Special Sense Anatomy PDF

# BAYELSA STATE COLLEGE OF NURSING SCIENCES ## TOMBIA-EKPETIAMA ### ANATOMY & PHYSIOLOGY #### SPECIAL SENSES The awareness and knowledge of happenings among human is due to the detection of stimuli transmitted from nerve endings (afferent sensory receptors which could be somatic or visceral) to the...

# BAYELSA STATE COLLEGE OF NURSING SCIENCES ## TOMBIA-EKPETIAMA ### ANATOMY & PHYSIOLOGY #### SPECIAL SENSES The awareness and knowledge of happenings among human is due to the detection of stimuli transmitted from nerve endings (afferent sensory receptors which could be somatic or visceral) to the brain and or spinal cord for processing and interpretation. Some of the stimuli are received by sensory receptors distributed throughout the body and others are received by highly complex receptor organs referred to as special general senses organs. The special senses have specialized sense organs devoted to them with specialized sensory receptors or nerve endings. There are five (5) senses in the human body, of which four (4) are considered special senses, with their sense organs located in the head having a connection with the brain. The sense of touch, carried mostly by the skin, is not regarded as a special sense, but a general sense. The distinction between special and general senses is used to classify nerve fibers running to and from the CNS: that is, information by somatic efferent and visceral afferent. In contrast, the other sense (touch) is a somatic sense that does not have a specialized organ, but comes from all over the body, especially the skin and also the visceral. In addition, from each special sense organ, impulses are sent to the cerebral cortex of the brain, through special cranial nerves. The stimuli, 80% of which comes from sensory stimuli, is perceived at this level as: 1. Sight 2. Smell 3. Taste 4. Hearing and balance In the brain, the incoming nerve impulses undergo a complex process of integration and co-ordination that results in allowing us to detect and analyze changes in our environment, providing information and a variety of responses for homeostasis. The special senses include: * **Sight** * Receptor organ: Eye * Stimulus: light * **Hearing and Balance** * Receptor organ: Ear (cochlear and membranous labyrinth) * Stimulus: sound wave and gravitational force * **Taste** * Receptor organ: Tongue (taste buds are also found in the mouth and throughout the length of the upper digestive tract, i.e., epithelial of the tongue, soft palate, pharynx and epiglottis) * Stimulus: fluid borne molecules. * **Smell** * Receptor organ: Nose (olfactory bulb) * Stimuli: Airborne molecules For the general sense, i.e. the sense of touch, the major receptor organ is the skin. # SENSE ORGAN FOR SIGHT (EYE) ## ANATOMY OF THE EYE **Location:** Orbital cavity surrounded by orbital bones and cushioned by pads of fats within the orbital socket. **Shape:** It is slightly asymmetrical. **Measurement:** It is about 2.5cm in diameter. Humans have a pair of eyes lying laterally to each other but are connected to function as a pair. ## WALLS OF THE EYE The walls of the eye surround the interior of the eyeball. It is made up of three layers of tissue, which can be referred to as tunics or Membranes. These include: * **A tough outermost protective fibrous layer/tunics:** Made up of the sclera and cornea. * **A middle vascular (nutritive) layer:** Uveal tract consisting of the vascular choroid, muscular ciliary body, iris and pupil. * **An inner neural (sensitive) tissue layer:** It is made of the retina. Note: Two fluids are found occupying and circulating through the three chambers of the eye, providing structure and nutrients to the eye. They are: * **Aqueous Humor (fluid):** Occupying the anterior and posterior chamber. Watery than vitreous fluid. * **Vitreous Humor (fluid):** Thick and jelly-like ## LAYERS OF THE EYE ### OUTER MOST LAYER: SCLERA AND CORNEA The sclera, also known as the white part of the eye, is a firm fibrous membrane that forms 5/6th of the outermost layer of the posterior lateral aspect of the eyeball. It continues anteriorly as a clear transparent epithelial membrane, i.e., the Cornea, that occupies anterior 1/6th of the eyeball. Its frontal surface, is covered by a stratified squamous non-keratinized epithelium which is continuous with the conjunctiva. The conjunctiva covers the sclera anteriorly and also lines the inner surface of the eyelid. Within the sclera, close to the sclera-cornea junction lies an oval canal, the canal of schlemm which passes around the circumference of the eyeball. The posterior pole of the sclera is pierced by the optic nerve (CNII) which carries nerve impulses originating from the retina into the brain. **Functions of the sclera and cornea** 1. The sclera maintains the shape of the eye and gives attachment to the extrinsic muscle. 2. It is the eye's primarily light focusing structure. 3. The cornea is sensitive to pain. 4. The cornea through which light rays enters the eye refracts i.e. bends the light to be focused on the retina. 5. The stratified squamous non-keratinized epithelium (SSNKE) helps to prevent the cornea from drying out thereby preventing wear and tear. ### MIDDLE (VASCULAR) LAYER: UVEAL TRACT It lies beneath the sclera and it's composed of: * Vascular choroid * Muscular ciliary body * Iris * Pupil **Vascular Choroid** It is a thin highly vascularized (rich in blood supply) pigmented layer beneath the sclera. It lines the posterior 5/6th of the inner surface of the sclera. **Functions of the vascular choroid** * It is the main nutritive tissue responsible for supply of nutrients, including Oxygen (O2), to the retina. * The pigment absorbs excess light preventing it from being reflected back to the retina, thus preventing blurred vision. **Muscular Ciliary Body (MCB)** It is the thickened continuation of the choroid, consisting of ciliary muscle fibres (smooth muscle fibres) from which the lens is suspended by fine ligaments and secretory epithelial cells. The structure by which the lens is attached to the ciliary body is known as suspensory ligaments, which is connected to the sclera and encircles the lens. **Functions of the muscular ciliary body:** * Contraction and relaxation of the ciliary smooth muscle fibres, controls the shape of the lens. * The epithelial cells secrete aqueous fluid into the anterior and posterior chambers. **Innervations of Ciliary Body** Parasympathetic branch of the cranial nerve three (CN III) oculomotor nerve. Its stimulation causes contraction of the ciliary muscle and accommodation of the eye. **Iris and Pupil** The iris is the pigmented anterior extension of the choroid lying posterior to the cornea. It is the structure that divides the anterior segment into the anterior and posterior chambers. The pigmented iris whose colour ranges from pale-blue to dark-brown, gives each individual eye their colour. The iris is also composed of two layers of smooth muscle fibres: * Circular muscle fibres * Radiating muscle fibres The pupil is seen as a hole surrounded by the iris, both surrounding an aperture called the pupil. **Nerve supply** The iris is innervated by both the parasympathetic and sympathetic. Parasympathetic stimulation constricts the pupils (the lens will bulge out for a close image). Sympathetic stimulation dilates the pupil, thereby controlling the amount of light that goes into the eye. The color of the iris is genetically determined and depends on the number of pigmented cells present. In albinos, there are no pigmented cells, and there are fewer pigmented cells in people with blue eyes than those with brown eyes. ## THE LENS The lens is a highly elastic ellipsoid, biconvex and transparent structure lying posterior to the pupil. It measures about 10mm across (lengthwise) and 4mm widthwise in adults. It is suspended from the ciliary body by suspensory ligaments. The lens consists of: * The lens capsule (outer) * Lens epithelium (middle) * Lens fibres (inner) **The lens capsule**: It is the smooth transparent outermost layer of the lens. **The lens epithelium:** It is between the capsule and the fiber responsible for the stable functioning of the lens. It also produces fibers for the lifelong growth of the lens. **The lens fibres:** Are long, thin and transparent cells that form the bulb at the lens. The thickness of the lens for proper focusing is regulated by the ciliary muscle through the suspensory ligaments. N/B: The lens is the only structure in the eye that can vary its refractive power achieved by changing its thickness ## INNER SENSORY/NEURAL TUNIC: THE RETINA The innermost layer known as the retina is the sensitive tunic or layer of the eyeball, well adapted for stimulation by light rays. It lines about 3/4th of the eyeball, thickest at the back and thins out anteriorly to terminate behind the ciliary body. The retina is further divided into: * Pigmented layer * Neural layer **Pigmented layer:** It is made up of epithelial cells lining the internal surface of the choroid, ciliary body and iris. It is void of photoreceptors. Its function is to absorb light rays preventing them from being scattered. **Neural layer:** It is the photosensitive layer containing photoreceptor neurons: rods and cones. They contain molecules called "opsins" that detect light rays. Hence, they are called transducers (meaning) they have the ability to convert light energy, which is electromagnetic, carried in the electromagnetic wave of light to electrochemical energy in the form of action potential (nerve impulses) that is taken to the brain for interpretation. The rods are more abundant, they are (about 120 million) and are more sensitive than cones (6.7 million). They are situated at the periphery of the neural layer. The cones which are 6.7 million are situated at the macula-luteal (yellow spot) superior to the blind spot. In the yellow spot is a centralized area called fovea centralis where the cones are mainly concentrated. It is rods free measuring about 0.33mm in diameter. The rods are stimulated by: * Dull light conditions * Black and white light While the cones are stimulated by: * Very bright light * Colored light Inferior to the macula luteal is the blind spot or optic disc. It is an opening where the optic nerve leaves the retina. ## INTERIOR OF THE EYE The interior of the eye is bounded by the walls of the eyeball and is divided into two segments by the lens. They include: * Anterior segment * Posterior segment ## ANTERIOR SEGMENT It extends from the cornea anteriorly to the lens posteriorly. It is incompletely divided into two chambers by the iris: * Anterior chamber * Posterior chamber Both chambers are filled with aqueous humor secreted by the secretory epithelial cells of the ciliary body. The fluid is transparent and about 98% of it is made up of water. **Content of aqueous humour** * Water * Glucose * Amino * Electrolytes (sodium, magnesium, bicarbonate, chloride, potassium, fluoride). * Immunoglobulin It has a pH of about 7.4 and osmolarity of about 304. **Functions of the aqueous humour** Aqueous humor plays an essential role in keeping the eye healthy. Therefore, its functions include: * Providing nourishment to the cornea and the lens by supplying amino acid and glucose to these structures. * Maintain a fairly constant intra-ocular pressure of the eye between 10-20mmHg which is a hydrostatic pressure that keeps the eyeball in a fairly spherical shape and keeps the walls of the eye taut (tensed). * Transporting vitamin C into the front segment to act as an anti-oxidant. * Providing inflation for expansion of the cornea which in turn increases protection against dust, wind and a number of pathogens. * Playing a role in immune response to defend against pathogens as a result of the presence of immunoglobulin ## DRAINAGE OF AQUEOUS HUMOUR The fluid produced by the ciliary glands into the posterior chamber, circulates in front of the lens through the pupil into the anterior chamber and returns into the venous circulation through the canal of schlemm. At the drainage angle (which is between the iris and the cornea) is a spongy tissue called "Trabecular meshwork. 80%-90% of aqueous humour makes its way into the circulation through it, and it goes further through the canal of schlemm back into the collector channels to the veins; interscleral plexus and aqueous veins, both drain the aqueous fluid into the episcleral and ciliary veins and eventually back into the body general circulatory system. The rate of aqueous humor production must be equal to its drainage, any variation in drainage will have a great influence on the intra-ocular pressure and increase in intra-ocular pressure can lead to glaucoma. ## POSTERIOR SEGMENT It extends from the lens anteriorly to the retina posteriorly. Its the largest of all the segments of the eye occupying about 80% of the entire eyeball. It is known as the vitreous body or humour, filled with vitreous fluid which is a soft colorless, transparent, jelly-like substance. **CONTENT OF VITREOUS HUMOUR** * Water (99%) * Protein (mucoprotein) * Salt * Collagen * Sugar Despite the water to collagen ratio, the vitreous humor has a firm jelly-like consistency. It is said to be the most prominent fluid in the eye. **Function of the vitreous humour** * It helps to maintain the spherical shape of the eye because of the intra-ocular pressure (IOP) exerted by the vitreous body. * It keeps the retina in place by the pressure it exerts on the retina. Note: The vitreous fluid begins to shrink from about age 50 and this can lead to disorder ranging from harmless floaters to a vision impairing retina detachment. ## EXTRA-OCCULAR MUSCLES OF THE EYE These muscles are located within the orbital cavity, though they are extrinsic (originate from outside) and separate from the eyeball, they act to control the movement of the eyeball. There are six extra ocular muscles: * 4 recti muscles * 2 oblique muscles ## RECTI AND OBLIQUE MUSCLES Recti is a Latin word for "straight" having a direct path from the origin to insertion. Oblique muscles have an angular approach to the eyeball from the origin. ## ACCESSORY STRUCTURES OF THE EYE These are the external protective appendages and supplementary components of the eye. They include: * Eyebrow * Eyelid and eyelashes * Lacrimal apparatus * Conjunctiva ### EYE BROW These are strips of hairs located on the ridges of the supra-orbital margin of the frontal bone superior to the eyelid. **FUNCTION** * Protect the eyeball from sweat, dust and other foreign bodies. * Shield the eyes from overhead light ### EYELID AND EYELASHES These are two thin folds of skin of subcutaneous connective/loose areolar tissue covering the anterior surface of the eye. The superiorly located eyelid is the superior palpabrae, while the inferiorly located eyelid is inferior palpabrae. On the free edges are short curved hairs called eyelashes. The layer of tissues forming the eyelid are: * A thin covering of skin * A thin sheet of subcutaneous connective (loose areolar tissue) * Three muscle fibres that is: * Orbicularis occuli * Levator palpebrae superioris * Muller muscle A thin sheet of dense connective tissue (the tarsal plate larger in the upper lid than the lower lid). A membranous lining of conjunctiva. The upper eyelid is more extensive and movable. Contraction of the orbicularis oculi closes the eye, while contraction of the levator palpebrae superioris opens the eye **FUNCTION** * The eyelashes help to keep airborne particles from reaching the eye surface. * They provide some protection from excessive light. * The palpebrae, through the process of blinking, spread tears, which is lacrimal fluid, and mucous over the anterior eye surface to keep it moist. * The eyelid and eyelashes also protect the eye from injury. ## CONJUCTIVA This is a thin transparent double layer mucous membrane, that lines the internal surface of the eyelid, anterior surface of the eyeball, and the anterior surface of the sclera. It is divided into three parts. Two parts are continuous with each other. They include: * Palpabrae conjunctiva * Bulbar conjunctiva * Corneal conjunctiva **PALPABRAE CONJUNCTIVA:** This lines the inner surface of both the upper and lower eyelid and the anterior surface of the eyeball. It consists of highly vascularized columnar epithelium. **BULBAR CONJUCTIVA:** It lines the anterior part of the sclera. It terminates anteriorly at the sclera-cornea junction and does not cover the cornea. **CORNEAL CONJUCTIVA:** It consists of avascular stratified epithelium found lining the cornea. The bulbar and palpabrae conjunctiva are continuous with each other. The conjunctiva has many small blood vessels that provide many nutrients to the eye and eyelid. It also contains special cells that secretes a component (of the tear film) to help prevent dry eye syndrome. There are also certain nociceptors in the conjunctiva that responds to pain stimuli. **Functions of the conjunctiva** Primarily, the conjunctiva function in the following ways: * Keeping the anterior surface of the eye moist and lubricated. * Keeping the inner surface of the eyelid moist and lubricated for easy closing without friction, or causing eye irritation. * Protecting the eye from dust, infections pathogens, debris and dust. ## LACRIMAL APPARATUS These are structures responsible for production and drainage of tears. Each eye consists of: * 1 lacrimal gland and its excretory ducts (6-10) * 2 lacrimal canaliculi * 1 lacrimal sac * 1 caruncle * 2 lacrimal punctum * Naso-lacrimal duct ### LACRIMAL GLAND: This is an ocinous type of exocrine gland situated laterally and superior to the eye in the frontal bone behind the supra orbital margin. It is about the shape and size of an almond, consisting of secretory epithelial cells. It is connected to the anterior surface of the eye by excretory ducts that are located inferior to it. * **Innervation** is by cranial nerve VII (facial nerve). * **Stimulation** of this nerve causes the gland to secret lacrimal fluid (tears). ### LACRIMAL PUNCUM: This is a small dark aperture located in a slight tissue elevation called lacrimal papillae. It is the opening of each of the lacrimal canaliculus. There are two (2) in each eye, which are: * Inferior punctum * Superior punctum They can be seen only if the eyelid edge is everted slightly. ### LACRIMAL CANALICULI: This is also referred to as lacrimal canal or duct. It is a small channel of about 1cm long. It commences at the point where the lacrimal punctum ends. There are two in each eye, which are: * The inferior canaliculi * The superior canaliculi The superior canaliculi- is smaller and shorter. It first ascends and bends at acute angle, and passes medially and downwards to the lacrimal sac. The inferior canaliculi- at first descends, and runs almost horizontally with the lacrimal sac. At this angle, they are dilated into ampulla. They are lined by non-keratinized stratified squamous surrounded by fibrous tissue. ### LACRIMAL SAC: This is the upper expanded end of the naso-lacrimal duct. It connects the lacrimal canaliculi and the naso-lacrimal duct. ### NASO-LACRIMAL DUCT: It is a major membranous drainage tube of approximately 2cm. It drains tears from the lower part of the lacrimal sac into the nasal cavity at the level of the inferior conchae where the tears mix with nasal mucus secretion and are evaporated. Normally, the rate of secretion of tears keeps pace with the rate of drainage. Secretion of tears increases with entrance of foreign body into the eye. It also increases with emotional states like crying, laughing, etc. ### LACRIMAL CARUNCLE: This is a small pinkish nodule at the inner corner (medial canthus) of the eye. It is made of skin covering sebaceous and sweat glands. This structure and a part of the conjunctiva may be inflamed and become pruritic, due to histamine release in the tissue and tear film. ### LACRIMAL FLUID: This consists of water, mucin, lipid, lysozymes, lactoferrin, lacritin, immunoglobulin, glucose, urea, sodium and potassium. **FUNCTIONS** * Moistening and lubricating the conjunctiva and the eye * Prevention of pathogenic microorganism attack on the eye. * Washing away irritants from the eye. * The oil nature delays evaporation and prevents drying of the conjunctiva ## DRAINAGE OF THE LACRIMAL FLUID Stimulation of the cranial nerve VII (facial nerve) causes the lacrimal gland to synthesize the lacrimal fluid and its release. This fluid in emptied into the anterior surface of the eye through its excretory duct associated with the lacrimal gland. It passes in front of the eye towards the median canthus where they drain into superior and inferior canaliculi through the inferior and superior punctum respectively. The fluid then drains into the lacrimal sac from which it drains into the nasolacrimal duct into the nasal cavity opening, at the level of the inferior conchae. # PHYSIOLOGY OF SIGHT (VISION) In order to achieve a clear vision, light reflected from objects within the visual field is focused on the retina of each eye. In producing a clear image, several processes are involved, which include: * Refraction of light rays entering the eye * Changing the size of the pupil * Focusing of image on the retina by accommodation * Convergence of image * Photochemical activities in the retina and conversion into neural impulse. ## REFRACTION When light rays (wave) traveling parallel to each other passes from a medium of one density into a medium of different density, they are bent. This phenomenon is called refraction. The degree of refraction depends on the comparative density of the two media. **Refractive index of some media in comparison with air and some eye structures** * Air ---- 1.00 * Cornea ---- 1.38 * Aqueous humour --- 1.33 * Lens ---- 1.40 The degree of refraction also depends on the curvature of the interface between the two media. The curvature of the cornea is constant, while that of the lens varies. Before reaching the retina, light rays passes successively through the conjunctiva, cornea, aqueous humor, lens, and vitreous humour. They are all denser than air and with the exception of the lens, they have a constant refractive power similar to that of water, so refraction take place in every medium before it falls on the retina. Note: In a normal eye, light ray is focused on the retina. However, in a myopic eye (short sightedness), it is focused in front of the retina, so no image is formed. This defect can be corrected by using a concave which diverges the light rays further to be focused on the retina. In the case of long sightedness (hypermetropia), light rays are focused behind the retina, so no image is formed. This defect can be corrected by using a convex lens which helps to converge light rays to be formed on the retina. The more a lens is dense, the greater the refractive power. Also, the greater the curvature of the lens, the greater the refractive power. ## FOCUSING OF IMAGE ON THE RETINA The refractive properties of the lens provide fine control for focusing light on the retina, and as a result, the image perceived on the retina is upside down and right to left. The brain adapts to this early in life, so objects are perceived the right way up. * Focus on a near object * Focus on a distant object ## HOW THE EYE FOCUSES **Mechanism of accommodation** Light from a distant object needs least refraction, as the object comes closer, there is increased refraction. To increase the refractive power of the lens. The following processes must take place: 1. Contraction of ciliary muscles 2. The ciliary body pulls forward and inward, relaxing the tension of the zonular fibre (of the suspensory ligament) of the lens. 3. The lens becomes thicker and round (bulge out), increasing its convexity due to its inherent elasticity. 4. Image is brought to focus on the retina. ## FOCUS ON DISTANT OBJECT * There is an increase in the refractive power. * Relaxation of ciliary muscles. * Ciliary body returns to normally. * Increase in tension on suspensory ligament of the lens. * Lens becomes thinner and flat. ## CHANGING THE SIZE OF THE PUPIL The pupil controls the amount of light that enters the eye. * In a bright light, the pupil constricts, to reduce the amount of light that enters the eye, and prevent the sensitive retina from getting damaged. * In a dim light, they are dilated to increase the amount of light entering the eye, to activate the light sensitive pigments in the rods and cones which stimulate the nerve endings in the retina. Pupil size influences accommodation by controlling the amount of light entering the eye, by the action of the autonomic sympathetic stimulation amount. ## CONSTRICTION OF THE PUPIL: This assists accommodation by reducing the width of the beam of light entering the eyes so that it passes through the central curved part of the lens. ## CONVERGENCE: Light rays from nearby objects enter the two eyes at different angles and for clear vision, they must stimulate corresponding areas of the two retinae. Extrinsic muscles move the eyes and to obtain a clear image, they also rotate them so that they converge on the object viewed. This co-ordinate muscle activity is under autonomic control. When there is voluntary movement of the eye, both eyes move and convergence is maintained. The nearer an object is to the eye, the greater the eye rotation needed to achieve convergence. If convergence is not complete, the eyes are focused on different objects or at different points of the same object. If convergence is not possible, the brain tends to ignore the impulse received from the divergent eye. ## CHANGING THE REFRACTORY POWER OF THE LENS: Changes in the thickness of the lens are made to focus light on the retina. The amount of adjustment depends on the distance of the objects from the eyes: that is, the lens is thicker for near vision and at its thinnest when focusing on objects more than 6m away. The lens loses its elasticity and stiffens with age, a condition known as presbyopia. ## DISTANT VISION Objects more than 6cm away are focused on the retina without adjustment of the lens or convergence of the eyes. # HEARING AND EQUILIBRIUM The ear has two basic functions: hearing and equilibrium. The ear can be used to alert, communicate fear and pleasure. The ear converts physical vibration into encoded nervous impulses which in turn is then processed by the central auditory pathway in the brain. The ear is a paired organ, one on each side of the head, with the sense organ itself, the cochlear deeply buried in the temporal bone. With the exception of the auricle (pinna), the structures that form the ear are enclosed within the petrous portion of the temporary bone. Part of the ear is concerned with conducting sound to the cochlear, which transduces vibration, performed by delicate hair cells, which when stimulated, initiate a nervous impulse. The hair cells are bathed in body fluid which provides them with nutrients and oxygen. ## STRUCTURE OF THE HUMAN EAR The ear is divided into three distinct parts: * outer/external ear * middle ear * inner/internal ear ## THE OUTER/EXTERNAL EAR The outer ear consists of auricle (pinna) and the external acoustic meatus (auditory canal). ### Auricle (Pinna): It is the external visible part of the ear that protrudes from the side of the head. It is composed of fibro-elastic cartilage covered with skin. The cartilage is molded to clearly defined hollows, ridges and furrows, that form an irregular shallow tunnel. The deepest depression which leads into the external acoustic meatus (EAM) is the concha. The concha is partly covered by two projections; the tongue-like tragus in front and the antitragus. Above the tragus is the helix. It is the most prominent outer ridge. It arises from the floor of the concha and continues as the incurred rim of the upper portion of the auricle. The antihelix is an inner concentric ridge surrounding the concha and is separated from the helix by a furrow called the scapula (scaphoid fossae). The pinna is angled so that it collects sound more from front than behind and channels it into the external acoustic meatus (EAM). The lobule (ear lobe) is the soft pliable part at the lower extremity of the auricle, composed of fibrous and adipose tissue, richly supplied with blood. It is the only area of the outer ear that does not contain cartilage. ### External Acoustic Meatus: This is a slightly S-shaped tube, oval in cross section and measuring about 2.5cm long extending from the auricle to the tympanic membrane (eardrum) medially. The lateral third (2) is embedded in cartilage and the remaining part (3) lies within the temporal bone, covered with skin. It contains numerous ceruminous glands and hair follicles with associated sebaceous glands. Ceruminous glands are modified sweat glands that secret cerumen (ear wax), which is a sticky material that contains protective substances. **Contents of cerumen (ear wax)** * Bactericidal enzymes, lysozymes, immunoglobulins. Foreign substances like dust, insects and microbes are prevented from reaching the tympanic membrane by the wax, hairs, and the curvature of the EAM. Movement of the temporomandibular joint (TMJ) during chewing and talking, massages the cartilaginous meatus, thereby moving the wax towards the exterior. The tympanic membrane completely separates the EAM from the middle ear. ## THE MIDDLE EAR This an irregular-shaped air-filled cavity located within the petrous portion of the temporal bone, connected to the back of the nose by a long thin tube called the eustachian (auditory/pharyngotympanic) tube. Vertically, it measures 15mm and also 15mm from front to back (anterior to posterior). It consists of the tympanic cavity with auditory ossicles, two small muscles and the auditory tube. The tympanic cavity, its content and the air sac that opens out of it are lined with either simple squamous or cuboidal epithelium. The tympanic cavity (tympanium) is a small narrow laterally compressed chamber situated within the temporal bone. It is separated from the EAM by the tympanic membrane. **BOUNDARIES OF THE MIDDLE EAR** * **Anteriorly:** It is connected with air in the nasopharynx via the pharyngotympanic tube. * **Posteriorly:** It is connected to the mastoid antrum and the mastoid air cells. * **Laterally:** by the tympanic membrane. * **The roof and floor are formed by the temporal bone** * **Medially:** by the inner ear, separated from the middle ear by a thin layer of temporal bone in which are two openings: * oval window * round window The oval window is covered by the footplate of the stapes, and the round window by a sheet of fibrous tissue. Air reaches the cavity through the pharyngotympanic tube which links the nasopharynx and the middle ear. ### The Auditory tube The auditory tube is about 4cm long and is lined with ciliated columnar epithelium. The presence of air at atmospheric pressure on both sides of the tympanic membrane is maintained by the pharyngotympanic (auditory) tube and this enables the membrane to vibrate when sound waves strike on it. The tube is usually closed, but when there is an unequal pressure across the tympanic membrane (e.g., at high altitude) contraction of the buccal and neck muscles (in swallowing and yawning) "pop-up" the tube, actively opening the tube and allow the air pressure in the middle ear and the nose to equalize. Sound is conducted from the tympanic membrane to the inner ear by the auditory ossicles. ### Auditory ossicles These are three small (miniature) bones, only a few millimeters in size arranged in a chain form, extending from the middle ear across the tympanic membrane to the oval window. They are held together in place by fine ligaments and are named according to their shapes. * **Malleus:** Called 'hammer'. This is the lateral hammer-shaped bone with a handle (manubrium) that is attached to the tympanic membrane, running from the centre upwards and the head also called capitallum, lying above the tympanic membrane, which forms a synovial joint with the incus. * **Incus:** Also called 'anvil'. It has a body, long process and a short process. The anterior surface articulates with the head of the malleus. The short process is attached to a ligament and the long process with the stapes. * **Stapes:** Also called 'stirrup'. It is the smallest bone in the human body. It consists of a head, neck anterior crus, posterior crus and a footplate. The head articulates with the incus, and the footplate fits into the oval window. ### Muscles Attached To The Auditory Ossicles 1. Tensor tympani muscle 2. Stapedius muscle **Tensor tympani muscle:** This muscle lies in a canal above and parallel to the pharyngotympanic tube. It originates from the cartilaginous wall of the tube and insert into the neck of the malleus which in turn, is attached to the tympanic membrane. This muscle pulls and keeps the tympanic membrane stretched or tensed constantly, which is essential for transmission of sound waves. Tensor tympani is supplied by mandibular division of the 5th cranial nerve (trigeminal nerve- CNV). Stimulation of this nerve contract the malleus inwards thereby making the tympanic membrane tensed. **Stapedius muscle:** This muscle arises from the posterior wall of the middle ear (tympanic cavity) and its tendon is inserted into the neck of the stapes. It is innervated by the 7th Cranial nerve (facial nerve-CNVII). This muscle contracts when stimulated, pulling the stapes away from the fenestra ovalis and reduces the movement of the footplate against the fluid in the cochlear. ## INNER EAR The inner ear, or labyrinth (meaning 'maze'), is a membranous structure enclosed by a bony labyrinth in the petrous (stone-like) of the temporal bone. It contains the sense organs of hearing (cochlear) and balance/equilibrium (vestibular apparatus). The inner ear is described in two parts, namely: * the bony labyrinth * membranous labyrinth The inner ear is formed from a network of channels and cavities in the temporal bone (the bony labyrinth). Within the bony labyrinth, like a tube within a tube, is the membranous labyrinth, a network of fluid-filled membranes that lines and fills the bony labyrinth. ### Bony labyrinth: This is lined with periosteum. It encloses the membranous labyrinth of the same shape, suspended in a watery, fluid, called perilymph. ### Membranous labyrinth: Is enclosed in the bony labyrinth, filled with endolymph. The sensory structures of the vestibular, apparatus and cochlear, are located within the membranous labyrinth. The inner ear is divided into three main regions: * the vestibule, containing the utricle and saccule * three semicircular canals * the cochlea ### Vestibule This is the expanded part nearest the middle ear. The oval and round windows are located in its lateral wall. It contains two membranous sacs, the utricle and the saccule, which are important in balance. ### Semicircular canals These are three tubes arranged so that one is situated in each of the three planes of space. Accordingly, there is an anterior, a posterior and a lateral semicircular canal in each internal ear. The anterior and posterior canals, are oriented at right angles to each other in the vertical plane, whereas the lateral canal lies horizontally. They are continuous with the vestibule and are also important in balance. ### Cochlea The cochlea is a coiled tube of about 35mm long and 3mm in diameter at its base. It resembles a snail's shell. It has a broad base, where it is continuous with the vestibule, and a narrow apex; and it spirals round a central bony column. A cross-section of the cochlea contains three compartments: * the scala vestibuli * the scala media, or cochlear duct * the scala tympani. In cross-section, the bony cochlea has two compartments containing perilymph: the scala vestibuli, which originates at the oval window, and the scala tympani, which ends at the round window. The two compartments are continuous with each other, and shows the relationship between these structures. The cochlea duct, which is an extension of the membranous labyrinth, lies between the Scala vestibuli and the Scala tympani. It is triangular in cross-section and contains the organs of hearing. Located along the length of the basilar membrane: from its base to the apex of the triangle, are supporting cells, and specialised cochlear hair cells containing auditory receptor cells. These cells form the spiral organ (of Corti): the sensory organ that responds to vibration by initiating nerve impulses that are then perceived as hearing within the brain. The auditory receptors are dendrites of affer