Human Anatomy and Physiology PDF

The outer ear consists of the auricle (pinna) and the external acoustic meatus (auditory canal). The auricle (pinna) The auricle is the visible part of the ear that projects from the side of the head. It is composed of fibroelastic cartilage covered with skin. It is deeply grooved and ridged; the m...

The outer ear consists of the auricle (pinna) and the external acoustic meatus (auditory canal). The auricle (pinna) The auricle is the visible part of the ear that projects from the side of the head. It is composed of fibroelastic cartilage covered with skin. It is deeply grooved and ridged; the most prominent outer ridge is the helix. The lobule (earlobe) is the soft pliable part at the lower extremity, composed of fibrous and adipose tissue richly supplied with blood. External acoustic meatus (auditory canal) This is a slightly ‘S’-shaped tube about 2.5 cm long extending from the auricle to the tympanic membrane (eardrum). The lateral third is cartilaginous and the remainder is a canal in the temporal bone. The meatus is lined with skin continuous with that of the auricle. There are numerous ceruminous glands and hair follicles, with associated sebaceous glands, in the skin of the lateral third. Ceruminous glands are modified sweat glands that secrete cerumen (earwax), a sticky material containing protective substances including the enzyme lysozyme and immunoglobulins. Foreign materials, e.g. dust, insects and microbes, are prevented from reaching the tympanic membrane by wax, hairs and the curvature of the meatus. Movements of the temporomandibular joint during chewing and speaking ‘massage’ the cartilaginous meatus, moving the wax towards the exterior. The tympanic membrane (eardrum) (Fig. 8.2) completely separates the external acoustic meatus from the middle ear. It is oval-shaped with the slightly broader edge upwards and is formed by three types of tissue: the outer covering of hairless skin, the middle layer of fibrous tissue and the inner lining of mucous membrane continuous with that of the middle ear. Figure 8.2 The tympanic membrane. Coloured scanning electron micrograph showing the malleus and the incus. Middle ear (tympanic cavity) This is an irregular-shaped air-filled cavity within the petrous portion of the temporal bone. The cavity, its contents and the air sacs which open out of it are lined with either simple squamous or cuboidal epithelium. The lateral wall of the middle ear is formed by the tympanic membrane. The roof and floor are formed by the temporal bone. The posterior wall is formed by the temporal bone with openings leading to the mastoid antrum through which air passes to the air cells within the mastoid process. The medial wall is a thin layer of temporal bone in which there are two openings: oval window round window (see Fig. 8.6). Figure 8.6 Passage of sound waves: A. The ear with cochlea uncoiled. B. Summary of transmission. The oval window is occluded by part of a small bone called the stapes and the round window, by a fine sheet of fibrous tissue. Air reaches the cavity through the pharyngotympanic (auditory or Eustachian) tube, which extends from the nasopharynx. It is about 4 cm 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 tube and enables the membrane to vibrate when sound waves strike it. The pharyngotympanic tube is normally closed but when there is unequal pressure across the tympanic membrane, e.g. at high altitude, it is opened by swallowing or yawning and the ears ‘pop’, equalising the pressure again. Auditory ossicles (Fig. 8.3) These are three very small bones only a few millimetres in size that extend across the middle ear from the tympanic membrane to the oval window (Fig. 8.1). They form a series of movable joints with each other and with the medial wall of the cavity at the oval window. They are named according to their shapes. Figure 8.3 The auditory ossicles. The malleus This is the lateral hammer-shaped bone. The handle is in contact with the tympanic membrane and the head forms a movable joint with the incus. The incus This is the middle anvil-shaped bone. Its body articulates with the malleus, the long process with the stapes, and it is stabilised by the short process, fixed by fibrous tissue to the posterior wall of the tympanic cavity. The stapes This is the medial stirrup-shaped bone. Its head articulates with the incus and its footplate fits into the oval window. The three ossicles are held in position by fine ligaments.  These are three tubes arranged so that one is situated in each of the three planes of space. They are continuous with the vestibule. Membranous labyrinth This is a network of delicate tubes, filled with endolymph. It comprises: the vestibule, which contains the utricle and saccule the cochlea three semicircular canals. The cochlea A cross-section of the cochlea (Fig. 8.5) contains three compartments: the scala vestibuli the scala media, or cochlear duct the scala tympani. Figure 8.5 A cross-section of the cochlea showing the spiral organ (of Corti). 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 Figure 8.6 shows the relationship between these structures. The cochlear duct is part of the membranous labyrinth and is triangular in shape. On the basilar membrane, or base of the triangle, are supporting cells and specialised cochlear hair cells containing auditory receptors. 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 efferent (sensory) nerves that combine forming the cochlear (auditory) part of the vestibulocochlear nerve (8th cranial nerve), which passes through a foramen in the temporal bone to reach the hearing area in the temporal lobe of the cerebrum (see Fig. 7.21, p. 150). Physiology of hearing Every sound produces sound waves or vibrations in the air, which travel at about 332 metres per second. The auricle, because of its shape, collects and concentrates the waves and directs them along the auditory canal causing the tympanic membrane to vibrate. Tympanic membrane vibrations are transmitted and amplified through the middle ear by movement of the ossicles (Fig. 8.6). At their medial end the footplate of the stapes rocks to and fro in the oval window, setting up fluid waves in the perilymph of the scala vestibuli. Some of the force of these waves is transmitted along the length of the scala vestibuli and scala tympani, but most of the pressure is transmitted into the cochlear duct. This causes a corresponding wave motion in the endolymph, resulting in vibration of the basilar membrane and stimulation of the auditory receptors in the hair cells of the spiral organ. The nerve impulses generated pass to the brain in the cochlear (auditory) portion of the vestibulocochlear nerve (8th cranial nerve). The fluid wave is finally expended into the middle ear by vibration of the membrane of the round window. The vestibulocochlear nerve transmits the impulses to the auditory nuclei in the medulla, where they synapse before they are conducted to the auditory area in the temporal lobe of the cerebrum (see Fig. 7.21, p. 150). Because some fibres cross over in the medulla and others remain on the same side, the left and right auditory areas of the cerebrum receive impulses from both ears. Sound waves have the properties of pitch and volume, or intensity (Fig. 8.7). Pitch is determined by the frequency of the sound waves and is measured in Hertz (Hz). Sounds of different frequencies stimulate the basilar membrane (Fig. 8.6A) at different places along its length, allowing discrimination of pitch. Figure 8.7 Behaviour of sound waves. A. Difference in frequency but of the same amplitude. B. Difference in amplitude but of the same frequency. The volume depends on the magnitude of the sound waves and is measured in decibels (dB). The greater the amplitude of the wave created in the endolymph, the greater is the stimulation of the auditory receptors in the hair cells in the spiral organ, enabling perception of volume. Long-term exposure to very loud noise causes hearing loss because it damages the sensitive hair cells of the spiral organ. Balance and the ear Learning outcome After studying this section, you should be able to: describe the physiology of balance. The semicircular canals and vestibule (Fig. 8.4) The semicircular canals have no auditory function although they are closely associated with the cochlea. Instead they provide information about the position of the head in space, contributing to maintenance of posture and balance. There are three semicircular canals, one lying in each of the three planes of space. They are situated above, beside and behind the vestibule of the inner ear and open into it. The semicircular canals, like the cochlea, are composed of an outer bony wall and inner membranous tubes or ducts. The membranous ducts contain endolymph and are separated from the bony wall by perilymph. The utricle is a membranous sac which is part of the vestibule and the three membranous ducts open into it at their dilated ends, the ampullae. The saccule is a part of the vestibule and communicates with the utricle and the cochlea. In the walls of the utricle, saccule and ampullae are fine, specialised epithelial cells with minute projections, called hair cells. Amongst the hair cells there are receptors on sensory nerve endings, which combine forming the vestibulocochlear nerve. Physiology of balance The semicircular canals and the vestibule (utricle and saccule) are concerned with balance. Any change of position of the head causes movement in the perilymph and endolymph, which bends the hair cells and stimulates the sensory receptors in the utricle, saccule and ampullae. The resultant nerve impulses are transmitted by the vestibular nerve, which joins the cochlear nerve to form the vestibulocochlear nerve. The vestibular branch passes first to the vestibular nucleus, then to the cerebellum. The cerebellum also receives nerve impulses from the eyes and proprioceptors (sensory receptors) in the skeletal muscles and joints. Impulses from these three sources are coordinated and efferent nerve impulses pass to the cerebrum and to skeletal muscles. This results in awareness of body position, maintenance of upright posture and fixing of the eyes on the same point, independently of head movements.  Sight and the eye Learning outcomes After studying this section, you should be able to: describe the gross structure of the eye describe the route taken by nerve impulses from the retina to the cerebrum explain how light entering the eye is focused on the retina state the functions of the extraocular eye muscles explain the functions of the accessory organs of the eye. The eye is the organ of sight. It is situated in the orbital cavity and supplied by the optic nerve (2nd cranial nerve). It is almost spherical in shape and about 2.5 cm in diameter. The space between the eye and the orbital cavity is occupied by adipose tissue. The bony walls of the orbit and the fat help to protect the eye from injury. Structurally the two eyes are separate but, unlike the ear, some of their activities are coordinated so that they function as a pair. It is possible to see with only one eye (monocular vision), but three- dimensional vision is impaired when only one eye is used, especially in relation to the judgement of speed and distance. Structure (Fig. 8.8) There are three layers of tissue in the walls of the eye: the outer fibrous layer: sclera and cornea the middle vascular layer or uveal tract: consisting of the choroid, ciliary body and iris the inner nervous tissue layer: retina. Figure 8.8 Section of the eye. Structures inside the eyeball include the lens, aqueous fluid and vitreous body. Sclera and cornea The sclera, or white of the eye, forms the outermost layer of the posterior and lateral aspects of the eyeball and is continuous anteriorly with the transparent cornea. It consists of a firm fibrous membrane that maintains the shape of the eye and gives attachment to the extrinsic muscles of the eye (p. 198). Anteriorly the sclera continues as a clear transparent epithelial membrane, the cornea. Light rays pass through the cornea to reach the retina. The cornea is convex anteriorly and is involved in refracting (bending) light rays to focus them on the retina. Choroid (Figs 8.8 and 8.9) The choroid lines the posterior five-sixths of the inner surface of the sclera. It is very rich in blood vessels and is deep chocolate brown in colour. Light enters the eye through the pupil, stimulates the sensory receptors in the retina and is then absorbed by the choroid. Figure 8.9 The choroid, ciliary body and iris. Viewed from the front. Figure 8.10 The lens and suspensory ligaments viewed from the front. The iris has been removed. Ciliary body The ciliary body is the anterior continuation of the choroid consisting of ciliary muscle (smooth muscle fibres) and secretory epithelial cells. As many of the smooth muscle fibres are circular, the ciliary muscle acts like a sphincter. The lens is attached to the ciliary body by radiating suspensory ligaments, like the spokes of a wheel (see Fig. 8.10). Contraction and relaxation of the ciliary muscle fibres, which are attached to these ligaments, control the shape of the lens. The epithelial cells secrete aqueous fluid into the anterior segment of the eye, i.e. the space between the lens and the cornea (anterior and posterior chambers) (Fig. 8.8). The ciliary body is supplied by parasympathetic branches of the oculomotor nerve (3rd cranial nerve). Stimulation causes contraction of the ciliary muscle and accommodation of the eye (p. 196). Iris The iris is the visible coloured part of the eye and extends anteriorly from the ciliary body, lying behind the cornea and in front of the lens. It divides the anterior segment of the eye into anterior and posterior chambers which contain aqueous fluid secreted by the ciliary body. It is a circular body composed of pigment cells and two layers of smooth muscle fibres, one circular and the other radiating (Fig. 8.9). In the centre is an aperture called the pupil. The iris is supplied by parasympathetic and sympathetic nerves. Parasympathetic stimulation constricts the pupil and sympathetic stimulation dilates it (see Figs 7.47 and 7.46, pp. 169 and 168). The colour of the iris is genetically determined and depends on the number of pigment cells present. Albinos have no pigment cells and people with blue eyes have fewer than those with brown eyes. Lens (Fig. 8.10) The lens is a highly elastic circular biconvex body, lying immediately behind the pupil. It consists of fibres enclosed within a capsule and it is suspended from the ciliary body by the suspensory ligament. Its thickness is controlled by the ciliary muscle through the suspensory ligament. When the ciliary muscle contracts, it moves forward, releasing its pull on the lens, increasing its thickness. The nearer is the object being viewed, the thicker the lens becomes to allow focusing. The lens bends (refracts) light rays reflected by objects in front of the eye. It is the only structure in the eye that can vary its refractory power, which is achieved by changing its thickness. Retina The retina is the innermost layer of the wall of the eye (Fig. 8.8). It is an extremely delicate structure and is well adapted for stimulation by light rays. It is composed of several layers of nerve cell bodies and their axons, lying on a pigmented layer of epithelial cells which attach it to the choroid. The light- sensitive layer consists of sensory receptor cells, rods and cones, which contain photosensitive pigments that convert light rays into nerve impulses. The retina lines about three-quarters of the eyeball and is thickest at the back. It thins out anteriorly to end just behind the ciliary body. Near the centre of the posterior part is the macula lutea, or yellow spot (Figs 8.11 and 8.12). In the centre of the yellow spot is a little depression called the fovea centralis, consisting of only cones. Towards the anterior part of the retina there are fewer cones than rods (Fig. 8.11). Figure 8.11 The retina. A. Magnified section. B. Light-sensitive nerve cells: rods and cones. C. Coloured scanning electron micrograph of rods (green) and cones (blue). Figure 8.12 The retina as seen through the pupil with an ophthalmoscope. About 0.5 cm to the nasal side of the macula lutea all the nerve fibres of the retina converge to form the optic nerve. The small area of retina where the optic nerve leaves the eye is the optic disc or blind spot. It has no light-sensitive cells. Blood supply to the eye The eye is supplied with arterial blood by the ciliary arteries and the central retinal artery. These are branches of the ophthalmic artery, one of the branches of the internal carotid artery. Venous drainage is by a number of veins, including the central retinal vein, which eventually empty into a deep venous sinus. The central retinal artery and vein are encased in the optic nerve, which enters the eye at the optic disc. Interior of the eye The anterior segment of the eye, i.e. the space between the cornea and the lens, is incompletely divided into anterior and posterior chambers by the iris (Fig. 8.8). Both chambers contain a clear aqueous fluid secreted into the posterior chamber by ciliary glands. It circulates in front of the lens, through the pupil into the anterior chamber and returns to the venous circulation through the scleral venous sinus (canal of Schlemm) in the angle between the iris and cornea (Fig. 8.8). There is continuous production and drainage but the intraocular pressure remains fairly constant between 1.3 and 2.6 kPa (10 to 20 mmHg). An increase in this pressure causes glaucoma (p. 204). Aqueous fluid supplies nutrients and removes wastes from the transparent structures in the front of the eye that have no blood supply, i.e. the cornea, lens and lens capsule. Behind the lens and filling the posterior segment (cavity) of the eyeball is the vitreous body. This is a soft, colourless, transparent, jelly-like substance composed of 99% water, some salts and mucoprotein. It maintains sufficient intraocular pressure to support the retina against the choroid and prevent the walls of the eyeball from collapsing. The eye keeps its shape because of the intraocular pressure exerted by the vitreous body and the aqueous fluid. It remains fairly constant throughout life. Optic nerves (second cranial nerves) (Fig. 8.13) The fibres of the optic nerve originate in the retina and they converge to form the optic nerve about 0.5 cm to the nasal side of the macula lutea. The nerve pierces the choroid and sclera to pass backwards and medially through the orbital cavity. It then passes through the optic foramen of the sphenoid bone, backwards and medially to meet the nerve from the other eye at the optic chiasma. Figure 8.13 The optic nerves and their pathways. Optic chiasma This is situated immediately in front of and above the pituitary gland, which is in the hypophyseal fossa of the sphenoid bone (see Fig. 9.2, p. 209). In the optic chiasma the nerve fibres of the optic nerve from the nasal side of each retina cross over to the opposite side. The fibres from the temporal side do not cross but continue backwards on the same side. This crossing over provides both cerebral hemispheres with sensory input from each eye. Optic tracts These are the pathways of the optic nerves, posterior to the optic chiasma (Fig. 8.13). Each tract consists of the nasal fibres from the retina of one eye and the temporal fibres from the retina of the other. The optic tracts pass backwards to synapse with nerve cells of the lateral geniculate bodies of the thalamus. From there the nerve fibres proceed backwards and medially as the optic radiations to terminate in the visual area of the cerebral cortex in the occipital lobes of the cerebrum (see Fig. 7.21, p. 150). Other neurones originating in the lateral geniculate bodies transmit impulses from the eyes to the cerebellum where, together with impulses from the semicircular canals of the ears and from the skeletal muscles and joints, they contribute to the maintenance of posture and balance. Physiology of sight Light waves travel at a speed of 300 000 kilometres (186 000 miles) per second. Light is reflected into the eyes by objects within the field of vision. White light is a combination of all the colours of the visual spectrum (rainbow), i.e. red, orange, yellow, green, blue, indigo and violet. This is demonstrated by passing white light through a glass prism which bends the rays of the different colours to a greater or lesser extent, depending on their wavelengths (Fig. 8.14). Red light has the longest wavelength and violet the shortest. Figure 8.14 Refraction: white light broken into the colours of the visible spectrum when it passes through a prism.  This range of colour is the spectrum of visible light. In a rainbow, white light from the sun is broken up by raindrops, which act as prisms and reflectors. The electromagnetic spectrum The electromagnetic spectrum is broad, but only a small part is visible to the human eye (Fig. 8.15). Beyond the long end are infrared waves (heat), microwaves and radio waves. Beyond the short end are ultraviolet (UV), X-rays and gamma rays. UV light is not normally visible because it is absorbed by a yellow pigment in the lens. Following removal of the lens (cataract extraction), UV light is visible and it has been suggested that long-term exposure may damage the retina. Figure 8.15 The electromagnetic spectrum. A specific colour is perceived when only one wavelength is reflected by the object and all the others are absorbed, e.g. an object appears red when only the red wavelength is reflected. Objects appear white when all wavelengths are reflected, and black when they are all absorbed. In order to achieve clear vision, light reflected from objects within the visual field is focused on to the retina of each eye. The processes involved in producing a clear image are refraction of the light rays, changing the size of the pupils and accommodation (adjustment of the lens for near vision). Although these may be considered as separate processes, effective vision is dependent upon their coordination. Refraction of the light rays When light rays pass from a medium of one density to a medium of a different density they are bent; for example, in the eye, the biconvex lens bends and focuses light rays (Fig. 8.16). This principle is used to focus light on the retina. Before reaching the retina, light rays pass successively through the conjunctiva, cornea, aqueous fluid, lens and vitreous body. They are all denser than air and, with the exception of the lens, they have a constant refractory power, similar to that of water. Figure 8.16 Refraction of light rays passing through a biconvex lens. Focussing of an image on the retina Light rays reflected from an object are bent (refracted) by the lens when they enter the eye in the same way as shown in Figure 8.16, although the image on the retina is actually upside down (Fig. 8.17). The brain adapts to this early in life so that objects are perceived ‘the right way up’. Figure 8.17 Section of the eye showing the focusing of light rays on the retina. Abnormal refraction within the eye is corrected using biconvex or biconcave lenses, which are shown on page 206. Lens The lens is a biconvex elastic transparent body suspended behind the iris from the ciliary body by the suspensory ligament. It is the only structure in the eye that changes its refractive power. Light rays entering the eye need to be refracted to focus them on the retina. Light from distant objects needs least refraction and, as the object comes closer, the amount of refraction needed is increased. To increase the refractive power, the ciliary muscle contracts. This moves the ciliary body inwards towards the lens (contracts the sphincter), relaxing the pull on the suspensory ligaments, and allows the lens to bulge, increasing its convexity. This focuses light rays from near objects on the retina. When the ciliary muscle relaxes it slips backwards, increasing its pull on the suspensory ligament,  making the lens thinner (Fig. 8.18). This focuses light rays from distant objects on the retina. Figure 8.18 Accommodation: action of the ciliary muscle on the shape of the lens. A. Distant vision. B. Near vision. Size of the pupils Pupil size influences accommodation by controlling the amount of light entering the eye. In a bright light the pupils are constricted. In a dim light they are dilated. If the pupils were dilated in a bright light, too much light would enter the eye and damage the sensitive retina. In a dim light, if the pupils were constricted, insufficient light would enter the eye to activate the light-sensitive pigments in the rods and cones which stimulate the nerve endings in the retina. The iris consists of one layer of circular and one of radiating smooth muscle fibres. Contraction of the circular fibres constricts the pupil, and contraction of the radiating fibres dilates it. The size of the pupil is controlled by the autonomic nervous system; sympathetic stimulation dilates the pupils and parasympathetic stimulation causes constriction. Accommodation Close vision In order to focus on near objects, i.e. within about 6 metres, accommodation is required and the eye must make the following adjustments: constriction of the pupils convergence lacrimal apparatus. Figure 8.21 Section of the eye and its accessory structures. Eyebrows These are two arched ridges of the supraorbital margins of the frontal bone. Numerous hairs (eyebrows) project obliquely from the surface of the skin. They protect the eyeball from sweat, dust and other foreign bodies. Eyelids (palpebrae) The eyelids are two movable folds of tissue situated above and below the front of each eye. On their free edges there are short curved hairs, the eyelashes. The layers of tissue forming the eyelids are: a thin covering of skin a thin sheet of subcutaneous connective (loose areolar) tissue two muscles – the orbicularis oculi and levator palpebrae superioris a thin sheet of dense connective tissue, the tarsal plate, larger in the upper than in the lower eyelid, which supports the other structures a lining of conjunctiva. Conjunctiva This is a fine transparent membrane that lines the eyelids and the front of the eyeball (Fig. 8.21). Where it lines the eyelids it consists of highly vascular columnar epithelium. Corneal conjunctiva consists of avascular stratified epithelium, i.e. epithelium without blood vessels. When the eyelids are closed the conjunctiva becomes a closed sac. It protects the delicate cornea and the front of the eye. When eyedrops are administered they are placed in the lower conjunctival sac. The medial and lateral angles of the eye where the upper and lower lids come together are called respectively the  medial canthus and the lateral canthus. Eyelid margins Along the edges of the lids are numerous sebaceous glands, some with ducts opening into the hair follicles of the eyelashes and some on to the eyelid margins between the hairs. Tarsal glands (Meibomian glands) are modified sebaceous glands embedded in the tarsal plates with ducts that open on to the inside of the free margins of the eyelids. They secrete an oily material, spread over the conjunctiva by blinking, which delays evaporation of tears. Functions The eyelids and eyelashes protect the eye from injury: reflex closure of the lids occurs when the conjunctiva or eyelashes are touched, when an object comes close to the eye or when a bright light shines into the eye – this is called the corneal reflex blinking at about 3- to 7-second intervals spreads tears and oily secretions over the cornea, preventing drying. When the orbicularis oculi contract, the eyes close. When the levator palpebrae contract, the eyelids open (see Fig. 16.58, p. 414). Lacrimal apparatus (Fig. 8.22) For each eye this consists of: 1 lacrimal gland and its ducts 2 lacrimal canaliculi 1 lacrimal sac 1 nasolacrimal duct.  Figure 8.22 The lacrimal apparatus. Arrows show the direction of the flow of tears. The lacrimal glands are exocrine glands situated in recesses in the frontal bones on the lateral aspect of each eye just behind the supraorbital margin. Each gland is approximately the size and shape of an almond, and is composed of secretory epithelial cells. The glands secrete tears composed of water, mineral salts, antibodies (immunoglobulins, see Ch. 15), and lysozyme, a bactericidal enzyme. The tears leave the lacrimal gland by several small ducts and pass over the front of the eye under the lids towards the medial canthus where they drain into the two lacrimal canaliculi; the opening of each is called the punctum. The two canaliculi lie one above the other, separated by a small red body, the caruncle. The tears then drain into the lacrimal sac, which is the upper expanded end of the nasolacrimal duct. This is a membranous canal approximately 2 cm long, extending from the lower part of the lacrimal sac to the nasal cavity, opening at the level of the inferior concha. Normally the rate of secretion of tears keeps pace with the rate of drainage. When a foreign body or other irritant enters the eye the secretion of tears is greatly increased and the conjunctival blood vessels dilate. Secretion of tears is also increased in emotional states, e.g. crying, laughing. Functions The fluid that fills the conjunctival sac is a mixture of tears and the oily secretion of tarsal glands, which is spread over the cornea by blinking. The functions of this fluid include: washing away irritating materials, e.g. dust, grit the bactericidal enzyme lysozyme prevents microbial infection its oiliness delays evaporation and prevents drying of the conjunctiva. Sense of smell Learning outcome After studying this section you should be able to: describe the physiology of smell. The sense of smell, or olfaction, originates in the nasal cavity, which also acts as a passageway for respiration (see Ch. 10). Olfactory nerves (first cranial nerves) These are the sensory nerves of smell. They originate as specialised olfactory nerve endings (chemoreceptors) in the mucous membrane of the roof of the nasal cavity above the superior nasal conchae (Fig. 8.23). On each side of the nasal septum nerve fibres pass through the cribriform plate of the ethmoid bone to the olfactory bulb where interconnections and synapses occur (Fig. 8.24). From the bulb, bundles of nerve fibres form the olfactory tract, which passes backwards to the olfactory area in the temporal lobe of the cerebral cortex in each hemisphere where the impulses are interpreted and odour perceived (see Fig. 7.21, p. 150). Figure 8.23 The olfactory structures. Figure 8.24 An enlarged section of the olfactory apparatus in the nose and on the inferior surface of the cerebrum. Physiology of smell The human sense of smell is less acute than in other animals. Many animals secrete odorous chemicals called pheromones, which play an important part in chemical communication in, for example, territorial behaviour, mating and the bonding of mothers and their newborn. The role of pheromones in human communication is unknown. All odorous materials give off volatile molecules, which are carried into the nose with inhaled air and even very low concentrations, when dissolved in mucus, stimulate the olfactory chemoreceptors. The air entering the nose is warmed, and convection currents carry eddies of inspired air to the roof of the nasal cavity. ‘Sniffing’ concentrates volatile molecules in the roof of the nose. This increases the number of olfactory receptors stimulated and thus perception of the smell. The sense of smell may affect the appetite. If the odours are pleasant the appetite may improve and vice versa. When accompanied by the sight of food, an appetising smell increases salivation and stimulates the digestive system (see Ch. 12). The sense of smell may create long-lasting memories, especially for distinctive odours, e.g. hospital smells, favourite or least-liked foods. Inflammation of the nasal mucosa prevents odorous substances from reaching the olfactory area of the nose, causing loss of the sense of smell (anosmia). The usual cause is a cold. Adaptation When an individual is continuously exposed to an odour, perception of the odour decreases and ceases within a few minutes. This loss of perception affects only that specific odour. Sense of taste Learning outcome After studying this section you should be able to: describe the physiology of taste. The sense of taste, or gustation, is closely linked to the sense of smell and, like smell, also involves stimulation of chemoreceptors by dissolved chemicals. Taste buds contain sensory receptors (chemoreceptors) that are found in the papillae of the tongue and widely distributed in the epithelia of the tongue, soft palate, pharynx and epiglottis. They consist of small sensory nerve endings of the glossopharyngeal, facial and vagus nerves (cranial nerves VII, IX and X). Some of the cells have hair-like cilia on their free border, projecting towards tiny pores in the epithelium (Fig. 8.25). The sensory receptors are stimulated by chemicals that enter the pores dissolved in saliva. Nerve impulses are generated and conducted along the glossopharyngeal, facial and vagus nerves before synapsing in the medulla and thalamus. Their final destination is the taste area in the parietal lobe of the cerebral cortex where taste is perceived (see Fig. 7.21, p. 150). Figure 8.25 Structure of taste buds. A. A section of a papilla. B. A taste bud – greatly magnified. C. Coloured scanning electron micrograph of a taste bud (centre) on the tongue. Physiology of taste Four fundamental sensations of taste have been described – sweet, sour, bitter and salt. This is probably an oversimplification because perception varies widely and many ‘tastes’ cannot be easily classified. It is thought that all taste buds are stimulated by all ‘tastes’. Taste is impaired when the mouth is dry, because substances can only be ‘tasted’ when in solution. The sense of taste triggers salivation and the secretion of gastric juice (see Ch. 12). It also has a protective function, e.g. when foul-tasting food is eaten, reflex gagging or vomiting may be induced.