcortical sight centres (lateral geniculate body and superior colliculus of midbrain). Then 4th neurons are located under pulvinar thalami. Their axons run through the posterior leg of internal capsule (visual radiation) and reach cortical visual analyser in calcarine sulcus (occipital lobe).
Axons of optic tract contact with cells in accessory oculomotor /parasympathetic/ nucleus (Yakubovych-Edinger-Westphal`s) by means of intermediate neuron. There are link for realising the pupillar reflex and accomodation. Axons of fifth neurons run in composition of oculomotor nerve get a ciliary ganglion, where sixth neurons positioned. Their axons pass with short ciliary nerves into eyeball and give innervating for ciliary and sphincter pupillae muscles. This reflex does not depend on our will and consciousness.
Fibres of III Oculomotor nerve starts from motor nucleus and accessory oculomotor /parasympathetic/ nucleus (Edinger-Westphal`s). They are found in lamina tecti of midbrain (superior colliculus). Nerve exites from brain in interpeduncular fossa then it passes through the superior orbital fissura. Here it subdivides into superior and inferior branches. Nervous fibres from superior branch innervate superior rectus muscle and levator of superior eyelid. Motor fibres from
inferior branch innervate inferior and medial rectus muscles and also inferior oblique muscle. The parasympathetic fibres form oculomotor radix and reach the ciliary ganglion. The postganglionic parasympathetic fibres from ganglion pass in composition of short ciliary nerves to ciliary and sphincter pupillae muscles.
The oculomotor nerve (775, 776, 777) supplies somatic motor fibers to all the ocular muscles, except the Obliquus superior and Rectus lateralis; it also supplies through its connections with the ciliary ganglion, sympathetic motor fibers to the Sphincter pupillæ and the Ciliaris muscles.
Plan of oculomotor nerve.
The fibers of the oculomotor nerve arise from a nucleus which lies in the gray substance of the floor of the cerebral aqueduct and extends in front of the aqueduct for a short distance into the floor of the third ventricle. From this nucleus the fibers pass forward through the tegmentum, the red nucleus, and the medial part of the substantia nigra, forming a series of curves with a lateral convexity, and emerge from the oculomotor sulcus on the medial side of the cerebral peduncle.
The nucleus of the oculomotor nerve does not consist of a continuous column of cells, but is broken up into a number of smaller nuclei, which are arranged in two groups, anterior and posterior. Those of the posterior group are six in number, five of which are symmetrical on the two sides of the middle line, while the sixth is centrally placed and is common to the nerves of both sides. The anterior group consists of two nuclei, an antero-medial and an antero-lateral (762).
The nucleus of the oculomotor nerve, considered from a physiological standpoint, can be subdivided into several smaller groups of cells, each group controlling a particular muscle.
On emerging from the brain, the nerve is invested with a sheath of pia mater, and enclosed in a prolongation from the arachnoid. It passes between the superior cerebellar and posterior cerebral arteries, and then pierces the dura mater in front of and lateral to the posterior clinoid process, passing between the free and attached borders of the tentorium cerebelli. It runs along the lateral wall of the cavernous sinus, above the other orbital nerves, receiving in its course one or two filaments from the cavernous plexus of the sympathetic, and a communicating branch from the ophthalmic division of the trigeminal. It then divides into two branches, which enter the orbit through the superior orbital fissure, between the two heads of the Rectus lateralis. Here the nerve is placed below the trochlear nerve and the frontal and lacrimal branches of the ophthalmic nerve, while the nasociliary nerve is placed between its two rami.
The superior ramus, the smaller, passes medialward over the optic nerve, and supplies the Rectus superior and Levator palpebræ superioris. The inferior ramus, the larger, divides into three branches. One passes beneath the optic nerve to the Rectus medialis; another, to the Rectus inferior; the third and longest runs forward between the Recti inferior and lateralis to the Obliquus inferior. From the last a short thick branch is given off to the lower part of the ciliary ganglion, and forms its short root. All these branches enter the muscles on their ocular surfaces, with the exception of the nerve to the Obliquus inferior, which enters the muscle at its posterior border.
IV Trochlear nerve has an own motor nucleus, which is disposed in quadrigeminal plate on level of inferior colliculus of midbrain. Their axons exite from brain from superior cerebral vellum, then pass laterally from lateral cerebral pedunculi and through the superior orbital fissura. Trochlear nerve supplies superior oblique muscle.
VI Abducens nerve has a motor nucleus, which is disposed superficially in facial colliculus (rhomboid fossa). Axon exites from brain in fissure between pyramids of medulla oblongata and pons, and from skull - through the superior orbital fissura. Abducens nerve supplies lateral oblique muscle.
Lesson No 21
Organs of hearing: external ear
EAR (HEARING AND EQUILIBRIUM ORGAN)
Ear subdivides on auricle (outer ear), middle ear and internal ear. Auricle and external auditory meatus belong to outer ear. Middle ear contains a tympanic cavity and auditory tube (Eustachian). Internal ear composes an osseous labyrinth and membranous labyrinth.
AURICLE contains a cartilage covered by skin. In inferior part a cartilage is absent there is auricular lobule (earlobe). Also auricle has a helix, triangular fossa, antihelix, concha, tragus, antitragus.
External auditory meatus is open outside, in depth from cavity of middle ear it dissociates by tympanic membrane. External auditory meatus has cartilaginous part and inner osseous part. Cartilaginous part composes one-third length of auditory meatus. Osseous part occupies two thirds of auditory meatus. Auditory meatus is curved S-like and for its straightening attached to examination of tympanic membrane necessary to draw off auricle posterior, up and outside.
The external ear consists of the expanded portion named the auricula or pinna, and the external acoustic meatus. The former projects from the side of the head and serves to collect the vibrations of the air by which sound is produced; the latter leads inward from the bottom of the auricula and conducts the vibrations to the tympanic cavity.
The Auricula
or Pinna (904) is
of an ovoid form, with its larger end directed upward. Its lateral surface is
irregularly concave, directed slightly forward, and presents numerous eminences
and depressions to which names have been assigned. The prominent rim of the
auricula is called the helix; where the helix turns downward behind, a
small tubercle, the auricular tubercle of
The cranial surface of the auricula presents elevations which correspond to the depressions on its lateral surface and after which they are named, e. g., eminentia conchæ, eminentia triangularis, etc.
The auricula. Lateral surface.
Structure.—The auricula is composed of a thin plate of yellow fibrocartilage, covered with integument, and connected to the surrounding parts by ligaments and muscles; and to the commencement of the external acoustic meatus by fibrous tissue.
The skin is thin, closely adherent to the cartilage, and covered with fine hairs furnished with sebaceous glands, which are most numerous in the concha and scaphoid fossa. On the tragus and antitragus the hairs are strong and numerous. The skin of the auricula is continuous with that lining the external acoustic meatus.
Cranial surface of cartilage of right auricula.
The cartilage of the auricula (cartilago auriculæ; cartilage of the pinna) (905, 906) consists of a single piece; it gives form to this part of the ear, and upon its surface are found the eminences and depressions above described. It is absent from the lobule; it is deficient, also, between the tragus and beginning of the helix, the gap being filled up by dense fibrous tissue. At the front part of the auricula, where the helix bends upward, is a small projection of cartilage, called the spina helicis, while in the lower part of the helix the cartilage is prolonged downward as a tail-like process, the cauda helicis; this is separated from the antihelix by a fissure, the fissura antitragohelicina. The cranial aspect of the cartilage exhibits a transverse furrow, the sulcus antihelicis transversus, which corresponds with the inferior crus of the antihelix and separates the eminentia conchæ from the eminentia triangularis. The eminentia conchæ is crossed by a vertical ridge (ponticulus), which gives attachment to the Auricularis posterior muscle. In the cartilage of the auricula are two fissures, one behind the crus helicis and another in the tragus.
The ligaments of the auricula (ligamenti auricularia [Valsalva]; ligaments of the pinna) consist of two sets: (1) extrinsic, connecting it to the side of the head; (2) intrinsic, connecting various parts of its cartilage together.
The extrinsic ligaments are two in number, anterior and posterior. The anterior ligament extends from the tragus and spina helicis to the root of the zygomatic process of the temporal bone. The posterior ligament passes from the posterior surface of the concha to the outer surface of the mastoid process.
The chief intrinsic ligaments are: (a) a strong fibrous band, stretching from the tragus to the commencement of the helix, completing the meatus in front, and partly encircling the boundary of the concha; and (b) a band between the antihelix and the cauda helicis. Other less important bands are found on the cranial surface of the pinna.
The muscles of the auricula (906) consist of two sets: (1) the extrinsic, which connect it with the skull and scalp and move the auricula as a whole; and (2) the intrinsic, which extend from one part of the auricle to another.
The extrinsic muscles are the Auriculares anterior, superior, and posterior.
The Auricularis anterior (Attrahens aurem), the smallest of the three, is thin, fan-shaped, and its fibers are pale and indistinct. It arises from the lateral edge of the galea aponeurotica, and its fibers converge to be inserted into a projection on the front of the helix.
The Auricularis superior (Attolens aurem), the largest of the three, is thin and fan-shaped. Its fibers arise from the galea aponeurotica, and converge to be inserted by a thin, flattened tendon into the upper part of the cranial surface of the auricula.
The Auricularis posterior (Retrahens aurem) consists of two or three fleshy fasciculi, which arise from the mastoid portion of the temporal bone by short aponeurotic fibers. They are inserted into the lower part of the cranial surface of the concha.
Actions.—In man, these muscles possess very little action: the Auricularis anterior draws the auricula forward and upward; the Auricularis superior slightly raises it; and the Auricularis posterior draws it backward.
The muscles of the auricula.
The intrinsic muscles are the:
Helicis major.
Antitragicus.
Helicis minor.
Transversus auriculæ.
Tragicus.
Obliquus auriculæ.
The Helicis major is a narrow vertical band situated upon the anterior margin of the helix.
It arises below, from the spina helicis, and is inserted into the anterior border of the helix, just where it is about to curve backward.
The Helicis minor is an oblique fasciculus, covering the crus helicis.
The Tragicus is a short, flattened vertical band on the lateral surface of the tragus.
The Antitragicus arises from the outer part of the antitragus, and is inserted into the cauda helicis and antihelix.
The Transversus auriculæ is placed on the cranial surface of the pinna. It consists of scattered fibers, partly tendinous and partly muscular, extending from the eminentia conchæ to the prominence corresponding with the scapha.
The Obliquus auriculæ, also on the cranial surface, consists of a few fibers extending from the upper and back part of the concha to the convexity immediately above it.
Nerves.—The Auriculares anterior and superior and the intrinsic muscles on the lateral surface are supplied by the temporal branch of the facial nerve, the Auricularis posterior and the intrinsic muscles on the cranial surface by the posterior auricular branch of the same nerve.
The arteries of the auricula are the posterior auricular from the external carotid, the anterior auricular from the superficial temporal, and a branch from the occipital artery.
The veins accompany the corresponding arteries.
The sensory nerves are: the great auricular, from the cervical plexus; the auricular branch of the vagus; the auriculotemporal branch of the mandibular nerve; and the lesser occipital from the cervical plexus.
External and middle ear, opened from the front. Right side.
The External
Acoustic Meatus (meatus acusticus externus; external auditory canal or
meatus) extends from the bottom of the concha to the tympanic membrane (907, 908). It
is about
The external acoustic meatus is formed partly by cartilage and membrane, and partly by bone, and is lined by skin.
The cartilaginous
portion (meatus acusticus externus cartilagineus) is about
The osseous
portion (meatus acusticus externus osseus) is about
Horizontal section through left ear; upper half of section.
The skin lining the meatus is very thin; adheres closely to the cartilaginous and osseous portions of the tube, and covers the outer surface of the tympanic membrane. After maceration, the thin pouch of epidermis, when withdrawn, preserves the form of the meatus. In the thick subcutaneous tissue of the cartilaginous part of the meatus are numerous ceruminous glands, which secrete the ear-wax; their structure resembles that of the sudoriferous glands.
Relations of the Meatus.—In front of the osseous part is the condyle of the mandible, which however, is frequently separated from the cartilaginous part by a portion of the parotid gland. The movements of the jaw influence to some extent the lumen of this latter portion. Behind the osseous part are the mastoid air cells, separated from the meatus by a thin layer of bone.
The arteries supplying the meatus are branches from the posterior auricular, internal maxillary, and temporal.
The nerves are chiefly derived from the auriculotemporal branch of the mandibular nerve and the auricular branch of the vagus.
MIDDLE EAR
The middle ear is an irregular, laterally compressed space in the petrous part of the temporal bone. It is lined with mucous membrane and filled with air, which reaches it from the nasopharynx via the pharyngotympanic tube (Figs 36.6, 36.7, 36.9). The middle ear contains three small bones, the malleus, incus and stapes, collectively called the auditory ossicles, which form an articulated chain connecting the lateral and medial walls of the cavity, and which transmit the vibrations of the tympanic membrane across the cavity to the cochlea.
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Fig. 36.9 A,
Axial CT scan at level of the ossicular mass (white arrow) in the
epitympanum, showing the mastoid antrum and its relationship to the lateral
(horizontal) semicircular canal (black arrow). B, Axial CT scan at the level
of the entrance of the pharyngotympanic tube, showing its relationship to the
internal carotid artery (black arrow). |
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The essential function of the middle ear is to transfer energy efficiently from relatively weak vibrations in the elastic, compressible air in the external acoustic meatus to the incompressible fluid around the delicate receptors in the cochlea. Mechanical coupling between the two systems must match their resistance to deformation or ‘flow', i.e. their impedance, as closely as possible. Aerial waves of low amplitude and low force per unit area arrive at the tympanic membrane, which has 15–20 times the area of the stapedial footplate that contacts the perilymph in the inner ear: the force per unit area generated by the footplate is increased by a similar amount, while the amplitude of vibration is almost unchanged.
Protective mechanisms incorporated into the design of the middle ear include the presence of the pharyngotympanic tube (to equalize pressure on both sides of the delicate tympanic membrane); the shape of the articulations between the ossicles; and the reflex contractions of stapedius and tensor tympani in response to sounds of fairly high intensity (preventing damage caused by sudden or excessive excursions of the ossicles).
The space within the middle ear can be subdivided into three parts. These are the mesotympanum or tympanic cavity proper, which is opposite the tympanic membrane; the epitympanum or attic, which is above the level of the membrane, and contains the head of the malleus and the body and short process of the incus; and the hypotympanum, which is in the floor of the cavity between the jugular bulb and the lower margin of the tympanic membrane. The vertical and anteroposterior diameters of the mesotympanum and hypotympanum are each approximately 15 mm; the transverse diameter is 6 mm superiorly and 4 mm inferiorly, narrowing to 2 mm opposite the umbo. The cavity is bounded laterally by the tympanic membrane and medially by the lateral wall of the internal ear, the promontory. It communicates posteriorly with the mastoid antrum and the mastoid air cells, and anteriorly with the nasopharynx via the pharyngotympanic tube (Figs 36.6, 36.7).
The tympanic cavity and mastoid antrum, auditory ossicles and structures of the internal ear are all almost fully developed at birth and subsequently alter little. In the fetus the cavity contains a gelatinous tissue which has practically disappeared by birth, when it is filled by a fluid which is absorbed when air enters via the pharyngotympanic tube. The tympanic cavity is a common site of infection in childhood.
BOUNDARIES OF THE TYMPANIC CAVITY
The tympanic cavity has a roof and a floor, and lateral, medial, posterior and anterior walls.
Roof
A thin plate of compact bone, the tegmen tympani, separates the cranial and tympanic cavities, and forms much of the anterior surface of the petrous temporal bone. It is prolonged posteriorly as the roof of the mastoid antrum and anteriorly it covers the canal for tensor tympani. In youth, the unossified petrosquamosal suture may allow the spread of infection from the tympanic cavity to the meninges. In adults, veins from the tympanic cavity traverse this suture to reach the superior petrosal or petrosquamous sinus and thus may also transmit infection to these structures through a process of thrombophlebitis.
Longitudinal fractures of the middle cranial fossa almost always involve the tympanic roof, accompanied by dislocation of the ossicular chain, rupture of the tympanic membrane, or a fractured roof of the osseous external acoustic meatus, which can be seen as a notch. Such injuries usually cause bleeding from the ear, with escape of cerebrospinal fluid if the dura mater has been torn (CSF otorrhea).
Floor
The floor of the tympanic cavity is a narrow, thin, convex plate of bone which separates the cavity from the superior bulb of the internal jugular vein. The bone may be patchily deficient, in which case the tympanic cavity and the vein are separated only by mucous membrane and fibrous tissue. Alternatively, the floor is sometimes thick and may contain some accessory mastoid air cells. A small aperture for the tympanic branch of the glossopharyngeal nerve lies near the medial wall.
Lateral wall
The lateral wall consists mainly of the tympanic membrane, but also contains the ring of bone to which the membrane is attached (see Figs 36.6B, 36.14A),. The lateral epitympanic bony wall is wedge-shaped in section and its sharp inferior portion is known as the outer attic wall or scutum. This part is easily eroded or blunted by cholesteatoma, a feature easily detected on CT scans (see Fig. 36.8). There is a deficiency or notch in the upper part of this ring, close to which are the small openings of the anterior and posterior canaliculi for the chorda tympani and the petrotympanic fissure. The posterior canaliculus for the chorda tympani is situated in the angle between the posterior and lateral walls of the tympanic cavity just behind the tympanic membrane, at a variable position approximately level with the upper end of the handle of the malleus. It leads into a minute canal that descends in front of the facial canal and ends in it about 6 mm above the stylomastoid foramen. The canaliculus transmits the chorda tympani and a branch of the stylomastoid artery to the tympanic cavity. The chorda tympani leaves the tympanic cavity through the anterior canaliculus, which opens at the medial end of the petrotympanic fissure.
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Fig. 36.14 The
chorda tympani nerve. A, Oblique vertical section through the left temporal
bone, to show roof and lateral wall of the middle ear, the chorda tympani and
the mastoid antrum. B, Chorda tympani crossing the tympanic membrane,
auroscopic view. |
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The petrotympanic fissure is a mere slit approximately 2 mm in length which opens just above and in front of the ring of bone to which the tympanic membrane is attached. It contains the anterior process and anterior ligament of the malleus and transmits the anterior tympanic branch of the maxillary artery to the tympanic cavity.
Tympanic membrane
The tympanic membrane separates the tympanic cavity from the external acoustic meatus (Fig. 36.10; see also Figs 38.6, 36.14). It is thin, semi-transparent, and almost oval, though somewhat broader above than below. It lies obliquely, at an angle of approximately 55° with the meatal floor. Its longest, anteroinferior diameter is 9 to 10 mm, and its shortest is 8 to 9 mm. Most of its circumference is a thickened fibrocartilaginous ring or anulus which is attached to the tympanic sulcus at the medial end of the meatus. The anulus contains radially orientated smooth muscle cells in several locations that possibly play a role in controlling blood flow or maintaining tension (Henson et al 2005). The sulcus is deficient superiorly, i.e. it is notched. Two bands, the anterior and posterior malleolar folds, pass from the ends of this notch to the lateral process of the malleus. The small triangular part of the membrane, the pars flaccida, lies above these folds and is lax and thin. The major part of the tympanic membrane, the pars tensa, is taut. The handle of the malleus is firmly attached to the internal surface of the tympanic membrane as far as its centre, which projects towards the tympanic cavity. The inner surface of the membrane is thus convex and the point of greatest convexity is termed the umbo. Although the membrane as a whole is convex on its inner surface, its radiating fibres are curved with their concavities directed inwards.
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Fig. 36.10
Auroscopic view of left tympanic membrane. Note that a bright cone of light
is seen in the anteroinferior quadrant of the membrane when it is
illuminated. |
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Microstructure
Histologically, the tympanic membrane is composed of an outer cuticular layer, an intermediate fibrous layer and an inner mucous layer.
The cuticular stratum is continuous with the thin skin of the meatus. It is keratinized, stratified squamous in type, devoid of dermal papillae and hairless. Its subepithelial tissue is vascularized and may develop a few peripheral papillae. Ultrastructurally, it is typically 10 cells thick and has two zones, a superficial layer of non-nucleated squames, and a deep zone which resembles the epidermal prickle cell layer (stratum spinosum). There are numerous desmosomes between cells, the deepest of which lie on a continuous basal lamina, but lack epithelial pegs and hemidesmosomes. The cells of this stratum have a propensity for lateral migration and differentiation not shared with any other stratified squamous epithelia in the body.
The fibrous stratum consists of an external layer of radiating fibres which diverge from the handle of the malleus, and a deep layer of circular fibres, which are plentiful peripherally, but sparse and scattered centrally. Ultrastructurally, the filaments are 10 nm in diameter, and are linked at 25 nm intervals. They have a distinctive amino acid composition, and may consist of a protein peculiar to the tympanic membrane. Small groups of collagen fibrils appear at 11 weeks in utero, interspersed with small bundles of elastin microfibrils. Older specimens contain more typically cross-banded collagen fibrils and an amorphous elastin component. The fibrous stratum is replaced by loose connective tissue in the pars flaccida.
The mucous stratum is a part of the mucosa of the tympanic cavity, and is thickest near the upper part of the membrane. It consists of a single layer of very flat cells, with overlapping interdigitating boundaries and desmosomes and tight junctions between adjacent cells. The cytoplasm contains only a few organelles: the luminal surfaces of these apparently metabolically inert cells have a few irregular microvilli and are covered by an amorphous electron-dense material. There are no ciliated columnar cells.
Innervation
The tympanic membrane is mainly innervated by the auriculotemporal nerve, and appears to perceive only pain. There is a minor, inconstant and overlapping sensory supply from cranial nerves VII, IX and X.
The auricular branch of the vagus arises from the superior vagal ganglion and is joined soon after by a ramus from the inferior ganglion of the glossopharyngeal nerve. It passes behind the internal jugular vein and enters the mastoid canaliculus on the lateral wall of the jugular fossa. It traverses the temporal bone and crosses the facial canal about 4 mm above the stylomastoid foramen. At this point it supplies an ascending branch to the facial nerve. Fibres of the nervus intermedius may pass to the auricular branch of the vagus here, which may explain the cutaneous vesiculation that sometimes accompanies geniculate herpes. The auricular branch then traverses the tympanomastoid fissure, and divides into two rami. One ramus joins the posterior auricular nerve and the other is distributed to the skin of part of the cranial surface of the auricle, the posterior wall and floor of the external acoustic meatus, and to the adjoining part of the outer surface of the tympanic membrane. The auricular branch therefore contains somatic afferent nerve fibres, which probably terminate in the spinal trigeminal nucleus. Stimulation of the vagus nerve, e.g. in syringing the ear, can have a reflex reaction on heart rate.
Myringoplasty
Persistent perforation of the tympanic membrane caused by infection or trauma causes hearing impairment and predisposes to continuing infection as a result of contamination with organisms from the external acoustic meatus. This condition is known as chronic suppurative otitis media of the tubo-tympanic type. Myringoplasty is a surgical procedure that uses a connective tissue scaffold or graft to support healing of the perforation. The commonest technique involves the elevation of the tympanic anulus and the placement of a piece of fibrous connective tissue, e.g. part of the fibrous deep fascia which invests the lateral surface of temporalis or the perichondrium of the tragal cartilage, onto the undersurface of the tympanic membrane to close the perforation. The healed edges of the perforation are stripped of epithelium to encourage healing and scar formation. The fibrous tissue supports the healing tympanic membrane and may in part be incorporated into the repair. Once the perforation is healed, the vibratory function of the tympanic membrane is usually restored to normal.
Medial wall
The medial wall of the tympanic cavity is also the lateral boundary of the internal ear. Its features are the promontory, fenestra vestibuli (fenestra ovalis, oval window), fenestra cochleae (fenestra rotunda, round window) and the facial prominence (Fig. 36.11).
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Fig. 36.11 Medial
wall of the left tympanic cavity, anterolateral aspect. A, Lateral wall and
adjacent parts of anterior and superior walls removed; facial canal and
carotid canal opened. B, Section along the axis of the petrous part of the
temporal bone. |
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The promontory is a rounded prominence furrowed by small grooves which lodge the nerves of the tympanic plexus. It lies over the lateral projection of the basal turn of the cochlea. A minute spicule of bone frequently connects the promontory to the pyramidal eminence of the posterior wall. The apex of the cochlea lies near the medial wall of the tympanic cavity, anterior to the promontory. A depression behind the promontory is known as the sinus tympani.
The fenestra vestibuli is a kidney-shaped opening situated above and behind the promontory, and leading from the tympanic cavity to the vestibule of the inner ear. Its long diameter is horizontal and its convex border is directed upwards. It is occupied by the base of the stapes, the footplate: the circumference of the footplate is attached to the margin of the fenestra by an anular ligament.
The fenestra cochleae is situated below and a little behind the fenestra vestibuli, from which it is separated by a posterior extension of the promontory, called the subiculum. Occasionally, another ridge of bone, the ponticulus, leaves the promontory above the subiculum and runs to the pyramid on the posterior wall of the cavity. The fenestra cochleae lies completely under the overhanging edge of the promontory in a deep hollow or niche, and is placed very obliquely. In dried specimens it opens anterosuperiorly from the tympanic cavity into the scala tympani of the cochlea, but in life it is closed by the secondary tympanic membrane. This is somewhat concave towards the tympanic cavity and convex towards the cochlea, and is bent so that its posterosuperior one-third forms an angle with its anteroinferior two-thirds. The membrane is composed of an external layer derived from the tympanic mucosa; an internal layer, derived from the cochlear lining membrane; and an intermediate, fibrous layer.
The prominence of the facial nerve canal indicates the position of the upper part of the bony facial canal (Fallopian canal) which contains the facial nerve. The canal crosses the medial tympanic wall from the cochleariform process anteriorly, runs just above the fenestra vestibuli, and then curves down into the posterior wall of the cavity. Its lateral wall may be partly deficient.
Posterior wall
The posterior wall of the tympanic cavity is wider above than below. Its main features are the aditus to the mastoid antrum, the pyramid, and the fossa incudis (Fig. 36.11).
The aditus to the mastoid antrum is a large irregular aperture which leads back from the epitympanic recess into the upper part of the mastoid antrum. A rounded eminence on the medial wall of the aditus, above and behind the prominence of the facial nerve canal, corresponds to the position of the lateral semicircular canal.
The pyramidal eminence is situated just behind the fenestra vestibuli and in front of the vertical part of the facial nerve canal. It is hollow and contains the stapedius muscle. Its summit projects towards the fenestra vestibuli and is pierced by a small aperture which transits the tendon of stapedius. The cavity in the pyramidal eminence is prolonged down and back in front of the facial nerve canal; it communicates with the canal by an aperture through which a small branch of the facial nerve passes to stapedius.
The fossa incudis is a small depression in the lower and posterior part of the epitympanic recess. It contains the short process of the incus, which is fixed to the fossa by ligamentous fibres.
Mastoid antrum
The mastoid antrum is an air sinus in the petrous part of the temporal bone. Its topographical relations are of considerable surgical importance. The aditus to the mastoid antrum, which leads back from the epitympanic recess, opens in the upper part of its anterior wall. The lateral semicircular canal lies medial to the aditus. The descending part of the facial nerve canal is anteroinferior. The medial wall is related to the posterior semicircular canal (see Ch. 37). The sigmoid sinus lies some distance posteriorly: the distance can be extremely variable and is dependent on the degree of pneumatization of the mastoid. The roof is formed by the tegmen tympani, and so the antrum lies below the middle cranial fossa and the temporal lobe of the brain. The floor has several openings which communicate with the mastoid air cells. The lateral wall, which offers the usual surgical approach to the cavity, is formed by the postmeatal process of the squamous part of the temporal bone. This is only 2 mm thick at birth but increases at an average rate of 1 mm a year, attaining a final thickness of 12–15 mm. In adults, the lateral wall of the antrum corresponds to the suprameatal triangle (Macewen's triangle) on the outer surface of the skull. This is palpable through the cymba conchae: the superior side of the triangle, the supramastoid crest, is level with the floor of the middle cranial fossa; the anteroinferior side, which forms the posterosuperior margin of the external acoustic meatus, indicates approximately the position of the descending part of the facial nerve canal; the posterior side, formed by a posterior vertical tangent to the posterior margin of the external acoustic meatus, is anterior to the sigmoid sinus.
The adult capacity of the mastoid antrum is variable, but on average is 1 mL, with a general diameter of 10 mm. Unlike the other air sinuses in the skull, it is present at birth, and is indeed then almost adult in size, although it is at a higher level relative to the external acoustic meatus than it is in adults. In the very young, the thinness of the lateral antral wall and the absence or under-development of the mastoid process means that the stylomastoid foramen and emerging facial nerve are very superficially situated.
Mastoid air cells
Though the mastoid process antrum is well developed at birth, the mastoid air cells are merely minute antral diverticula at this stage. As the mastoid develops in the second year, the air cells gradually extend into it and by the fourth year they are well formed, although their greatest growth occurs at puberty. They vary considerably in number, form and size. Usually, they interconnect and are lined by a mucosa with squamous non-ciliated epithelium, continuous with that in the mastoid antrum and tympanic cavity. They may fill the mastoid process, even to its tip, and some may be separated from the sigmoid sinus and posterior cranial fossa only by extremely thin bone, which is occasionally deficient (Fig. 36.11). Some may lie superficial to, or even behind, the sigmoid sinus, and others may be present in the posterior wall of the descending part of the facial nerve canal. Those in the squamous part of the temporal bone may be separated from deeper cells in the petrous part by a plate of bone in the line of the squamomastoid suture (Körner's septum). Sometimes they extend only minimally into the mastoid process, in which case the process consists largely of dense bone or trabecular bone containing bone marrow. Varieties of the mastoid process are recognized. The three types most commonly described are pneumatized (with many air cells); sclerotic or diploic (with few or no air cells); and mixed (contain both air cells and bone marrow).
The mastoid process may have no air cells at all in 20% of skulls. Alternatively, air cells may extend beyond the mastoid process into the squamous part of the temporal bone above the supramastoid crest; into the posterior root of the zygomatic process of the temporal bone; into the osseous roof of the external acoustic meatus just below the middle cranial fossa; or into the floor of the tympanic cavity very close to the superior jugular bulb. Rarely, a few may excavate the jugular process of the occipital bone. An important group may extend medially into the petrous part of the temporal bone, even to its apex, and are related to the pharyngotympanic tube, carotid canal, labyrinth and abducens nerve. Some investigators maintain that these are not continuous with the mastoid cells, but grow independently from the tympanic cavity. All of these extensions of the mastoid air cells are pathologically important since infection may spread to the structures around them.
Innervation
The mastoid air cells are innervated by a meningeal branch of the mandibular division of the trigeminal nerve.
Mastoiditis
Mastoiditis is a potentially dangerous, life-threatening condition that develops as a result of the spread of bacterial infection from the tympanic cavity via the aditus to the mastoid antrum and associated mastoid air cells. In particular, the infection may spread through the tegmen tympani to the dura mater of the middle cranial fossae, to cause an extradural collection. This in turn may cause necrosis of the adjacent dura mater and infection may spread to form a subdural empyema in the subarachnoid space, or an abscess in the substance of the adjacent temporal lobe. Bacterial meningitis may also develop. Similar spread may be seen into the posterior cranial fossa. In both sites the infection may prove fatal.
Infection may spread laterally through the cortical bone of the lateral aspect of the mastoid process to form a subperiosteal postauricular abscess (Bezold's abscess), or through the cortical bone of the tip of the mastoid process to the attachment of the posterior belly of digastric and sternocleidomastoid, which stimulates painful muscular contraction and torticollis.
Anterior wall
The inferior, larger area of the anterior wall of the tympanic cavity is narrowed by the approximation of the medial and lateral walls of the cavity (Fig. 36.6). It is a thin lamina and forms the posterior wall of the carotid canal. It is perforated by the superior and inferior caroticotympanic nerves and the tympanic branch or branches of the internal carotid. The canals for tensor tympani and the osseous part of the pharyngotympanic tube open above it, the canal for tensor tympani being superior to that for the pharyngotympanic tube. Both canals incline downwards and anteromedially, to open in the angle between the squamous and petrous parts of the temporal bone, and are separated by a thin, osseous septum. The canal for tensor tympani and the bony septum runs posterolaterally on the medial tympanic wall, and ends immediately above the fenestra vestibuli. Here, the posterior end of the septum is curved laterally to form a pulley, the processus trochleariformis (cochleariform process), which is a surgical landmark for the identification of the geniculate ganglion of the facial nerve. The tendon of tensor tympani turns laterally over the pulley before attaching to the upper part of the handle of the malleus.
Pharyngotympanic tube blockage in children
The pharyngotympanic tube serves to ventilate the middle ear, exchanging nasopharyngeal air with the air in the middle ear, which has been altered in its composition via transmucosal gas exchange with the haemoglobin in the blood vessels of the mucosa. The tube also carries mucus from the middle ear cleft to the nasopharynx as a result of ciliary transport.
In children, the pharyngotympanic tube is relatively narrow. It is prone to obstruction when the mucosa swells in response to infection or allergic challenge: obstruction results in a relative vacuum being created in the middle ear secondary to transmucosal gas exchange, and this in turn promotes mucosal secretion and the formation of a middle ear effusion. Because of the collapsibility of the pharyngotympanic tube, the vacuum thus created can overcome the distending effect of the muscles of the tube and ‘lock’ the tube shut. The resultant persistent middle ear effusion, otitis media with effusion (glue ear), can cause hearing loss by splinting the tympanic membrane and impeding its vibration. It can also provide an ideal environment for the proliferation of bacteria, with the result that an acute otitis media may develop. It is possible to relieve the vacuum and unlock the tube, and then remove the effusion by myringotomy, i.e. by surgically creating a hole in the tympanic membrane. This hole will generally heal rapidly and it is common practice to insert a flanged ventilation tube (a grommet or tympanostomy tube) to keep the hole open. Migration of the outer squamous layer of the tympanic membrane eventually displaces the tube and the myringotomy heals.
It is assumed that acute otitis media usually arises as a result of ascending infection from the nasopharynx via the pharyngotympanic tube to the middle ear cleft. From there it may extend to the mastoid aditus and antrum. Swelling secondary to the infection may result in the closure of both exits from the middle ear, i.e. the pharyngotympanic tube and the aditus, with subsequent accumulation of pus under pressure, which causes lateral bulging and inflammation of the tympanic membrane. The latter may burst, releasing mucopurulent discharge into the external acoustic meatus, which results in a release of the pressure in the middle ear and a diminution in the levels of pain. After a brief period the discharge dries up, and for the most part the resultant perforation of the tympanic membrane heals. Normal ventilation and drainage of mucus from the middle ear is restored once the swelling in the pharyngotympanic tube resolves. On occasion the process will fail to produce a perforation of the tympanic membrane and the inflammatory exudates will not drain. The immune defence system sterilizes the exudates of organisms, resulting in a sterile mucoid effusion, otitis media with effusion (see above).
Middle ear. Tympanic cavity. Auditory tube
Tympanic cavity positioned in thickness of temporal pyramid and has the following walls:
1. tegmental wall (superior);
2. jugular wall (inferior);
3. labyrinthic wall (medial), where found 2 windows: vestibular (oval) window and cochlear (round) window. Vestibular window is closed by base stapes. Round window is tightened by secondary tympanic membrane;
4. mastoid wall (posterior). On it located stapedius muscle. Superiorly posterior wall continues into mastoid cave, the mastoid cells open in it;
5. carotid wall (anterior), a tympanic foramen of auditory tube and muscle-tensor of tympanic membrane are found here;
6. membranous wall (lateral) is formed tympanic membrane. Epitympanic recess contains a head of malleus and body of the incus.
The middle ear or tympanic cavity is an irregular, laterally compressed space within the temporal bone. It is filled with air, which is conveyed to it from the nasal part of the pharynx through the auditory tube. It contains a chain of movable bones, which connect its lateral to its medial wall, and serve to convey the vibrations communicated to the tympanic membrane across the cavity to the internal ear.
The
tympanic cavity consists of two parts: the tympanic cavity proper,
opposite the tympanic membrane, and the attic or epitympanic recess,
above the level of the membrane; the latter contains the upper half of the malleus
and the greater part of the incus. Including the attic, the vertical and
antero-posterior diameters of the cavity are each about
The Tegmental Wall or Roof (paries tegmentalis) is formed by a thin plate of bone, the tegmen tympani, which separates the cranial and tympanic cavities. It is situated on the anterior surface of the petrous portion of the temporal bone close to its angle of junction with the squama temporalis; it is prolonged backward so as to roof in the tympanic antrum, and forward to cover in the semicanal for the Tensor tympani muscle. Its lateral edge corresponds with the remains of the petrosquamous suture.
The Jugular Wall or Floor (paries jugularis) is narrow, and consists of a thin plate of bone (fundus tympani) which separates the tympanic cavity from the jugular fossa. It presents, near the labyrinthic wall, a small aperture for the passage of the tympanic branch of the glossopharyngeal nerve.
Right tympanic membrane as seen through a speculum.
The Membranous or Lateral Wall (paries membranacea; outer wall) is formed mainly by the tympanic membrane, partly by the ring of bone into which this membrane is inserted. This ring of bone is incomplete at its upper part, forming a notch (notch of Rivinus), close to which are three small apertures: the iter chordæ posterius, the petrotympanic fissure, and the iter chordæ anterius.
The iter chordæ posterius (apertura tympanica canaliculi chordæ) is situated in the angle of junction between the mastoid and membranous wall of the tympanic cavity immediately behind the tympanic membrane and on a level with the upper end of the manubrium of the malleus; it leads into a minute canal, which descends in front of the canal for the facial nerve, and ends in that canal near the stylo-mastoid foramen. Through it the chorda tympani nerve enters the tympanic cavity.
The petrotympanic
fissure (fissura petrotympanica; Glaserian fissure) opens just above
and in front of the ring of bone into which the tympanic membrane is inserted;
in this situation it is a mere slit about
The iter
chordæ anterius (
The Tympanic
Membrane (membrana tympani) (909, 910)
separates the tympanic cavity from the bottom of the external acoustic meatus.
It is a thin, semitransparent membrane, nearly oval in form, somewhat broader
above than below, and directed very obliquely downward and inward so as to form
an angle of about fifty-five degrees with the floor of the meatus. Its longest
diameter is downward and forward, and measures from 9 to
The tympanic membrane viewed from within. (Testut.) The malleus has been resected immediately beyond its lateral process, in order to show the tympanomalleolar folds and the membrana flaccida. 1. Tympanic membrane. 2. Umbo. 3. Handle of the malleus. 4. Lateral process. 5. Anterior tympanomalleolar fold. 6. Posterior tympanomalleolar fold. 7. Pars flaccida. 8. Anterior pouch of Tröltsch. 9. Posterior pouch of Tröltsch. 10. Fibrocartilaginous ring. 11. Petrotympanic fissure. 12. Auditory tube. 13. Iter chordæ posterius. 14. Iter chordæ anterius. 15. Fossa incudis for short crus of the incus. 16. Prominentia styloidea.
Structure.—The tympanic membrane is composed of three strata: a lateral (cutaneous), an intermediate (fibrous), and a medial (mucous). The cutaneous stratum is derived from the integument lining the meatus. The fibrous stratum consists of two layers: a radiate stratum, the fibers of which diverge from the manubrium of the malleus, and a circular stratum, the fibers of which are plentiful around the circumference but sparse and scattered near the center of the membrane. Branched or dendritic fibers, as pointed out by Grüber, are also present especially in the posterior half of the membrane.
Vessels and Nerves.—The arteries of the tympanic membrane are derived from the deep auricular branch of the internal maxillary, which ramifies beneath the cutaneous stratum; and from the stylomastoid branch of the posterior auricular, and tympanic branch of the internal maxillary, which are distributed on the mucous surface. The superficial veins open into the external jugular; those on the deep surface drain partly into the transverse sinus and veins of the dura mater, and partly into a plexus on the auditory tube. The membrane receives its chief nerve supply from the auriculotemporal branch of the mandibular; the auricular branch of the vagus, and the tympanic branch of the glossopharyngeal also supply it. 150
The right membrana tympani with the hammer and the chorda tympani, viewed from within, from behind, and from above.
The Labyrinthic or Medial Wall (paries labyrinthica; inner wall) (913) is vertical in direction, and presents for examination the fenestræ vestibuli and cochleæ, the promontory, and the prominence of the facial canal.
The fenestra vestibuli (fenestra ovalis) is a reniform opening leading from the tympanic cavity into the vestibule of the internal ear; its long diameter is horizontal, and its convex border is upward. In the recent state it is occupied by the base of the stapes, the circumference of which is fixed by the annular ligament to the margin of the foramen.
The fenestra cochleæ (fenestra rotunda) is situated below and a little behind the fenestra vestibuli, from which it is separated by a rounded elevation, the promontory. It is placed at the bottom of a funnel-shaped depression and, in the macerated bone, leads into the cochlea of the internal ear; in the fresh state it is closed by a membrane, the secondary tympanic membrane, which is concave toward the tympanic cavity, convex toward the cochlea. This membrane consists of three layers: an external, or mucous, derived from the mucous lining of the tympanic cavity; an internal, from the lining membrane of the cochlea; and an intermediate, or fibrous layer.
Coronal section of right temporal bone.
The promontory (promontorium) is a rounded hollow prominence, formed by the projection outward of the first turn of the cochlea; it is placed between the fenestræ, and is furrowed on its surface by small grooves, for the lodgement of branches of the tympanic plexus. A minute spicule of bone frequently connects the promontory to the pyramidal eminence.
The prominence of the facial canal (prominentia canalis facialis; prominence of aqueduct of Fallopius) indicates the position of the bony canal in which the facial nerve is contained; this canal traverses the labyrinthic wall of the tympanic cavity above the fenestra vestibuli, and behind that opening curves nearly vertically downward along the mastoid wall.
The mastoid or posterior wall (paries mastoidea) is wider above than below, and presents for examination the entrance to the tympanic antrum, the pyramidal eminence, and the fossa incudis.
The entrance to the antrum is a large irregular aperture, which leads backward from the epitympanic recess into a considerable air space, named the tympanic or mastoid antrum (see page 142). The antrum communicates behind and below with the mastoid air cells, which vary considerably in number, size, and form; the antrum and mastoid air cells are lined by mucous membrane, continuous with that lining the tympanic cavity. On the medial wall of the entrance to the antrum is a rounded eminence, situated above and behind the prominence of the facial canal; it corresponds with the position of the ampullated ends of the superior and lateral semicircular canals.
The pyramidal eminence (eminentia pyramidalis; pyramid) is situated immediately behind the fenestra vestibuli, and in front of the vertical portion of the facial canal; it is hollow, and contains the Stapedius muscle; its summit projects forward toward the fenestra vestibuli, and is pierced by a small aperture which transmits the tendon of the muscle. The cavity in the pyramidal eminence is prolonged downward and backward in front of the facial canal, and communicates with it by a minute aperture which transmits a twig from the facial nerve to the Stapedius muscle.
The fossa incudis is a small depression in the lower and back part of the epitympanic recess; it lodges the short crus of the incus.
The Carotid or Anterior Wall (paries carotica) is wider above than below; it corresponds with the carotid canal, from which it is separated by a thin plate of bone perforated by the tympanic branch of the internal carotid artery, and by the deep petrosal nerve which connects the sympathetic plexus on the internal carotid artery with the tympanic plexus on the promontory. At the upper part of the anterior wall are the orifice of the semicanal for the Tensor tympani muscle and the tympanic orifice of the auditory tube, separated from each other by a thin horizontal plate of bone, the septum canalis musculotubarii. These canals run from the tympanic cavity forward and downward to the retiring angle between the squama and the petrous portion of the temporal bone.
The semicanal for the Tensor tympani (semicanalis m. tensoris tympani) is the superior and the smaller of the two; it is cylindrical and lies beneath the tegmen tympani. It extends on to the labyrinthic wall of the tympanic cavity and ends immediately above the fenestra vestibuli.
The septum canalis musculotubarii (processus cochleariformis) passes backward below this semicanal, forming its lateral wall and floor; it expands above the anterior end of the fenestra vestibuli and terminates there by curving laterally so as to form a pulley over which the tendon of the muscle passes.
The auditory
tube (tuba auditiva; Eustachian tube) is the channel through which
the tympanic cavity communicates with the nasal part of the pharynx. Its length
is about
The osseous
portion (pars osseo tubæ auditivæ) is about
The cartilaginous
portion (pars cartilaginea tubæ auditivæ), about
Three auditory ossicles (malleus, incus and stapes), and two muscles are placed in tympanic cavity. Malleus has a head and manubrium with anterior and lateral processes. Muscle-tensor of tympanic membrane handle fastened to malleus. Incus consists of body, short and long legs. Body of incus adjoins to head of malleus, forming incusоmalleus jont. Long leg unites with stapes. Stapes has a head, anterior leg and posterior leg and base stapes, which closes a vestibular window. Musculus stapedius fastens to posterior leg of stapes. Muscles of tympanic cavity regulate auditory ossicles and prevent their oscillation during loud sounds.
Structure of internal ear. 8th cranial nerve
Osseous labyrinth consists of cochlea, vestibulum and semicircular canals.
Vestibulum represents by cavity, its lateral wall carries vestibular and cochlear fenestrae (windows). Vestibular (oval) fenestra containes a base of stapes, and a cochlear (round) window is closed by the secondary tympanic membrane. There are 5 foramens of the semicircle canals in posterior wall of vestibulum, anterior wall has a big foramen conducting into cochlear canal. Crest of internal wall separates a spherical recess from elliptic recess. Internal foramen of vestibular canalicule opens in elliptic recess.
Osseous semicircular canals represented by three arched tubes lying in three mutually perpendicular planes. There are anterior, posterior and lateral semicircular canals. Each semicircular canal has broadened part in its base anterior, posterior and lateral osseous ampule. Semicircle canals join the vestibulum by the medium of osseous legs. Those legs containing ampule are called ampular legs. The legs of the anterior and posterior semicircular canal fuse together into one. As result the semicircle osseous canals unite with vestibulum by five foramina.
Cochlea (snail shell) lies anteriorly from vestibulum, represented by osseous tube forming two and half turns round cochlear axis (modiolus). There is osseous spiral plate inside the cochlea, apex of the cochlea called cupula. In cochlear base internal foramen of cochlear canaliculi is found.
Membranous labyrinth is inserted in osseous labyrinth, has the lesser dimensions and repeats the course of osseous labyrinth. The perilymphatic space is situated between internal surface of osseous labyrinth and external surface of membranous one and filled with liquid - perilymph. Membranous labyrinth is filled with the endolymph. Membranous labyrinth consists of vestibular part (a), semicircular canals (b) and cochlear duct (c).
A. Vestibular labyrinth consists of utriculus and sacculus. Utriculus lies in elliptic recess of osseous labyrinth and connects with semicircular ducts, and a sacculus lies in spherical recess of osseous labyrinth and connects with cochlear duct by communicating duct. Utriculus and saccule communicate each other by the medium of utriculosaccular duct. From last endolymphatic duct starts that passes in vestibular canalicule. Endolymphatic duct passing from external foramen of vestibular canaliculus on posterior surface of pyramide of temporal bone, reaches endolymphatic sacculus placed under cerebral dura mater.
B. Semicircular ducts are inserted in osseous semicircular canals. So there are anterior, posterior and lateral semicircular ducts. They carry anterior, posterior and lateral membranous ampulae. Receptors of balance of rotating located in cristae ampullares. Receptors of balance located in static maculae in utriculus and sacculus.
C. Cochlear duct positioned in spiral canal and starts from vestibular osseous labyrinth and finishes in blind end. On transversal cut a cochlear duct has triangle shape and enclosed by external, superior and inferior walls. External wall fused together with periosteum of spiral canal; an inferior wall is a tympanic wall, it supplementes the spiral plate; superior wall is vestibular wall. Cochlear duct occupies middle part of osseous spiral canal and separates tympanic scala from vestibular scala. Spiral organ (Corti) localised in cochlear duct on spiral membrane, which belongs to peripheral part of auditory analyser.
Sound waves are received by tympanic membrane from auricle and external acoustic meatus. Oscillation of tympanic membrane is transfered to auditory ossicles - malleus, incus and stapes. Base of stapes, which covers a window vestibularа, begins oscillates the perilymph. Oscillation passes through vestibular scala, helicotrema and tympanic scala. Then vestibular wall (Reisner`s membrane) starts to vibrate. Last forces the oscillation of the endolymph in cochlear duct. This vibration is received by sensory haircells of spiral (Korti) organ.
The internal ear is the essential part of the organ of hearing, receiving the ultimate distribution of the auditory nerve. It is called the labyrinth, from the complexity of its shape, and consists of two parts: the osseous labyrinth, a series of cavities within the petrous part of the temporal bone, and the membranous labyrinth, a series of communicating membranous sacs and ducts, contained within the bony cavities.
The
Vestibule (vestibulum).—The vestibule is the central part of the
osseous labyrinth, and is situated medial to the tympanic cavity, behind the
cochlea, and in front of the semicircular canals. It is somewhat ovoid in
shape, but flattened transversely; it measures about
Interior of right osseous labyrinth.
The Bony
Semicircular Canals (canales semicirculares ossei).—The
bony semicircular canals are three in number, superior, posterior, and lateral,
and are situated above and behind the vestibule. They are unequal in length,
compressed from side to side, and each describes the greater part of a circle.
Each measures about
Position of the right bony labyrinth of the ear in the skull, viewed from above. The temporal bone is considered transparent and the labyrinth drawn in from a corrosion preparation.
The superior
semicircular canal (canalis semicircularis superior), 15 to
The posterior
semicircular canal (canalis semicircularis posterior), also
vertical, is directed backward, nearly parallel to the posterior surface of the
petrous bone; it is the longest of the three, measuring from 18 to
The lateral
or horizontal canal (canalis semicircularis lateralis; external
semicircular canal) is the shortest of the three. It measures from 12 to
The cochlea and vestibule, viewed from above. All the hard parts which form the roof of the internal ear have been removed with the saw.
The Cochlea
(922, 923).—The
cochlea bears some resemblance to a common snail-shell; it forms the anterior
part of the labyrinth, is conical in form, and placed almost horizontally in
front of the vestibule; its apex (cupula) is directed forward and
lateralward, with a slight inclination downward, toward the upper and front
part of the labyrinthic wall of the tympanic cavity; its base
corresponds with the bottom of the internal acoustic meatus, and is perforated
by numerous apertures for the passage of the cochlear division of the acoustic
nerve. It measures about
The modiolus is the conical central axis or pillar of the cochlea. Its base is broad, and appears at the bottom of the internal acoustic meatus, where it corresponds with the area cochleæ; it is perforated by numerous orifices, which transmit filaments of the cochlear division of the acoustic nerve; the nerves for the first turn and a half pass through the foramina of the tractus spiralis foraminosus; those for the apical turn, through the foramen centrale. The canals of the tractus spiralis foraminosus pass up through the modiolus and successively bend outward to reach the attached margin of the lamina spiralis ossea. Here they become enlarged, and by their apposition form the spiral canal of the modiolus, which follows the course of the attached margin of the osseous spiral lamina and lodges the spiral ganglion (ganglion of Corti). The foramen centrale is continued into a canal which runs up the middle of the modiolus to its apex. The modiolus diminishes rapidly in size in the second and succeeding coil.
The
bony canal of the cochlea takes two turns and three-quarters around the
modiolus. It is about
The osseous spiral lamina (lamina spiralis ossea) is a bony shelf or ledge which projects from the modiolus into the interior of the canal, and, like the canal, takes two-and three-quarter turns around the modiolus. It reaches about half-way toward the outer wall of the tube, and partially divides its cavity into two passages or scalæ, of which the upper is named the scala vestibuli, while the lower is termed the scala tympani. Near the summit of the cochlea the lamina ends in a hook-shaped process, the hamulus laminæ spiralis; this assists in forming the boundary of a small opening, the helicotrema, through which the two scalæ communicate with each other. From the spiral canal of the modiolus numerous canals pass outward through the osseous spiral lamina as far as its free edge. In the lower part of the first turn a second bony lamina, the secondary spiral lamina, projects inward from the outer wall of the bony tube; it does not, however, reach the primary osseous spiral lamina, so that if viewed from the vestibule a narrow fissure, the vestibule fissure, is seen between them.
The osseous labyrinth is lined by an exceedingly thin fibro-serous membrane; its attached surface is rough and fibrous, and closely adherent to the bone; its free surface is smooth and pale, covered with a layer of epithelium, and secretes a thin, limpid fluid, the perilymph. A delicate tubular process of this membrane is prolonged along the aqueduct of the cochlea to the inner surface of the dura mater.
The Membranous Labyrinth (labyrinthus membranaceus) (924, 925, 926).—The membranous labyrinth is lodged within the bony cavities just described, and has the same general form as these; it is, however, considerably smaller, and is partly separated from the bony walls by a quantity of fluid, the perilymph. In certain places it is fixed to the walls of the cavity. The membranous labyrinth contains fluid, the endolymph, and on its walls the ramifications of the acoustic nerve are distributed.
Within the osseous vestibule the membranous labyrinth does not quite preserve the form of the bony cavity, but consists of two membranous sacs, the utricle, and the saccule.
The Utricle (utriculus).—The utricle, the larger of the two, is of an oblong form, compressed transversely, and occupies the upper and back part of the vestibule, lying in contact with the recessus ellipticus and the part below it. That portion which is lodged in the recess forms a sort of pouch or cul-de-sac, the floor and anterior wall of which are thickened, and form the macula acustica utriculi, which receives the utricular filaments of the acoustic nerve. The cavity of the utricle communicates behind with the semicircular ducts by five orifices. From its anterior wall is given off the ductus utriculosaccularis, which opens into the ductus endolymphaticus.
The Saccule (sacculus).—The saccule is the smaller of the two vestibular sacs; it is globular in form, and lies in the recessus sphæricus near the opening of the scala vestibuli of the cochlea. Its anterior part exhibits an oval thickening, the macula acustica sacculi, to which are distributed the saccular filaments of the acoustic nerve. Its cavity does not directly communicate with that of the utricle. From the posterior wall a canal, the ductus endolymphaticus, is given off; this duct is joined by the ductus utriculosaccularis, and then passes along the aquæductus vestibuli and ends in a blind pouch (saccus endolymphaticus) on the posterior surface of the petrous portion of the temporal bone, where it is in contact with the dura mater. From the lower part of the saccule a short tube, the canalis reuniens of Hensen, passes downward and opens into the ductus cochlearis near its vestibular extremity (924).
The membranous labyrinth.
VIDEO 1
VIDEO 2
The Semicircular Ducts (ductus semicirculares; membranous semicircular canals), (925, 926).—The semicircular ducts are about one-fourth of the diameter of the osseous canals, but in number, shape, and general form they are precisely similar, and each presents at one end an ampulla. They open by five orifices into the utricle, one opening being common to the medial end of the superior and the upper end of the posterior duct. In the ampullæ the wall is thickened, and projects into the cavity as a fiddle-shaped, transversely placed elevation, the septum transversum, in which the nerves end.
The utricle, saccule, and semicircular ducts are held in position by numerous fibrous bands which stretch across the space between them and the bony walls.
Structure (927).
http://intranet.tdmu.edu.ua/data/teacher/video/fiz_pat/Video1/F199077_innerear.mpg
Diagrammatic longitudinal section of the cochlea.
Covering the sulcus spiralis internus and the spiral organ of Corti is the tectorial membrane, which is attached to the limbus laminæ spiralis close to the inner edge of the vestibular membrane. Its inner part is thin and overlies the auditory teeth of Huschke; its outer part is thick, and along its lower surface, opposite the inner hair cells, is a clear band, named Hensen’s stripe, due to the intercrossing of its fibers. The lateral margin of the membrane is much thinner. Hardesty 151 considers the tectorial membrane as the vibrating mechanism in the cochlea. It is inconceivably delicate and flexible; far more sensitively flexible in the transverse than in the longitudinal direction and the readiness with which it bends when touched is beyond description. It is ectodermal in origin. It consists of fine colorless fibers embedded in a transparent matrix (the matrix may be a variety of soft keratin), of a soft collagenous, semisolid character with marked adhesiveness. The general transverse direction of the fibers inclines from the radius of the cochlea toward the apex.
The vestibular nerve (n. vestibularis) supplies the utricle, the saccule, and the ampullæ of the semicircular ducts. On the trunk of the nerve, within the internal acoustic meatus, is a ganglion, the vestibular ganglion (ganglion of Scarpa); the fibers of the nerve arise from the cells of this ganglion. On the distal side of the ganglion the nerve splits into a superior, an inferior, and a posterior branch. The filaments of the superior branch are transmitted through the foramina in the area vestibularis superior, and end in the macula of the utricle and in the ampullæ of the superior and lateral semicircular ducts; those of the inferior branch traverse the foramina in the area vestibularis inferior, and end in the macula of the saccule. The posterior branch runs through the foramen singulare at the postero-inferior part of the bottom of the meatus and divides into filaments for the supply of the ampulla of the posterior semicircular duct.