Special Senses - Block 3 Module 3 Cases 1 & 2 (PDF)

SPECIAL SENSES SYSTEM COORDINATOR: DR. VINCENT CABAHUG BLOCK 3 MODULE 3 (CASES 1 AND 2) BASIC BIOMEDICAL SCIENCES I NOT FOR SALE | DO NOT UPLOAD IN...

SPECIAL SENSES SYSTEM COORDINATOR: DR. VINCENT CABAHUG BLOCK 3 MODULE 3 (CASES 1 AND 2) BASIC BIOMEDICAL SCIENCES I NOT FOR SALE | DO NOT UPLOAD IN ONLINE SITES ------------------------------------------------------------------------------------------------------------------------------------------------------- CASE 1  Inferior orbital fissure: Located posteriorly, between the maxilla ORBIT and the greater wing of the sphenoid; it communicates with the Boundaries pterygopalatine fossa. It transmits the maxillary nerve and its o The orbit is a pyramidal cavity with its base anterior and its apex infraorbital and zygomatic branches, the inferior ophthalmic vein, posterior. Parts of three bones form the orbital margin, which and sympathetic nerves. circumscribes the base:  Superior orbital fissure: Located posteriorly, between the  Superior margin: frontal bone greater and lesser wings of the sphenoid; it communicates with the  Lateral margin: processes of the frontal and zygomatic bones middle cranial fossa. It transmits the oculomotor, trochlear,  Inferior margin: zygomatic bone and the maxilla ophthalmic, and abducens nerves and the superior ophthalmic  Medial margin: processes of the maxilla and the frontal bone vein. o The orbital margins lead back to four orbital walls that are composed  Optic canal: Located posteriorly, in the lesser wing of the of multiple elements: sphenoid, at the apex of the orbit; it communicates with the middle  Roof (superior wall): Formed by the orbital plate of the frontal bone cranial fossa. It transmits the optic nerve and the ophthalmic artery. and the lesser wing of the sphenoid. These separate the orbital Contents cavity from the anterior cranial fossa and the frontal lobe of the o Eyeballs cerebral hemisphere.  Embryology  Lateral wall: Formed by the zygomatic bone and the greater wing  The first indication of the eyes is the optic grooves in the neural of the sphenoid. folds at the cranial end of the embryo. The grooves form at the  Floor (inferior wall): Formed largely by the orbital plate of the beginning of the fourth week and deepen to form hollow optic maxilla (which separates the orbital cavity from the maxillary sinus) vesicles that project from the forebrain. and to a smaller degree by the zygomatic and palatine bones.  The optic vesicles contact the surface ectoderm and induce  Medial wall: Formed anteroposteriorly by the frontal process of the development of the lens placodes. maxilla, the lacrimal bone, the orbital plate of the ethmoid (which  As the lens placode thickens to form a lens pit and lens vesicle, separates the orbital cavity from the ethmoid sinuses), and the body the optic vesicle invaginates to form the optic cup. The retina of the sphenoid. forms from the two layers of the optic cup.  The retina, optic nerve fibers, muscles of the iris, and epithelium of the iris and ciliary body are derived from the neuroectoderm of the forebrain. The sphincter and dilator muscles of the iris develop from the ectoderm at the rim of the optic cup. The surface ectoderm gives rise to the lens and the epithelium of the lacrimal glands, eyelids, conjunctiva, and cornea. The mesenchyme gives rise to the eye muscles, except those of the iris, and to all connective and vascular tissues of the cornea, iris, ciliary body, choroid, and sclera.  Layers  The eyeball consists of three coats or layers.  From external to internal, these are (1) the external fibrous coat, (2) the middle vascular coat, and (3) the inner nervous coat.  Fibrous Coat  The external, protective fibrous coat is made up of a posterior opaque part, the sclera, and an anterior transparent part, the cornea.  Sclera  The opaque sclera is composed of dense fibrous tissue and is - white. The optic nerve pierces the sclera posteriorly, and the nerve's dural sheath fuses with the sclera.  The lamina cribrosa is the area of the sclera that is pierced o Openings Into the Orbital Cavity by the nerve fibers of the optic nerve.  Several openings connect the orbit with immediately neighboring  The ciliary arteries and nerves and their associated veins, the areas and convey multiple neurovascular structures. venae vorticosae, also pierce the sclera.  Orbital opening: This Is the large anterior opening that forms the  The sclera is directly continuous in front with the cornea at the base of the orbit and communicates directly with the face. About corneoscleral junction, or limbus. one sixth of the eyeball protrudes through the base and is  Cornea exposed; the walls of the orbit protect the remainder.  The transparent cornea is largely responsible for the refraction  Supraorbital notch (often a supraorbital foramen): Situated in of the light entering the eye and is the most important the frontal bone, on the superior orbital margin. It transmits the refractive medium of the eye. This refractive power occurs on supraorbital nerve and blood vessels. the anterior surface of the cornea, where the refractive index  Infraorbital groove and canal: Situated on the floor of the orbit of the cornea differs greatly from that of the air. in the orbital plate of the maxilla. They transmit the infraorbital  The film of tears secreted from the lacrimal gland is critical in nerve (a continuation of the maxillary nerve) and blood vessels. maintaining the normal environment for the corneal epithelial  Nasolacrimal canal: Located anteriorly on the medial wall, in the cells. The cornea is in contact with the aqueous humor lacrimal bone. It transmits the nasolacrimal duct and posteriorly. communicates with the inferior meatus of the nose.  Blood supply. The cornea is avascular and devoid of  Anterior and posterior ethmoidal foramina: Two small lymphatic drainage. It is nourished by diffusion from the openings on the medial wall, typically in the ethmoid bone. These aqueous humor and from the capillaries at its edge. transmit the anterior and posterior ethmoid nerves (both branches  Nerve supply. Long ciliary nerves from the ophthalmic of the ophthalmic nerve), respectively. division of the trigeminal nerve  Zygomaticotemporal and zygomaticofacial foramina: Two  Vascular Pigmented Coat small openings on the lateral wall, usually in the zygomatic bone.  The middle vascular pigmented coat consists, from behind These convey the zygomaticotemporal and zygomaticofacial forward, of the choroid, the ciliary body, and the iris. nerves (both branches of the maxillary nerve), respectively.  Choroid  The optic disc is slightly depressed at its center, where the The choroid Is composed of an outer pigmented layer and  central artery of the retina pierces it. an inner, highly vascular layer.  There is a complete absence of rods and cones at the optic  Ciliary Body disc, so that it is insensitive to light and is referred to as the  The ciliary body is continuous posteriorly with the choroid, and blind spot. anteriorly, it lies behind the peripheral margin of the iris. It is  On ophthalmoscopic examination, the optic disc appears pale composed of the ciliary ring, the ciliary processes, and the pink in color, much paler than the surrounding retina. ciliary muscle.  The ciliary ring is the posterior part of the body; its surface has shallow grooves, the ciliary striae.  The ciliary processes are radially arranged folds, or ridges, to the posterior surfaces of which are connected the suspensory ligaments of the lens.  The ciliary muscle is composed of meridianal and circular fibers of smooth muscle. The meridianal fibers run backward from the region of the corneoscleral junction to the ciliary processes. The circular fibers are fewer in number and lie internal to the meridianal fibers.  Nerve Supply: The ciliary muscle is supplied by parasympathetic fibers from the oculomotor nerve. After synapsing in the ciliary ganglion, the postganglionic fibers pass forward to the eyeball in the short ciliary nerves.  Action: Contraction of the ciliary muscle, especially the meridianal fibers, pulls the ciliary body forward. This relieves the tension in the suspensory ligaments, and the elastic lens becomes more convex. This increases the refractive power of  Refractive Media the lens.  Aqueous Humor  Iris and Pupil  The aqueous humor is a clear fluid that fills the anterior and  The iris is a thin, contractile. pigmented diaphragm with a posterior chambers of the eyeball. It is believed to be a secretion central aperture, the pupil. It is suspended in the aqueous from the ciliary processes, from which it enters the posterior humor between the cornea and the lens. The periphery of the chamber. It then flows into the anterior chamber through the iris is attached to the anterior surface of the ciliary body. It pupil and is drained away through the spaces at the iridocorneal divides the space between the lens and the cornea into an angle into the canal of Schlemm. Obstruction to the draining of anterior and a posterior chamber. the aqueous humor results in a rise in intraocular pressure called  The muscle fibers of the iris are involuntary and consist of glaucoma. This can produce degenerative changes in the retina, circular and radiating fibers. The circular fibers form the with consequent blindness. sphincter (constrictor) pupillae and are arranged around  The function of the aqueous humor is to support the wall of the the margin of the pupil. The radial fibers form the dilator eyeball by exerting internal pressure and thus maintaining its pupillae and consist of a thin sheet of radial fibers that lie optical shape. It also nourishes the cornea and the lens and close to the posterior surface. removes the products of metabolism; these functions are  Nerve Supply: important because the cornea and the lens do not possess a The sphincter pupillae is supplied by parasympathetic blood supply. fibers from the oculomotor nerve. After synapsing in the  Vitreous Body ciliary ganglion, the postganglionic fibers pass forward to the  The vitreous body is a transparent gel that fills the eyeball eyeball in the short ciliary nerves. behind the lens. The hyaloid canal is a narrow channel that The dilator pupillae is supplied by sympathetic fibers, runs through the vitreous body from the optic disc to the which pass forward to the eyeball in the long ciliary nerves. posterior surface of the lens. In the fetus, it is filled by the  Action: hyaloid artery, which disappears before birth. The sphincter pupillae constricts the pupil in the presence  The function of the vitreous body is to contribute slightly to the of bright light and during accommodation. magnifying power of the eye. It supports the posterior surface of The dilator pupillae dilates the pupil in the presence of light the lens and assists in holding the neural part of the retina of low intensity or in the presence of excessive sympathetic against the pigmented part of the retina. activity such as occurs in fright.  Lens  Nervous Coat  The lens is a transparent, biconvex structure enclosed in a  The inner nervous (sensory) coat consists of the retina. The transparent capsule. It is situated behind the iris and in front of retina consists of an outer pigmented layer and an inner the vitreous body and is encircled by the ciliary processes. nervous layer. Its outer surface is in contact with the choroid,  The lens consists of an elastic capsule, which envelops the and its inner surface is in contact with the vitreous body. structure; a cuboidal epithelium, which is confined to the  The posterior three quarters of the retina is the receptor organ. anterior surface of the lens; and lens fibers, which are formed Its anterior edge forms a wavy ring, the ora serrata, where the from the cuboidal epithelium at the equator of the lens. The lens nervous tissues end. The anterior part of the retina is fibers make up the bulk of the lens. nonreceptive and consists of pigment cells, with a deeper  The elastic lens capsule is under tension, causing the lens layer of columnar epithelium. This anterior part of the retina constantly to endeavor to assume a globular rather than a disc covers the ciliary processes and the back of the iris. shape. The equatorial region, or circumference, of the lens  At the center of the posterior part of the retina is an oval, attaches to the ciliary processes of the ciliary body by the yellowish area, the macula lutea, which is the area of the retina suspensory ligament. The pull of the radiating fibers of the for the most distinct vision. It has a central depression, the fovea suspensory ligament tends to keep the elastic lens flattened so centralis. that the eye can be focused on distant objects.  The optic nerve leaves the retina about 3 mm to the medial side of the macula lutea by the optic disc.   Chambers  The orbital septum is thickened at the margins of the lids to form  The anterior chamber of the eye develops from a cleft-like space the superior and inferior tarsal plates. that forms in the mesenchyme located between the developing  The lateral palpebral ligament is a band that connects the lens and cornea. lateral ends of the tarsal plates to a bony tubercle just within the  The mesenchyme superficial to this space forms the orbital margin. substantia propria (transparent connective tissue) of the  The medial palpebral ligament attaches the medial ends of the cornea and the mesothelium of the anterior chamber. plates to the crest of the lacrimal bone.  After the lens is established, it induces the surface ectoderm  The tarsal glands are embedded in the posterior surface of the to develop into the epithelium of the cornea and conjunctiva. tarsal plates.  The posterior chamber of the eye develops from a space that  The superficial surface of the tarsal plates and the orbital septum forms in the mesenchyme posterior to the developing iris and are covered by the palpebral fibers of the orbicularis oculi anterior to the developing lens. muscle.  When the pupillary membrane disappears and the pupil forms,  The aponeurosis of insertion of the levator palpebrae the anterior and posterior chambers of the eye are able to superioris muscle pierces the orbital septum to reach the communicate with each other through the scleral venous anterior surface of the superior tarsal plate and the skin. sinus. This vascular structure encircling the anterior chamber  A thin smooth muscle band, the superior tarsal muscle, of the eye is the outflow site of aqueous humor from the underlies the levator. anterior chamber to the venous system.  Extraocular muscles o Accessory visual structures  Eyelids-anatomy and physiology  The eyelids protect the eye from injury and excessive light. The upper eyelid is larger and more mobile than the lower, and they meet each other at the medial and lateral angles.  The palpebral fissure is the elliptical opening between the eyelids and Is the entrance into the conjunctival sac.  When the eye is closed, the upper eyelid completely covers the cornea of the eye.  When the eye is open and looking straight ahead, the upper lid just covers the upper margin of the cornea. The lower lid lies just below the cornea when the eye is open and rises only slightly when the eye is closed.  The superficial surface of the eyelids is covered by skin, and the deep surface is covered by a mucous membrane called the conjunctiva.  The eyelashes are short, curved hairs on the free edges of the eyelids. They are arranged in double or triple rows at the mucocutaneous junction.  The sebaceous glands (glands of Zeis) open directly into the eyelash follicles.  The ciliary glands (glands of Moll) are modified sweat glands that open separately between adjacent lashes.  Six skeletal muscles (superior rectus, inferior rectus, medial  The tarsal glands are long, modified sebaceous glands that rectus, lateral rectus, superior oblique, inferior oblique) run pour their oily secretion onto the margin of the lid; their openings from the bony walls of the orbit to insertions on the exterior of the lie behind the eyelashes. This oily material prevents the overflow eyeball. of tears and helps make the closed eyelids airtight.  The four rectus muscles arise from a common tendinous ring  The more rounded medial angle is separated from the eyeball by surrounding the optic foramen. a small space, the lacus lacrimalis, in the center of which is a  The superior oblique arises nearby. Only the inferior oblique small, reddish yellow elevation, the caruncula lacrimalis. originates from the anterior floor of the orbit.  A reddish semilunar fold, called the plica semilunaris, lies on  Because the superior and the inferior recti are inserted on the the lateral side of the caruncle. medial side of the vertical axis of the eyeball, they raise and  A small elevation, the papilla lacrimalis, is present near the depress the pupil, respectively, but also rotate it medially. medial angle of the eye.  For the superior rectus muscle to raise the pupil directly upward,  On the summit of the papilla is a small hole, the punctum the inferior oblique muscle must assist; for the inferior rectus to lacrimale, which leads into the canaliculus lacrimalis. depress the pupil directly downward, the superior oblique  The papilla lacrimalis projects into the lacus, and the punctum muscle must assist. and canaliculus carry tears down into the nose.  Note that the tendon of the superior oblique muscle passes  The conjunctiva is a thin mucous membrane that lines the eyelids through a fibrocartilaginous pulley (trochlea) attached to the and is reflected at the superior and inferior fornices onto the frontal bone. Here, the tendon turns backward and laterally and anterior surface of the eyeball. inserts into the sclera beneath the superior rectus muscle.  Its epithelium is continuous with that of the cornea.  Three cranial nerves (oculomotor, trochlear, and abducens)  The ducts of the lacrimal gland pierce the upper lateral part of control the six extraocular muscles. the superior fornix.  The trochlear nerve supplies only the superior oblique muscle.  The conjunctiva thus forms a potential space, the conjunctival  The abducens nerve innervates only the lateral rectus muscle. sac, which is open at the palpebral fissure.  The oculomotor nerve governs all the remaining muscles.  The subtarsal sulcus is a groove on the deep side of the eyelid  A convenient way to recall this motor organization is with the that runs close to and parallel with the margin of the lid. The chemical formula SO4(LR6)3. That is, the fourth cranial nerve sulcus tends to trap small foreign particles introduced into the (trochlear) supplies the superior oblique (SO), the sixth cranial conjunctival sac and is thus clinically important. nerve (abducens) supplies the lateral rectus (LR), and the third  The orbital septum is a fibrous sheet that forms the framework of cranial nerve (oculomotor) supplies all the others. the eyelids. This is attached to the periosteum at the orbital margins.  Nerves and vessels branch at the lower border of the nasal bone and supplies  Six cranial nerves (optic, oculomotor, trochlear, trigeminal, the skin of the nose down as far as the tip. abducens, facial) plus additional autonomic components supply  Frontal Nerve the orbit.  The frontal nerve enters the orbit through the upper part of the  Optic Nerve (CN I) superior orbital fissure and passes forward on the upper  The optic nerve enters the orbit from the middle cranial fossa by surface of the levator palpebrae superioris beneath the roof of passing through the optic canal. It is accompanied by the the orbit. ophthalmic artery, which lies on its lower lateral side.  It divides into the supratrochlear and supraorbital nerves  Sheaths of pia mater, arachnoid mater, and dura mater surround that wind around the upper margin of the orbital cavity to the nerve. It runs forward and laterally within the cone of the recti supply the skin of the forehead; the supraorbital nerve also muscles and pierces the sclera at a point medial to the posterior supplies the mucous membrane of the frontal air sinus. pole of the eyeball.  Lacrimal Nerve  Here, the meninges fuse with the sclera so that the  The lacrimal nerve enters the orbit through the upper part of subarachnoid space with its contained cerebrospinal fluid the superior orbital fissure and passes forward along the upper extends forward from the middle cranial fossa, around the optic border of the lateral rectus muscle. nerve, and through the optic canal, as far as the eyeball. Thus,  It joins a branch of the zygomaticotemporal nerve, which later a rise in pressure of the cerebrospinal fluid within the cranial leaves it to enter the lacrimal gland (parasympathetic cavity is transmitted to the back of the eyeball. secretomotor fibers). The lacrimal nerve ends by supplying the  Oculomotor Nerve (CN Ill) skin of the lateral part of the upper lid.  The oculomotor nerve divides into two parts, a superior and an  Abducens Nerve (CN VI) inferior ramus, as it enters the orbit through the lower part of the  The abducens nerve enters the orbit through the lower part of superior orbital fissure. the superior orbital fissure. It supplies the lateral rectus muscle.  The superior ramus supplies the superior rectus muscle, then  Facial Nerve (CN VII) pierces it, and supplies the levator palpebrae superioris muscle.  The facial nerve does not enter the orbit. However, its temporal  The inferior ramus supplies the inferior rectus, the medial and zygomatic branches supply the orbicularis oculi muscle and rectus, and the inferior oblique muscles. other facial muscles that influence the eyelids.  The nerve to the inferior oblique gives off a branch that passes  Also, it initiates the parasympathetic pathway that eventually to the ciliary ganglion and carries parasympathetic fibers to the connects to the lacrimal nerve and supplies secretomotor fibers sphincter pupillae and the ciliary muscle. to the lacrimal gland.  Trochlear Nerve (CN IV)  Ciliary Ganglion  The trochlear nerve enters the orbit through the upper part of the  The ciliary ganglion is a parasympathetic ganglion about the size superior orbital fissure. of a pinhead and situated in the posterior part of the orbit.  It runs forward and supplies the superior oblique muscle.  It receives its preganglionic parasympathetic fibers from the  Trigeminal Nerve (CN V) oculomotor nerve via the nerve to the inferior oblique. The  The ophthalmic division of the trigeminal nerve passes through postganglionic fibers leave the ganglion in the short ciliary the superior orbital fissure and immediately divides into three nerves, which enter the back of the eyeball and supply the branches. From medial to lateral, these are the nasociliary, sphincter pupillae and the ciliary muscle. frontal, and lacrimal nerves.  A number of sympathetic fibers pass from the internal carotid  The maxillary division of the trigeminal nerve crosses the inferior plexus into the orbit and run through the ganglion without orbital fissure and gives off the infraorbital nerve, which enters interruption. the infraorbital canal in the floor of the orbit.  Orbital fascia  Nasociliary Nerve  The orbital fascia is the periosteum of the bones that form the walls  The nasociliary nerve enters the orbit through the lower part of the orbit. It is loosely attached to the bones and is continuous of the superior orbital fissure. It crosses above the optic nerve, through the foramina and fissures with the periosteum covering runs forward along the upper margin of the medial rectus the outer surfaces of the bones muscle, and ends by dividing into the anterior ethmoidal and  Mucous membrane (Conjunctiva) infratrochlear nerves.  Lines the eyelids and anterior aspect of the eyeballs, and most of  Branches the lacrimal apparatus, which lubricates it. The communicating branch to the ciliary ganglion is a LACRIMAL APPARATUS sensory nerve. The sensory fibers from the eyeball pass to The lacrimal apparatus consists of the structures that secrete and collect the ciliary ganglion via the short ciliary nerves, pass tears, that is, the lacrimal gland and the lacrimal ducts. Proper through the ganglion without interruption, and then join the function of this system is critical in ensuring the cornea remains moist. nasociliary nerve by means of the communicating branch. Anatomical parts The long ciliary nerves, two or three in number, arise from o Lacrimal Gland the nasociliary nerve as it crosses the optic nerve. They  The lacrimal gland consists of a large orbital part and a small contain sympathetic fibers for the dilator pupillae muscle. palpebral part, which are continuous with each other around the The nerves pass forward with the short ciliary nerves and lateral edge of the aponeurosis of the levator palpebrae superioris. pierce the sclera of the eyeball. They continue forward  The gland is situated above the eyeball in the upper anterolateral between the sclera and the choroid to reach the iris. part of the orbit posterior to the orbital septum. The posterior ethmoidal nerve supplies the ethmoidal and  The gland opens into the lateral part of the superior fornix of the sphenoidal air sinuses. conjunctiva by approximately 12 ducts. The infratrochlear nerve passes forward below the pulley  The parasympathetic secretomotor nerve supply is derived from of the superior oblique muscle and supplies the skin of the the lacrimal nucleus of the facial nerve. medial part of the upper eyelid and the adjacent part of the  The preganglionic fibers reach the pterygopalatine ganglion via nose. the nervus intermedius and its great petrosal branch and via the The anterior ethmoidal nerve passes through the anterior nerve of the pterygoid canal. ethmoidal foramen and enters the anterior cranial fossa on  The postganglionic fibers leave the ganglion and join the maxillary the upper surface of the cribriform plate of the ethmoid. It nerve. They then pass into its zygomatic branch and the enters the nasal cavity through a slit-like opening alongside zygomaticotemporal nerve. They reach the lacrimal gland within the crista galli. After supplying an area of mucous the lacrimal nerve. membrane, It appears on the face as the external nasal   The sympathetic postganglionic nerve supply is from the internal a total refractive power of 59 diopters when the lens is carotid plexus and travels in the deep petrosal nerve, the nerve of accommodated for distant vision. the pterygoid canal, the maxillary nerve, the zygomatic nerve, the  About two-thirds of the 59 diopters of refractive power of the eye is zygomaticotemporal nerve, and finally the lacrimal nerve. provided by the anterior surface of the cornea (not by the eye lens). o Lacrimal Ducts  The principal reason for this phenomenon is that the refractive  Tears secreted from the lacrimal gland circulate across the cornea index of the cornea is markedly different from that of air, whereas and accumulate in the lacus Iacrimalis. From here, the tears enter the refractive index of the eye lens is not greatly different from the the canaliculi lacrimales through the puncta lacrimalis. The indices of the aqueous humor and vitreous humor. canaliculi lacrimales pass medially and open into the lacrimal sac,  The total refractive power of the internal lens of the eye, as it which lies in the lacrimal groove behind the medial palpebral normally lies in the eye surrounded by fluid on each side, is only 20 ligament and Is the upper blind end of the nasolacrimal duct. diopters, about one third the total refractive power of the eye.  The nasolacrimal duct is about 0.5 ln. (1.3 cm) long and emerges  However, the importance of the internal lens is that in response to from the lower end of the lacrimal sac. The duct descends downward, nervous signals from the brain, its curvature can be increased backward, and laterally in a bony canal and opens into the inferior markedly to provide “accommodation.” nasal meatus in the inferior lateral wall of the nasal passage. A fold o Formation of an Image on the Retina. of mucous membrane, the lacrimal fold, guards the opening. This  In the same manner that a glass lens can focus an image on a sheet prevents air from being forced up the duct into the lacrimal sac on of paper, the lens system of the eye can focus an image on the retina. blowing the nose.  The image is inverted and reversed with respect to the object. Secretory system  However, the mind perceives objects in the upright position despite o The lacrimal apparatus is made up of the excretory system and the upside-down orientation on the retina because the brain is secretory system. trained to consider an inverted image as normal. o The secretory system is composed of the basic receptors and the reflex secretors. The glands that produce the basic secretions form the precorneal tear film that cover the corneal epithelium and bulbar conjunctiva are:  Inner layer- which is made up of mucin (high-molecular-weight glycoproteins) is derived from the goblet cells in the conjunctiva, the crypts of Henle, and the glands of Manz Refractive errors of the eye  Middle layer- which is watery or aqueous or lacrimal is derived from o Emmetropia (Normal Vision) the accessory lacrimal glands of Krause and accessory glands of  The eye is considered to be normal, or emmetropic, if parallel light Wolfring. rays from distant objects are in sharp focus on the retina when the  The glands of Krause are located in the superior conjunctival fornix. ciliary muscle is completely relaxed. This means that the  The glands of Wolfring are found at the non-marginal border of the emmetropic eye can see all distant objects clearly with its ciliary tarsal plate. muscle relaxed.  Outer layer- made up of oily secretions derived from the tarsal  However, to focus objects at close range, the eye must contract its Meibomian glands, glands of Zeis, and glands of Mall. ciliary muscle and thereby provide an appropriate degree of o Basic secretion is the fundamental, indispensable part of the secretory accommodation. system. The reflex secretors are made up of the lacrimal gland and o Hyperopia (Farsightedness) accessory lacrimal glands.  Hyperopia or “farsightedness” is usually due to either an eyeball that o The average normal secretion of tears is between 0.9 and 2.2 ul/min. is too short or, occasionally, to a lens system that is too weak. The average glucose in tears is 2.5-4.1 mg/100ml. The albumin is  In this condition, parallel light rays are not bent sufficiently by the 60% of total protein in tears, the remainder divided equally between relaxed lens system to come to focus by the time they reach the globulin and lysozyme fractions. The dominant immunoglobulin in retina. To overcome this abnormality, the ciliary muscle must tears, saliva, and lysozyme is IgA. contract to increase the strength of the lens. Excretory system  By using the mechanism of accommodation, a farsighted person is o Composed of the lacrimal canaliculi, lacrimal sac, nasolacrimal duct. capable of focusing distant objects on the retina. Tears drain from the upper and lower puncta to the canaliculi and  If the person has used only a small amount of strength in the ciliary then to the lacrimal sac then to the nasolacrimal duct which empties muscle to accommodate for the distant objects, he or she still has into the inferior meatus of the nasal cavity. much accommodative power left, and objects closer and closer to EYE AS AN OPTICAL SYSTEM the eye can also be focused sharply until the ciliary muscle has Refractive surfaces of the eye contracted to its limit. o The lens system of the eye is composed of four refractive interfaces:  In old age, when the lens becomes “presbyopic,” a farsighted  The interface between air and the anterior surface of the cornea; person is often unable to accommodate the lens sufficiently to focus  The interface between the posterior surface of the cornea and the even on distant objects, much less on near objects. aqueous humor o Myopia (Nearsightedness)  The interface between the aqueous humor and the anterior surface  In myopia, or “nearsightedness,” when the ciliary muscle is of the lens of the eye completely relaxed, the light rays coming from distant objects are  The interface between the posterior surface of the lens and the focused in front of the retina. vitreous humor.  This condition is usually due to too long an eyeball but also can result o The internal index of air is 1, the cornea, 1.38, the aqueous humor, from too much refractive power in the lens system of the eye. 1.33, the crystalline lens (on average), 1.40, and the vitreous humor,  No mechanism exists whereby the eye can decrease the strength of 1.34. its lens to less than that which exists when the ciliary muscle is o Consideration of All Refractive Surfaces of the Eye as a Single completely relaxed. Lens-The “Reduced” Eye.  A myopic person has no mechanism to focus distant objects  If all the refractive surfaces of the eye are added together sharply on the retina. algebraically and then considered to be one single lens, the optics  However, as an object moves nearer to the person’s eye, it finally of the normal eye may be simplified and represented schematically gets close enough that its image can be focused. as a “reduced eye.” This representation is useful in simple  Then, when the object comes still closer to the eye, the person can calculations. use the mechanism of accommodation to keep the image focused  In the reduced eye, a single refractive surface is considered to clearly. A myopic person has a definite limiting “far point” for clear exist, with its central point 17 millimeters in front of the retina and vision. o Correction of Myopia and Hyperopia Through Use of Lenses PHYSIOLOGY OF VISION  If the refractive surfaces of the eye have too much refractive power, Definition of vision as in myopia, this excessive refractive power can be neutralized by o These are structures that the light pass through before reaching the placing a concave spherical lens in front of the eye, which will retina, namely (a) cornea, (b) aqueous humor, (c) crystalline lens, and diverge rays. (d) vitreous humor  Conversely, in a person who has hyperopia- that is, someone who o Normal vision happens in the following order: has too weak a lens system- the abnormal vision can be corrected  Light enters the eye through the cornea by adding refractive power using a convex lens in front of the eye.  Clear, dome-shaped surface that covers the front of the eye  One usually determines the strength of the concave or convex lens  From the cornea, the light passes through the pupil needed for clear vision by “trial and error”- that is, by first trying a  Iris, or the colored part of the eye, controls the amount of light strong lens and then a stronger or weaker lens until the one that passing through gives the best visual acuity is found.  From there, it then hits the lens o Astigmatism  Clear structure inside the eye that focuses light rays onto the retina  Astigmatism is a refractive error of the eye that causes the visual  Next, light passes through the vitreous humor image in one plane to focus at a different distance from that of the  Clear, jelly-like substance that fills the center of the eye plane at right angles.  Helps to keep the eye round in shape  Astigmatism usually results from too great a curvature of the  Finally, the light reaches the retina cornea in one plane of the eye. An example of an astigmatic lens  Light-sensitive nerve layer that lines the back of the eye would be a lens surface like that of an egg lying sidewise to the  From here the image is inverted incoming light. Conversion of light stimulus to electrical stimulus in the retina  The degree of curvature in the plane through the long axis of the o Vision is the process initiated when the outer segments of the retina egg is not nearly as great as the degree of curvature in the plane convert the portion of the electromagnetic spectrum that constitutes through the short axis. visible light (400-700 nm) to an electrical signal.  Because the curvature of the astigmatic lens along one plane is less o The visual impulse is initiated when the quantum of electromagnetic than the curvature along the other plane, light rays striking the energy (a photon) transfers its oscillation to a molecule of a peripheral portions of the lens in one plane are not bent nearly as photopigment of a rod or cone with which it shares the same natural much as the rays striking the peripheral portions of the other plane. frequency. All visual pigments made up of a protein called an opsin  This effect is demonstrated in Figure 50-14, which shows rays of and 11-cis retinal, the aldehyde of vitamin A, an alcohol (retinol) light originating from a point source and passing through an oblong, o Human photoreceptor outer segment disks contains 4 different opsins astigmatic lens. (light-absorbing proteins).  The light rays in the vertical plane, indicated by plane BD, are  Rhodopsin, the photopigment of rods, has a maximum absorption refracted greatly by the astigmatic lens because of the greater at 496 nm, an absorption spectrum similar to that of the retina in dim curvature in the vertical direction than in the horizontal direction. illuminations.  By contrast, the light rays in the horizontal plane, indicated by  The outer segment of cone disks contain 1 of 3 photopigments: plane AC, are not bent nearly as much as the light rays in vertical  Short wavelength sensitive (blue) plane BD. Therefore, light rays passing through an astigmatic lens  Middle wavelength sensitive (green) do not all come to a common focal point because the light rays  Long wavelength sensitive (red) passing through one plane focus far in front of those passing o Vision is divided into surround (or ambient) vision and focal vision. through the other plane.  Surround vision is mediated primarily by the peripheral retina and  The accommodative power of the eye can never compensate for provides information concerning spatial localization astigmatism because, during accommodation, the curvature of the  Focal vision is mediated primarily by the fovea centralis and eye lens changes approximately equally in both planes; therefore, in subserves form perception, identification, and color vision astigmatism, each of the two planes requires a different degree of o The nervous impulse that originates in the retina passes in the optic accommodation. nerve, optic chiasm, and the optic tract to the lateral geniculate body  Thus, without the aid of glasses, a person with astigmatism never of the thalamus. Here, axons carrying visual impulse synapse with sees in sharp focus. axons that terminate in the visual cortex o Correction of Astigmatism With a Cylindrical Lens Components of the visual pathway  One may consider an astigmatic eye as having a lens system made up of two cylindrical lenses of different strengths and placed at right angles to each other.  To correct for astigmatism, the usual procedure is to find a spherical lens by trial and error that corrects the focus in one of the two planes of the astigmatic lens. Then, an additional cylindrical lens is used to correct the remaining error in the remaining plane. To do this, both the axis and the strength of the required cylindrical lens must be determined. o Figure 51-12 presents the essentials of the retina’s neural connections, showing the circuit in the peripheral retina at the left and the circuit in the foveal retina at the right. The different neuronal cell types are as follows:  The photoreceptors (the rods and cones)- which transmit signals to the outer plexiform layer, where they synapse with bipolar cells and horizontal cells  The horizontal cells, which transmit signals horizontally in the outer plexiform layer from the rods and cones to bipolar cells   The bipolar cells, which transmit signals vertically from the rods, o Interpretation of Color in the Nervous System cones, and horizontal cells to the inner plexiform layer, where they  In Figure 51-10, one can see that an orange monochromatic light synapse with ganglion cells and amacrine cells with a wavelength of 580 nanometers stimulates the red cones to  The amacrine cells, which transmit signals in two directions, either a value of about 99 (99% of the peak stimulation at optimum directly from bipolar cells to ganglion cells or horizontally within the wavelength); it stimulates the green cones to a value of about 42, inner plexiform layer from axons of the bipolar cells to dendrites of but the blue cones are not stimulated at all. Thus, the ratios of the ganglion cells or to other amacrine cells stimulation of the three types of cones in this case are 99:42:0. The  The ganglion cells, which transmit output signals from the retina nervous system interprets this set of ratios as the sensation of through the optic nerve into the brain orange.  A sixth type of neuronal cell in the retina, which is not very prominent  Conversely, a monochromatic blue light with a wavelength of 450 and is not shown in the figure, is the interplexiform cell. This type nanometers stimulates the red cones to a stimulus value of 0, the of cell transmits signals in the retrograde direction from the inner green cones to a value of 0, and the blue cones to a value of 97. plexiform layer to the outer plexiform layer. These signals are This set of ratios- 0:0:97- is interpreted by the nervous system as inhibitory and are believed to control lateral spread of visual signals blue. by the horizontal cells in the outer plexiform layer. Their role may be  Likewise, ratios of 83:83:0 are interpreted as yellow, and ratios of to help control the degree of contrast in the visual image. 31:67:36 are interpreted as green. o The Visual Pathway From the Cones to the Ganglion Cells o Perception of White Light Functions Differently From the Rod Pathway  About equal stimulation of all the red, green, and blue cones gives  As is true for many of our other sensory systems, the retina has both one the sensation of seeing white. Yet, there is no single wavelength an old type of vision based on rod vision and a new type of vision of light corresponding to white; instead, white is a combination of all based on cone vision. The neurons and nerve fibers that conduct the wavelengths of the spectrum. the visual signals for cone vision are considerably larger than those  Furthermore, the perception of white can be achieved by stimulating that conduct the visual signals for rod vision, and the signals are the retina with a proper combination of only three chosen colors that conducted to the brain 2-5 times as rapidly. Also, the circuitry for the stimulate the respective types of cones about equally. two systems is slightly different. o Color Blindness  To the right in Figure 51-12 is the visual pathway from the foveal  Red-Green Color Blindness portion of the retina, representing the new, fast cone system.  When a single group of color-receptive cones is missing from the  This illustration shows three neurons in the direct pathway: (1) eye, the person is unable to distinguish some colors from others. cones; (2) bipolar cells; and (3) ganglion cells.  For example, one can see in Figure 51-10 that green, yellow,  In addition, horizontal cells transmit inhibitory signals laterally in orange, and red colors, which are the colors between the the outer plexiform layer, and amacrine cells transmit signals wavelengths of 525 and 675 nanometers, are normally laterally in the inner plexiform layer. distinguished from one another by the red and green cones.  To the left in Figure 51-12 are the neural connections for the  If either of these two cones is missing, the person cannot use peripheral retina, where both rods and cones are present. this mechanism for distinguishing these four colors; the person  Three bipolar cells are shown; the middle of these connects only is especially unable to distinguish red from green and is to rods, representing the type of visual system present in many therefore said to have red-green color blindness. lower animals. The output from the bipolar cell passes only to  A person with loss of red cones is called a protanope; the overall amacrine cells, which relay the signals to the ganglion cells. visual spectrum is noticeably shortened at the long wavelength  Thus, for pure rod vision, there are four neurons in the direct visual end because of a lack of the red cones. pathway: (1) rods; (2) bipolar cells; (3) amacrine cells; and (4)  A colorblind person who lacks green cones is called a ganglion cells. deuteranope; this person has a perfectly normal visual spectral  In addition, horizontal and amacrine cells provide lateral width because red cones are available to detect the long connectivity. wavelength red color. However, a deuteranope can only  The other two bipolar cells shown in the peripheral retinal circuitry of distinguish 2 or 3 different hues, whereas somebody with normal Figure 51-12 connect with both rods and cones; the outputs of these vision sees 7 unique hues. bipolar cells pass both directly to ganglion cells and by way of  Red-green color blindness is a genetic disorder that occurs almost amacrine cells. exclusively in males. Mechanism of color perception and deficiencies  That is, genes in the female X chromosome code for the o Different cones are sensitive to different colors of light. The retina respective cones. Yet, color blindness almost never occurs in detects the different gradations of color in the visual spectrum. females because at least one of the two X chromosomes almost o Tricolor Mechanism of Color Detection always has a normal gene for each type of cone. Because the  All theories of color vision are based on the well-known observation male has only one X chromosome, a missing gene can lead to that the human eye can detect almost all gradations of colors when color blindness. only red, green, and blue monochromatic lights are appropriately  Because the X chromosome in the male is always inherited from mixed in different combinations. the mother, never from the father, color blindness is passed from o Spectral Sensitivities of the Three Types of Cones mother to son, and the mother is said to be a color blindness  On the basis of color vision tests, the spectral sensitivities of the carrier. About 8% of all women are color blindness carriers. three types of cones in humans have proved to be essentially the  Blue Weakness same as the light absorption curves for the three types of pigment  Only rarely are blue cones missing, although sometimes they are found in the cones. These curves are shown in Figure 51-8 and underrepresented in a genetically inherited condition called blue slightly differently in Figure 51-10. They can explain most of the weakness. phenomena of color vision  Color Test Charts  A rapid method for determining color blindness is based on the use of spot charts such as those shown in Figure 51-11. These charts are arranged with a mixture of spots of several different colors.  In the top chart, a person with normal color vision reads “74,” whereas a red-green color-blind person reads “21.”  In the bottom chart, a person with normal color vision reads “42,” whereas a red-blind person reads “2,” and a green-blind person reads “4.”  Anatomical Parts o External Ear  The external ear has an auricle and an external auditory meatus. The auricle (pinna) has a characteristic shape and collects air vibrations.  It consists of a thin plate of elastic cartilage covered by skin. Its main components are the helix (the elevated margin of the auricle), tragus (a projection from the anterior margin of the auricle that extends over the opening of the external acoustic meatus), lobule (earlobe, which does not contain cartilage), and concha (the deepest depression within the auricle that leads into the external meatus).  The auricle is an effective sound collecting and localizing device In many mammals. However, that function is highly questionable in humans. Mechanism of light and dark adaptation  The auricular muscles are essentially vestigial in humans but are o If a person has been in bright light for hours, large portions of the variably entertaining to those able to wiggle their ears. photochemicals in both the rods and the cones will have been reduced  The external auditory (acoustic) meatus is a slender curved tube to retinal and opsins. Furthermore, much of the retinal of both the rods that leads from the concha of the auricle to the tympanic membrane. and the cones will have been converted into vitamin A. It conducts sound waves from the auricle to the tympanic membrane.  Because of these two effects, the concentrations of the  The outer (lateral) third of the meatus is elastic cartilage. The inner photosensitive chemicals remaining in the rods and cones are (medial) two thirds is bony and is formed by the tympanic plate of considerably reduced, and the sensitivity of the eye to light is the temporal bone. correspondingly reduced.  The meatus is lined by skin, and its outer third is provided with  This process is called light adaptation. hairs and sebaceous and ceruminous glands. The latter are o Conversely, if a person remains in darkness for a long time, the retinal modified sweat glands that secrete a yellowish brown wax and opsins in the rods and cones are converted back into the light- (cerumen: earwax). The hairs and the wax provide a sticky barrier sensitive pigments. Furthermore, vitamin A is converted back into that prevents the entrance of foreign bodies. retinal to increase light-sensitive pigments, the final limit being  The lesser occipital and great auricular nerves (branches of the determined by the amounts of opsins in the rods and cones to combine cervical plexus) are the main sensory nerves of the auricle. These with the retinal. are supplemented by small components of the facial and  This process is called dark adaptation. glossopharyngeal nerves.  The auriculotemporal nerve and the auricular branch of the CASE 2 vagus nerve are the primary sensory nerves for the external EARS meatus and the external surface of the tympanic membrane. Embryology  The lymph drainage is to the superficial parotid, mastoid, and o The otic vesicle develops from the surface ectoderm during the fourth superficial cervical lymph nodes. week. The vesicle develops into the membranous labyrinth of the  Tympanic Membrane internal ear.  The tympanic membrane (eardrum) is a thin, fibrous structure o The otic vesicle divides into a dorsal utricular part, which gives rise that forms the interface between the external and middle ears. to the utricle, semicircular ducts, and endolymphatic duct, and a  The membrane is obliquely oriented, facing inferiorly, anteriorly, ventral saccular part, which gives rise to the saccule and cochlear and laterally. It is slightly concave laterally, with a small duct. The cochlear duct gives rise to the spiral organ. depression (the umbo) at the depth of the concavity. The tip of o The bony labyrinth develops from the mesenchyme adjacent to the the handle of the malleus produces the umbo. membranous labyrinth. The epithelium lining the tympanic cavity,  When the membrane is illuminated through an otoscope, the mastoid antrum, and pharyngotympanic tube is derived from the concavity produces a cone of light, which is a bright, light- endoderm of the tubotympanic recess, which develops from the first reflecting area that radiates anteriorly and inferiorly from the pharyngeal pouch. umbo. o The auditory ossicles develop from the dorsal ends of the cartilages in  The tympanic membrane is circular and measures about 1 cm the first two pharyngeal arches. The epithelium of the external diameter. The circumference is thickened and is slotted into a acoustic meatus develops from the ectoderm of the first pharyngeal groove in the bone. groove.  The groove (tympanic sulcus) is deficient superiorly, which o The tympanic membrane is derived from three sources: endoderm of forms a notch. From the sides of the notch, two bands, termed the first pharyngeal pouch, ectoderm of the first pharyngeal groove, the anterior and posterior malleolar folds, pass to the lateral and mesenchyme between the previous two layers. process of the malleus. o The auricle develops from the fusion of six auricular hillocks, which  The small triangular area on the tympanic membrane that is form from mesenchymal prominences around the margins of the first bounded by the folds is slack and is called the pars flaccida. pharyngeal groove.  The remainder of the membrane is tense and is called the pars o Congenital deafness may result from abnormal development of the tensa. The handle of the malleus is bound to the inner surface membranous labyrinth, bony labyrinth, or auditory ossicles. of the tympanic membrane by the mucous membrane.  Inheritance of a recessive trait is the most common cause of  The tympanic membrane is extremely sensitive to pain. congenital deafness, but a rubella virus infection near the end of  The auriculotemporal nerve and the auricular branch of the the embryonic period is a major cause of abnormal development of vagus nerve innervate its outer surface, and the tympanic the spiral organ and defective hearing. plexus of the glossopharyngeal nerve supplies its inner o There are many minor anomalies of the auricle; however, some of aspect. them may alert clinicians to the possible presence of associated major o Middle Ear anomalies (e.g., defects of middle ear). Low-set, severely malformed  The middle ear is an air-containing cavity within the petrous part of ears are often associated with chromosomal abnormalities, the temporal bone and is lined with a mucous membrane. It is a particularly trisomy 13 and trisomy 18. narrow, oblique, slit-like cavity whose long axis lies approximately parallel to the plane of the tympanic membrane.  It communicates with the nasopharynx anteriorly and with the from the anterior wall onto the medial wall supports the tensor mastoid antrum posteriorly. The middle ear has four main tympani muscle. components: (1) the tympanic cavity, (2) the auditory ossicles and  Its posterior end is curved upward and forms a pulley, the their muscles, (3) the auditory tube, and (4) the mastoid area. processus cochleariformis, around which the tendon of the  Tympanic Cavity tensor tympani bends laterally to reach its insertion on the  The tympanic cavity is the main chamber of the middle ear. It handle of the malleus. consists of two parts: (1) the tympanic cavity proper and (2) the  The lateral (membranous) wall is largely formed by the tympanic epitympanic recess. membrane. This wall has a superior part formed by the  The tympanic cavity proper is the area directly medial to the epitympanic recess and an inferior part formed by the eardrum. tympanic membrane.  Auditory Ossicles and Associated Muscles  The epitympanic recess is the upper portion of the chamber  The three auditory ossicles are the malleus, incus, and stapes. located superior to the tympanic membrane. These form a mobile osseous bridge from the tympanic membrane  Conceptually, think of the tympanic cavity as a six-sided box with to the oval window. The joints between ossicles are synovial type. a roof (superior wall), floor (inferior wall), anterior wall, posterior  The malleus is the largest ossicle and possesses a head, a neck, wall, medial wall, and lateral wall. a long process (handle), an anterior process, and a lateral process.  The roof is a thin plate of bone, the tegmen tympani, which is  The head is rounded and articulates posteriorly with the incus. part of the petrous temporal bone.  The neck is the constricted part below the head.  It separates the epitympanic recess of the tympanic cavity from  The handle passes downward and backward and is firmly the dura mater lining the floor of the middle cranial fossa. attached to the medial surface of the tympanic membrane. It Is  Fracture of the tegmen tympani may cause leaking of visible through the tympanic membrane on otoscopic cerebrospinal fluid into the tympanic cavity. examination.  The floor (jugular wall) is a thin plate of bone, which may be  The anterior process is a spicule of bone that connects to the partly replaced by fibrous tissue. anterior wall of the tympanic cavity by a ligament.  It separates the tympanic cavity from the superior bulb of the  The lateral process projects laterally and attaches to the internal jugular vein. anterior and posterior malleolar folds of the tympanic membrane.  The tympanic nerve (a branch of CN IX) pierces the floor to  The incus possesses a large body and two processes. enter the tympanic cavity.  The body is rounded and articulates anteriorly with the head of  The anterior (carotid) wall has two openings in its upper part. the malleus. The head of the malleus and the body of the incus  The upper, smaller opening conveys the tensor tympani occupy much of the epitympanic recess. muscle. The lower, larger opening leads into the auditory tube.  The long process descends behind and parallel to the handle  The thin, bony septum that separates the canals is prolonged of the malleus. Its lower end bends medially and articulates with backward on the medial wall, where it forms a shelflike the head of the stapes. Its shadow on the tympanic membrane projection. can sometimes be recognized on otoscopic examination.  The carotid canal (containing the internal carotid artery) lies  The short process projects backward and attaches to the immediately adjacent to the thin, lower part of the anterior wall, posterior wall of the tympanic cavity by a ligament. beneath the bony wall of the auditory tube.  The stapes has a head, a neck. two limbs, and a base.  The posterior (mastoid) wall has a large, irregular opening, the  The head is small and articulates with the long process of the aditus ad antrum (aditus to the mastoid antrum), in its upper incus. The neck is narrow and receives the insertion of the part. This opening is the entrance to the cavity (antrum) of the stapedius muscle. mastoid process and the mastoid air cells.  The two limbs diverge from the neck and attach to the oval base.  It connects the epitympanic recess to the mastoid antrum.  The edge of the base (footplate) attaches to the margin of the  A small, hollow, conical projection, the pyramid (pyramidal oval window by a ring of fibrous tissue, the annular ligament. eminence), sits below the aditus.  Two muscles, the tensor tympani and the stapedius, attach to  The pyramid houses the stapedius muscle; the tendon of the the ossicles. The stapedius is the smallest skeletal muscle in the stapedius emerges from its apex. human body.  The main stem of the facial nerve (CN VII) descends within  Auditory Ossicle Movement the facial canal immediately adjacent to the posterior wall. This  The malleus and incus rotate on an anteroposterior axis that close relationship forms the elevated prominence of the facial runs through the ligament connecting the anterior process of the canal along the posterior wall. malleus to the anterior wall of the tympanic cavity, the anterior  A small opening inferior to the pyramid transmits the chorda process of the malleus and the short process of the incus, and tympani nerve from the facial nerve into the tympanic cavity. the ligament connecting the short process of the incus to the  The medial (labyrinthine) wall separates the tympanic cavity posterior wall of the tympanic cavity. from the internal ear. Thus, the medial wall of the tympanic cavity  When the tympanic membrane moves medially, the handle of is the lateral wall of the internal ear. the malleus also moves medially. The head of the malleus and  The greater part of this wall shows a rounded projection, the the body of the incus move laterally. The long process of the promontory, which results from the underlying first turn of the incus moves medially with the stapes. The base of the stapes is cochlea. pushed medially in the fenestra vestibuli, and the motion ls  The fenestra vestibuli (oval window), which is closed by the communicated to the perilymph in the scala vestibuli of the footplate of the stapes, lies above and behind the promontory. internal ear. Liquid being incompressible, the perilymph causes  The fenestra cochleae (foramen tympani, round window), an outward bulging of the secondary tympanic membrane in the which is round and closed by the secondary tympanic fenestra cochleae at the lower end of the scala tympani. The membrane, sits below the posterior end of the promontory. above movements are reversed if the tympanic membrane  The prominence of the facial canal is an elevation above the moves laterally. Excessive lateral movements of the head of the oval window formed by the adjacent facial nerve within the facial malleus cause a temporary separation of the articular surfaces canal. It appears on both the medial and posterior walls. It runs between the malleus and incus so that the base of the stapes is horizontally above the promontory and oval window and then not pulled laterally out of the fenestra vestibuli. curves downward on the posterior wall behind the pyramid.  During passage of the vibrations from the tympanic membrane  The prominence of the lateral semicircular canal is an to the perilymph via the small ossicles, the leverage increases elevation above the prominence of the facial canal that is formed at a rate of 1.3 to 1. Moreover, the area of the tympanic by the lateral semicircular canal. The bony shelf that extends membrane is about 17 times greater than that of the base of the  stapes, causing the effective pressure on the perilymph to to develop during the 2nd year of life, in response to the influence increase by a total of 22 to 1. of the sternocleidomastoid muscle.  Nerves  Two cranial nerves, the fascial nerve (CN VII) and the glossopharyngeal nerve (CN IX), have significant relations to the middle ear.  Fascial Nerve  Auditory Tube  The facial nerve enters the facial canal at the bottom of the  The auditory (pharyngotympanic) tube connects the anterior internal auditory meatus. The nerve runs laterally above the wall of the tympanic cavity to the nasopharynx. As the tube vestibule of the internal ear until It reaches the medial wall of the descends, it passes over the upper border of the superior middle ear. Here, the nerve expands to form the sensory constrictor muscle. geniculate ganglion. The nerve then bends sharply backward  The tube functions to equalize air pressure in the middle ear with above the promontory. atmospheric pressure, thus balancing pressure on both sides of  On arriving at the posterior wall of the middle ear, it curves the tympanic membrane. downward on the medial side of the aditus of the mastoid  This balance allows the eardrum to move easily. antrum. It descends in the posterior wall of the middle ear,  The tube also functions as a drainage route for the serous behind the pyramid, and finally emerges through the secretions of the mucous membrane lining the middle ear. stylomastoid foramen at the exterior base of the skull.  The lateral third of the auditory tube is bony and the medial two  Important branches: Intrapetrous part thirds is cartilaginous.  The greater petrosal nerve arises from the facial nerve at the  The entire tube is lined with a mucous membrane that is geniculate ganglion. continuous with the epithelium of the tympanic cavity and the It contains preganglionic parasympathetic fibers that pass to nasopharynx. the pterygopalatine ganglion and are there relayed through  The cartilage part is roughly C shaped. the zygomatic and lacrimal nerves to the lacrimal gland;  The mucous membrane of the open part of the "C" is normally other postganglionic fibers pass through the nasal and collapsed and in apposition with the mucous membrane lining palatine nerves to the glands of the mucous membrane of the cartilage. Therefore, in order to function, the auditory tube the nose and palate. It may also contain taste fibers from the must be actively opened. mucous membrane of the palate.  Contractions of the tensor veli palatini and levator veli palatini The nerve emerges on the superior surface of the petrous muscles cause the collapsed mucous membrane to separate from part of the temporal bone, travels forward, and drops into the cartilage wall and open the auditory tube. the foramen lacerum.  Because these are muscles of the soft palate, opening of the It joins the deep petrosal nerve from the sympathetic auditory tube occurs during palatal activity, for example, during plexus on the internal carotid artery, and the two form the swallowing and yawning. Thus, these are optimal activities for nerve of the pterygoid canal at the anterior lip of the equalizing pressure and draining the middle ear. Think about foramen lacerum. This passes forward and enters the what you must do in order to "pop" your ears when changing pterygopalatine fossa, where it ends in the pterygopalatine altitude. ganglion.  Because of the continuity of the mucous membrane from  The nerve to the stapedius arises from the facial nerve as it nasopharynx to auditory tube to middle ear, the auditory tube may descends in the facial canal behind the pyramid. It supplies serve as a route for spread of infection from the nasopharynx to the the muscle within the pyramid. middle ear.  The chorda tympani nerve arises from the facial nerve just  Mastoid Area inside the stylomastoid foramen.  The mastoid area consists of two main parts: (1) the mastoid It enters the middle ear close to the posterior border of the antrum and (2) the mastoid air cells. tympanic membrane, runs forward across the inner surface  The mastoid antrum is the main cavity within the mastoid process of the tympanic membrane, and crosses the root of the of the temporal bone. handle of the malleus.  The aditus ad antrum in the posterior wall of the tympanic cavity It lies in the interval between the mucous membrane and the is the doorway that leads from the epitympanic recess into the fibrous layers of the tympanic membrane, which explains the antrum. naming of the nerve.  The relations of the mastoid antrum are important in The nerve leaves the middle ear through the petrotympanic understanding the spread of infection. fissure and enters the infratemporal fossa, where it joins the  Anterior wall: Related to the tympanic cavity and contains the lingual nerve. aditus to the mastoid antrum The chorda tympani contains taste fibers from the mucous  Posterior wall: Separates the antrum from the sigmoid venous membrane covering the