Chapter 8
The special senses
Hearing and the ear
The ear is the organ of hearing and is also involved in balance. It is supplied by the 8th cranial nerve, i.e. the cochlear part of the vestibulocochlear nerve, which is stimulated by vibrations caused by sound waves.
With the exception of the auricle (pinna), the structures that form the ear are encased within the petrous portion of the temporal bone.
Structure
The ear is divided into three distinct parts (Fig. 8.1): the outer ear, middle ear (tympanic cavity) and inner ear.
The outer ear collects the sound waves and directs them to the middle ear, which in turn transfers them to the inner ear, where they are converted into nerve impulses and transmitted to the hearing area in the cerebral cortex.
Outer ear
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 embedded in cartilage and the remainder lies within 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 bacteriocidal 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.
Middle ear (tympanic cavity)
This is an irregular-shaped air-filled cavity within the petrous portion of the temporal bone (Figs 8.1 and 8.3). 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:
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 links the nasopharynx and middle ear. It is about 4 cm long and 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 extends 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. The ossicles are held in place by fine ligaments and are named according to their shapes.
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.
Inner ear (Fig. 8.4)
The inner ear or labyrinth (meaning ‘maze’) contains the organs of hearing and balance. It is described in two parts, the bony labyrinth and the membranous labyrinth and is divided into three main regions:
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 (Fig. 8.4).
The bony labyrinth.
This is lined with periosteum. Within the bony labyrinth, the membranous labyrinth is suspended in a watery fluid called perilymph.
The membranous labyrinth.
This is filled with endolymph.
The 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 (p. 196).
The semicircular canals
These are three tubes arranged so that one is situated in each of the three planes of space. They are continuous with the vestibule and are also important in balance (p. 196).
The cochlea
This 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 (Fig. 8.5) contains three compartments:
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. 7.20, p. 157).
Physiology of hearing 
8.1
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. 7.20, p. 157). 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 excessive noise causes hearing loss because it damages the sensitive hair cells of the spiral organ.
Balance and the ear
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, or equilibrium. The arrangement of the three semicircular canals, one in each plane, not only allows perception of the position of the head in space but also the direction and rate of any movement. 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. The cerebellum coordinates incoming impulses from the vestibular nerve, the eyes and proprioceptors. Thereafter, impulses are transmitted to the cerebrum and skeletal muscles enabling perception of body position and any adjustments needed to maintain posture and balance. This maintains upright posture and fixing of the eyes on the same point, independently of head movements.
Sight and 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 protect the eye from injury.
Structurally the two eyes are separate but, unlike the ears, some of their activities are coordinated so that they normally 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
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 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 (see Table 8.1, p. 204).
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 (p. 198) and is then absorbed by the choroid.
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 size and thickness 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. 202).
Iris
The iris is the visible coloured ring at the front 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 7.44 and 7.43, respectively, pp. 174 and 175).
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 is suspended from the ciliary body by the suspensory ligament. Its thickness is controlled by the ciliary muscle through the suspensory ligament. 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.
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 (see Fig. 8.18).
Retina
The retina is the innermost lining of the eye (Fig. 8.8). It is an extremely delicate structure and 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. 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 8.11A 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.
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