Chapter 16-定稿 - 图文

Chapter 16 The Sensory System

16.1 Sense and Sensory Mechanisms

The word “sense” might be defined as “the interpretation, by the specialized areas of the cerebral cortex, of an impulse arising from the receptors which are designed to report changes taking place either within the body or outside of it.” These sensory receptors consist of the endings of the dendrites of afferent neurons. Some receptors are designed to respond only to special stimuli (sound waves, light rays) while others respond to such general sensations as pain or pressure.

The senses have been said to number five. Actually there are more than that. A partial list includes the following:

1. Visual sense from the eye. 2. Hearing sense from the ear.

3. Taste sense from the tongue receptors. 4. Smell sense from the upper nasal cavities.

5. Pressure, heat, cold, pain and touch senses from the skin.

6. Position and balance sense from the muscles, the joints and the semicircular canals in

the ear.

7. Hunger and thirst senses from various internal parts of the body.

16.2 The Eye

16.2.1 Protection of the Eyeball and its Parts

In the embryo the eye develops as an out-pocketing of the brain. As in the case of the brain, the eye is a delicate organ. Therefore, nature has carefully protected the eye by means of the following:

1. The skull bones that form the eye orbit (cavity) serve to protect more than half of the

eyeball at the back (dorsally).

2. The lids and the eyelashes aid in protecting the eye at the front part (anteriorly). 3. The tears wash away small foreign objects that may enter the lid area.

4. A sac lined with an epithelial membrane separates the front of the eye from the eyeball

proper and aids in the destruction of some of the pathogenic bacteria that may enter from the outside.

16.2.2 Coats of the Eyeball

The eyeball has three separate coats or tunics. The outermost layer is called the sclera and is made of firm, tough, connective tissue. It is commonly referred to as the white of the eye. The second tunic of the eyeball is known as the choroid coat. It is heavily pigmented, preventing light rays from scattering and reflecting (bouncing) off the inner surface of the eye. The choroid coat might be compared to the dull black lining of a camera. It is made of a delicate network of connective tissue containing much dark brown pigment and interlaced with many blood vessels. The innermost coat, called the retina, includes some ten different layers of nerve cells, including the end organs commonly called the rods and cones. These are the receptors for the sense of vision. The rods are sensitive to white and black. The cones are sensitive to color. As far as is known, there are three types of cones, each of which is sensitive to one of the primary colors (red, yellow and blue). Persons who completely lack cones are totally color blind; those who lack one type of cone are partially color blind. Color blindness is an inherited condition and occurs almost exclusively in males. 16.2.3 Pathway of Light Rays

Light rays pass through a series of transparent, colorless eye parts. On the way they undergo a process of bending known as refraction. This refracting of the light rays makes it possible for light from a very large area to be focused upon a very small surface, the retina, where the receptors are located. The following are, in order from outside in, the transparent refracting parts, or media, of the eye:

1. The cornea is a forward continuation of the outer coat, but it is transparent and

colorless, whereas the sclera is opaque and white.

2. The aqueous humor, a watery fluid which fills much of the eyeball in front of the lens,

helps to maintain the slight forward curve in the cornea.

3. The crystalline lens is a circular structure made of a jellylike material.

4. The vitreous body fills the entire space behind the lens and is necessary to keep the

eyeball in its spherical shape.

The cornea is referred to frequently as the “window” of the eye. It bulges forward slightly and is the most important refracting structure. Injuries caused by foreign objects or by infection may result in scar formation in the cornea and a resulting area of opacity through which light rays cannot pass. If such an injury involves the central area in front of the pupil (the hole in the center of the colored part of the eye), blindness may be the result. The cornea may be transplanted; eye banks store corneas obtained from donors immediately after death, or in some cases, before death.

The next light-bending medium is the aqueous humor, followed by the crystalline lens. The lens has two bulging surfaces, so it may be best described as biconvex. During youth the lens is elastic and therefore is an important part of the system of accommodation to near vision. In the process of accommodation the lens becomes thicker and thus bends the light rays a greater amount, as is required for near objects. With aging the lens loses, its elasticity, and therefore its ability to adjust by thickening, resulting in what is known as the old eye, or presbyopia.

The last of these transparent refracting parts of the eye is the vitreous body. As in the case of tile aqueous humor it is important in maintaining the ball-like shape of the eyeball as well as aiding in refraction. The vitreous body is not replaceable; an injury that causes a loss of an appreciable amount of the jellylike vitreous material will cause collapse of the eyeball. This will require the removal of the eyeball, an operation called enucleation. 16.2.4 Muscles of the Eye

Certain muscles are inside the eyeball itself, and therefore are described as intrinsic, while

others are attached to bones of the eye orbit as well as to the sclera and are called extrinsic muscles.

The intrinsic muscles are found in two circular structures:

1. The iris, the colored or pigmented part of the eye, which has a central opening called

the pupil. The size of the pupil is governed by the action of two sets of muscles, one of which is arranged in a circular fashion, while the other extends in a radial manner resembling the spokes of a wheel.

2. The ciliary body, which is shaped somewhat like a flattened ring with a hole that is the

size of the outer edge of the iris. This muscle alters the shape of the lens.

The purpose of the iris is to regulate the amount of light entering the eye. If a strong light is flashed in the eye, the circular muscle fibers of the iris, which form a sphincter, contract and thus reduce the size of the pupil. On the other hand, if the light is very dim, the radial involuntary iris muscles, which are attached at the outer edge, contract; the opening is pulled outward and thus enlarged. This pupillary enlargement is known as dilation.

The pupil changes size, too, according to whether one is looking at a near object or a distant one. Viewing a near object causes the pupil to become smaller; a far view will cause it to enlarge.

The muscle of the ciliary body is similar in direction and method of action to the radial muscle of the iris. When the ciliary muscle contracts, it removes the tension on the suspensory ligament of the lens. The elastic lens then recoils and becomes thicker in much the same way that a rubber band would thicken if a pull on it were released. This action changes the focus of the lens, and thus adjusts the eye for either long views or close-ups.

The six extrinsic muscles connected with each eye are ribbonlike and extend forward from the apex of the orbit behind the eyeball. One end of each muscle is attached to a bone of the skull, while the other end is attached to the white (sclera) of the eye. These muscles pull on the eyeball in a coordinated fashion that causes the two eyes to move together in order to center on

one visual field. There is another muscle located within the orbit which is attached to the upper eyelid. When this muscle contracts, it keeps the eye open (Fig. 16.1).

Figure 16.1 The eye

16.2.5 Nerve Supply to the Eye

The two sensory nerves of the eye are:

1. The optic nerve, which carries visual impulses received by the rods and cones in the

retina to the brain. This, it will be recalled, is the second cranial nerve.

2. The ophthalmic nerve which carries impulses of pain, touch and temperature from the

eye and surrounding parts. It is a branch of the fifth (trigeminal) cranial nerve. The optic nerve is connected with the eyeball a little toward the medial or nasal side of the eye at the back. At this region there are no rods and cones; and so this part, which is a circular white area, is called the blind spot, known also as the optic disk. There is a tiny depressed area in the retina called the fovea centralis, which is the clearest point of vision.

There are three nerves that carry motor fibers to the muscles of the eyeball. The largest is the oculomotor nerve, which supplies motor fibers, voluntary and involuntary, to all the muscles but two. The other two nerves, the trochlear and the abducens, supply one voluntary muscle each.

16.2.6 The Lacrimal Apparatus and the Conjunctival Sac

The lacrimal gland produces tears and is located above the eye toward one side; that is, it is superior and lateral to the eyeball. Tiny tubes carry the tears to the front surface of the eyeball, where they serve to constantly wash the sac that separates the front part of the eyeball from the larger back portion (Fig. 16.2). This sac is lined with a membrane called the conjunctiva. The conjunctiva lines the eyelids and is reflected onto the front of the eyeball. It is kept moist by the tears flowing across the front of the eye. Tears then are carried into tiny openings near the nasal corner of the eye. From these openings tears are carried by tubes that drain into the nose via the nasolacrimal duct. A slight excess of tears, or lacrlmation, causes nose blowing; and a greater overproduction of tears results in an overflow onto the face.

Figure 16.2 Lacrimal apparatus

16.2.7 Eye Infections

Inflammation of the membrane that lines the eyelids and covers the front of the eyeball is called conjunctivitis. It may be acute or chronic, and may be caused by a variety of irritants and pathogens. “Pinkeye” is an acute conjunctivitis that is highly contagious and is caused by cocci or bacilli in most cases. Sometimes irritants such as wind and excessive glare, for example from snow, may cause an inflammation that then may cause a susceptibility to bacterial infection. In the case of the contagious epidemic form, children should be kept at home until the infection has been cured.

Trachoma, sometimes referred to as granular conjunctivitis, is caused by Chlamydia trachomatis, which is a type of bacteria. This disease was formerly quite common in the mountains of the southern United States, and among the American Indians. It is still prevalent in the Far East, in Egypt and in southern Europe. This disease is characterized by the formation of granules on the lids, which may cause such serious irritation of the cornea that blindness can result. Better hygiene and the use of antibiotic drugs have reduced the prevalence and seriousness of this infection.

An eye infection of the newborn infant, called ophthalmia neonatorum, is caused by the entrance of gonococci into the conjunctival sac. Neglect of this infection may cause blindness.

Prevention by the instillation of an appropriate antiseptic such as a silver nitrate solution or penicillin is routine in hospitals at the time of delivery of the infant.

The iris, the choroid coat, the ciliary body and other parts of the eyeball may become infected by a number of different organisms. Such disorders are likely to be very serious fortunately they are not very common. Syphilis spirochetes, tubercle bacilli and a variety of cocci may cause these painful infections. They may follow sinus infections, tonsillitis, conjunctivitis and numerous other disorders. The care of these conditions usually should be in the hands of an ophthalmologist, a physician who specializes in disorders of the eye. 16.2.8 Eye Defects and Eyestrain

One defect that is often responsible for eyestrain in children is farsightedness, or hyperopia. In this condition the light rays are not bent sharply enough to focus on the retina, with the result that the eye cannot focus properly on nearby objects. The eyeball may be too short, so that the actual focal point is behind the retina. This is normal in the infant, but usually corrects itself by the time the child uses his eyes more for near vision. To a certain extent it is possible to use the ciliary muscle in the process of thickening the lens to focus objects on the too-near retina. However, this causes constant strain. Visual tests may not show that the condition exists unless drops which paralyze the ciliary muscles are used. Hence any suggestion of eyestrain should lead to consulting a specialist who has a license to practice medicine and who will use the drops as necessary.

Myopia or nearsightedness is another defect of development. In this case the eyeball is too long, or the bending of the light rays is too sharp, so that the focal point is in front of the retina. Objects that are a distance away appear blurred, and may appear clear only if brought very near the eye. Only by the use of lenses that will throw the point of focus back can this disorder be corrected. In some young people this nearsightedness becomes worse each year until the person reaches his twenties. It was thought by some that much use of the eyes for school work may have played a part, but it has been found that people who do not do any reading or other close

work suffer from this problem to as great a degree. Careful attention to good hygiene and changing of glasses as indicated by the ophthalmologist are desirable.

Another rather common visual defect is astigmatism. This condition is due to irregularity in the curvature of the cornea or the lens. The surfaces do not bend the light rays the same amount, resulting in blurred vision with severe eyestrain. Astigmatism often is found in combination with hyperopia or myopia, so a careful eye examination and properly fitted glasses will reduce or prevent eyestrain.

The scientific name for cross-eyedness is internal strabismus. Strabismus means that the muscles of the eyeballs do not coordinate, so that the two eyes do not work together. There are several different kinds of strabismus (in another sense it means squint), but the cross-eye type in which the eyeball is pulled inward (medially) is fairly common and is found early in life. Care by a skilled ophthalmologist as soon as possible may result in restoration of muscle balance. Each patient is treated as his needs indicate. In some, glasses and exercises may correct the defect, while in others surgery may be required. If correction is not accomplished early, the affected eye may become blinded, since the brain has a way of cutting out the confusing double image, and the eye suffers from disuse.

Some of the symptoms of eyestrain include:

1. Inflammation and infection of structures in the eyelids, as, for example, formation, in

which oil glands on lid edges become infected.

2. Excessive tear formation (lacrimation) and pain in the eyes. 3. Headaches and other nervous disturbances.

4. Digestive disturbances and loss of appetite with malnutrition.

Eyestrain is so important that more attention to hygiene of the eyes is to be highly recommended. Some points to remember are:

1. Smaller children should begin reading books in which the type is larger and the letters

are spaced relatively far apart to make them easier to differentiate.

2. Be certain that there is enough light without glare.

3. The table or desk on which the work is being done should be neither too low nor too

high.

4. Proper examination of the eyes and the use of adequate lenses are very important. The

notion that glasses will weaken the eyes has absolutely no basis in fact.

16.2.9 Blindness and its Causes

The most common causes of blindness are those which largely affect the elderly. Number one on the list is cataract formation. A cataract involves the lens or its capsule so that it loses its transparency. Sometimes the areas of opacity can be seen through a pupil which becomes greatly enlarged because of the reduction in the amount of light that can reach the retina. In other cases there is very gradual loss of vision, and frequent changes in glasses may aid in maintaining useful vision for some time. Removal of the lens may restore some vision, but the addition of a contact lens usually is required to achieve satisfactory visual acuity, as well as binocular vision which is desirable for driving a car, for example. Most persons will need reading glasses for close work.

A second very important cause of blindness, particularly in older persons, is glaucoma, a condition characterized by excess pressure of the eye fluid. Aqueous humor is being produced constantly from the blood; and after circulation it is reabsorbed into the blood stream. Interference with the normal reentry of this fluid to the blood stream leads to an increase in pressure inside the eyeball. As in the case of cataract, glaucoma usually progresses rather slowly, with vague visual disturbances and gradual impairment of vision. Halos around lights, headaches and the need for frequent changes of glasses (particularly by people over 40) are symptoms that should be investigated by an ophthalmologist. There are different forms of glaucoma, some occurring in the very young; and each type requires a different management. Since continued high pressure of the aqueous humor may cause destruction of the optic nerve fibers, it is important to obtain continuous treatment beginning early in the disease to avoid blindness.

Diabetes as a cause of blindness is increasing in the United States. Disorders of the eye directly related to diabetes include 1) optic atrophy in which the optic nerve fibers die, 2) cataracts, which occur earlier and with greater frequency among diabetics, and 3) diabetic retinopathy, in which the retina can be damaged by blood vessel hemorrhages and other causes. Diabetics also are extremely susceptible to atherosclerosis，fatty deposits in the arteries.

There are many other causes of blindness, and frequently these could have been prevented. Injuries by pieces of glass and other sharp objects are an important cause of eye damage. Industrial accidents involving the eye have been greatly reduced by the use of protective goggles. If an injury should occur, it is then very important to prevent infection. Even a tiny scratch can become so seriously infected that blindness will result.

The retina may become detached from the choroid and float into the vitreous body. If neglected, blindness may result. Treatment includes a sort of “spot welding” with an electric current or a weak laser beam. A series of pinpoint scars reattach the retina.

16.3 The Ear

The ear is a combination sensory organ, related to both hearing and equilibrium (Fig.16.3). It may be divided into three main sections namely:

1. The external ear, which includes the outer projection and a canal. 2. The middle ear, which is an air space containing three small bones.

3. The internal ear, which is the most important part, since it contains the sensory end

organs or receptors for hearing and equilibrium.

Figure 16.3 The ear

16.3.1 The Eeternal Ear

The projecting part of the ear is known as the pinna，or the auricle. From a functional point of view it is probably of little importance in the human. Then follows the opening itself, the external auditory canal, which extends medially for about 1 inch or more, depending upon which wall of the canal is measured. The skin lining this tube is very thin, and in the first part of the canal contains many wax, or ceruminous, glands. The cerumen，or wax, may become dried and impacted in the canal so that removal by a physician is required. The same kinds of disorders that involve the skin elsewhere also may affect the skin of the external auditory canal: eczema, boils and other infections.

At the end of the auditory canal is the tympanic membrane, or eardrum. It serves as a boundary between the external auditory canal, or meatus, and the middle ear cavity. It may be injured by inserted objects such as bobby pins or toothpicks. Normally the air pressure on the two sides of the drum is equalized by means of the eustachian tube connecting the middle ear cavity and the throat (pharynx), allowing the eardrum to vibrate freely with the incoming sound waves. Sudden great changes in the pressure on either side of the eardrum may cause excessive stretching and inflammation of the membrane. There may even be perforation of the drum. In some cases pressure from pus or exudate in the middle ear cavity can be relieved only by cutting the eardrum, a procedure called a myringotomy.

16.3.2 The Middle Ear

The middle ear cavity is a small flattehed space that contains air and three small bones, or ossicles. Air is brought into the cavity through the eustachian tube (also called the auditory tube; it should not be confused with the external auditory canal). The eustachlan tube connects the lower part of the middle ear cavity with the pharynx. The mucous membrane of the pharynx is continuous through the eustachian tube into the middle ear cavity, and infection travels along the membrane, causing middle ear disease. This happens more often in children, partly because the tube is more horizontal in the child, while in the adult the tube tends to slant toward the pharynx. At the back of the middle ear cavity is an opening into the mastoid air cells, which are spaces inside a part of the temporal bone, one of the major bones of the skull.

The three ossicles are joined in such a way that they amplify the sound waves received by the tympanic membrane and then transmit the sounds to the fluid in the internal ear. The handlelike part of the first bone, or malleus, is attached, to the tympanic membrane, while the headlike portion connects with the second bone, which is called the incus. The innermost of the ossicles is shaped somewhat like a stirrup and is called the stapes. It is connected with the membrane of the oval window which in turn vibrates and conducts these waves to the fluid of the internal ear. 16.3.3 The Internal Ear

The most complicated and important part of the ear is the internal portion. It includes three separate spaces hollowed out inside the temporal bone. Because they are rather complex, they constitute what has been called the bony labyrinth. Next to the oval window is the vestibule. This entrance area then communicates with the bony tube shaped like a snail shell, called the cochlea, toward the front, and with the semicircular canals toward the back. These spaces all contain a fluid called perilymph. In the fluid of the bony semicircular canals are the membranous canals, which contain another fluid called endolymph. In a similar fashion a membranous cochlea is situated in the perilymph of the bony cochlea, and it also is filled with endolymph.

The organ of hearing, made of receptors connected with nerve fibers in the cochlear nerve (a part of the acoustic nerve), is located inside the membranous cochlea, or cochlear duct. The sound waves enter the external auditory canal and cause the eardrum to vibrate. These vibrations are amplified by the ossicles and transmitted by them to the perilymph. They then are conducted by the perilymph through the membrane to the endolymph. The waves of the endolymph are transmitted to the tiny hairlike receptors, which are thereby stimulated and which conduct nerve impulses through the nerve fibers to the brain for interpretation (Fig. 16.4).

The semicircular canals contain the sensory organs related to equilibrium. The membranous canals are connected with two small sacs in the vestibule, and one of these sacs contains sensory end organs for obtaining information with relation to the position of the head. Nerve fibers from these sacs and from the canals form the vestibular nerve which joins the cochlear nerve to form the acoustic nerve, which latter, as we learned, is one of the 12 cranial nerves.

Figure 16.4 The internal ear, including a section showing the crista where the sensory receptors for balance are located 16.3.4 Disorders of the Ear

Infection of the middle ear cavity is rather common and is called otitis media. A variety of bacteria as well as viruses may cause otitis media. It is also a frequent complication of measles,

influenza, scarlet fever and other infections. Antibiotic drugs have reduced complications and have caused a marked reduction of the amount of surgery done. However, some cases cannot be cured without surgery of some kind. A complication formerly quite common was mastoid infection. It is less common now, but it still does occur, and it requires the care of a specialist in most cases.

Deafness, either partial or complete, may be caused by a variety of conditions. Obstruction of the external auditory canal by wax or foreign bodies may interfere with the conduction of sound waves to the tympanic membrane. Inflammation and scarring of the tympanic membrane or of the joints between the ossicles may prevent normal vibration and amplification of sound waves. In children, the most common problem is related to an excess of lymphoid tissue near the opening of the eustachian tube into the pharynx. This prevents the equalization of air pressure on both sides of the eardrum.

Otosclerosis is a type of deafness found in adults between 18 and 40. Bone changes prevent normal vibration of the third ossicle (stapes). There seems to be a hereditary predisposition to the disease. Surgery includes an operation to release the stapes so that it will move again. If this fails, a new window can be made between the middle and inner ears. Hearing aids help most of these victims. Learning of lipreading should be encouraged.

There are many other causes of deafness, and many other kinds. Injuries of the internal ear, the acoustic nerve or of the parts of the brain that conduct or interpret auditory messages may cause deafness. They may be due to infections such as measles or syphilis. Alcohol, quinine, arsenic or mercury compounds are also damaging.

16.4 Other Organs of Special Sense

16.4.1 Taste Sense

The sense of taste involves receptors in the tongue and two different nerves that carry taste impulses to the brain. The taste receptors are known as taste buds and are located along the

edges of small depressed areas called fissures. These taste buds are stimulated only if the substance to be tasted is in solution. Tastes have been described as essentially of four kinds, namely:

1. Sweet tastes, which are most acutely experienced at the tip of the tongue.

2. Sour tastes, most effectively detected by the taste buds located at the sides of the tongue.

3. Salty tastes, which, as in the case of sweet tastes, are most acute at the tip of the tongue.

4. Bitter tastes, which are detected at the back (dorsal) part of the tongue.

The nerves of taste include the facial and the glossopharyngeal nerves. The interpretation of taste impulses probably is accomplished by the lower front portion of the brain, although there may not be a sharply separate taste or gustatory center (Fig. 16.5).

Figure 16.5 Organs of taste and smell

16.4.2 Sense of Smell

The sensory end organs, or receptors, for smell are located in the olfactory epithelium of the upper part of the nasal cavity. Because they are high in the nasal cavity, an animal or a person

“sniffs” in order to bring the gases responsible for an odor upward in the nose. The pathway of the impulses from the receptors for smell is the olfactory nerve. This leads to the olfactory center in the brain. The interpretation of smell is closely related to the sense of taste. The smell of foods is just as important in stimulating appetite and the flow of digestive juices as is the sense of taste. 16.4.3 Hunger and Appetite

Hunger includes intermittent sensation coming from the region of the stomach. It is due, in part, to contractions of the stomach muscle, and is not continuous; that is, if a person is starving, the hunger pangs diminish instead of becoming more acute. Appetite differs from hunger in that although it is basically a desire for food, it often has no relationship to the need for food. Hunger may have been relieved by an adequate meal, but the person may still have an appetite for additional food. A loss of appetite is known as anorexia, and may be due to a great variety of physical and mental disorders. The location of the nerve receptors which transmit hunger impulses is still uncertain. They are probably in the stomach muscles. 16.4.4 Sense of Thirst

Although thirst may be due to a generalized lack of water in the tissues, the sense of thirst seems to be largely localized in the mouth, the tongue and the pharynx. It is a very unpleasant sensation and is continuous up to relief or death. If there is an excessive excretion of water, as in diabetes, there may be excessive thirst, which is called polydipsia.

16.5 General Senses

As opposed to the special senses, in which the receptors are limited to a relatively small area in the body, the general senses are scattered throughout the body. These may be said to include pressure, heat, cold, pain, touch, position and balance senses, all of which are rather widely distributed. 16.5.1 Pressure Sense

It has been found that even though the skin is anesthetized, there still is consciousness of

pressure. These end organs for deep sensibility are located in the subcutaneous and deeper tissues. They are sometimes referred to as receptors for deep touch. 16.5.2 Temperature Sense

Heat and cold receptors have separate nerve fiber connections. Each has its type of end organ structure peculiar to it, and the distribution of each varies considerably. A warm object will stimulate only the heat receptors, while a cool object affects only the cold terminals. More heat receptors are found in the lips than in the hands, so that they are more sensitive to heat than are the hands. As in the case of other sensory receptors, continued stimulation results in adaptation; that is, the receptors adjust themselves in such a way that one does not feel a sensation so acutely if the original stimulus is continued. For example, the initial immersion of a hand in hot water may give rise to an uncomfortable sensation; however, if the immersion is prolonged, the water very soon will not feel as hot as it did at first (even if it has not cooled appreciably). 16.5.3 Sense of Touch

The touch receptors are small rounded bodies called tactile corpuscles. They are found mostly in the dermis and are especially close together in the tips of the fingers and the toes. The tip of the tongue also contains many of these receptors and so is very sensitive to touch, whereas the back of the neck is relatively insensitive. 16.5.4 Pain Sense

Pain is the most important protective sense. The receptors for pain are the most widely distributed sensory end organs. They are found in the skin, the muscles and the joints, and to a lesser extent in most internal organs (including the blood vessels and viscera). Pain receptors are not oval bodies as are many of the other sensory end organs, but apparently are merely branchings of the nerve fiber, called free nerve endings. Referred pain is a term used in cases in which pain that seems to be in an outer part of the body, particularly the skin, actually originates in an internal organ located near that particular area of skin. These areas of referred pain have been mapped out on the basis of much experience and many experiments. It has been found, for

example, that liver and gallbladder disease often cause referred pain in the skin over the right shoulder. Spasm of the coronary arteries that supply the heart may cause pain in the left shoulder and the left arm. One reason for this is that some neurons have the twofold duty of conducting impulses both from visceral pain receptors and from pain receptors in neighboring areas of the skin. The brain cannot differentiate between these two possible sources; but since most pain sensations originate in the skin, the brain automatically assigns the pain to this more likely place of origin.

Pain sense differs from other senses in that continued stimulation does not result in adaptation. This is nature’s way of being certain that the warnings of the pain sense are heeded. Sometimes the cause cannot be remedied quickly, and occasionally not at all. Then it is necessary to relieve pain. Some methods that have been found to be effective include:

1. Application of cold, especially crushed ice in ice caps, for headaches; or in bags for

localized areas of injury or inflammation; or cold compresses made by wringing out a towel (or gauze for small compresses) in cold water.

2. Compression of the painful area so that the nerve does not carry the pain impulses to the

brain. This may help relieve pain for a short time only.

3. Analgesic drugs, which are mild pain relievers. Examples are phenacetin, acetaminophen

and aspirin.

4. Narcotic drugs, which produce stupor and sleep. These are often very effective pain

relievers. An example of a narcotic drug is morphine.

5. Anesthetics, which may be either local (i.e., that render only a certain area insensitive) or

general, producing total unconsciousness. These are used largely to prevent pain during surgery.

16.5.5 Sense of Position

Receptors located in muscles, tendons and joints relay impulses that aid in judging the position and changes in the locations of parts with respect to each other. They also inform the

brain of the amount of muscle contraction and tendon tension. These rather widely spread end organs, which are known as proprioceptors, are aided in this function by the semicircular canals and related internal ear structures. Information received by these receptors is needed for coordination of muscles and is important in such activities as walking, running and many more complicated skills such as playing a musical instrument. These muscle sense end organs also play an important part in maintaining muscle tone and good posture, as well as allowing for the adjustment of the muscles for the particular kind of work to be done. The nerve fibers that carry impulses from these receptors enter the spinal cord and ascend to the brain in the back (posterior) part of the cord.

Syphilis and certain other diseases may involve the posterior part of the spinal cord, causing degeneration of it and loss of the position sense, giving rise to a condition known as tabes dorsalis. The lower part of the body usually is affected first, with the result that the victim gradually loses not only position sense but muscular coordination. Certain activities such as walking, which these two faculties hitherto made possible without the aid of sight, and with little thought, now become difficult---particularly in the dark, when the victim cannot see how he plants his feet.

As the disease progresses, the patient may lose the ability to walk. Early treatment of possible causes as a means of prevention is important, since once the nerve tissue is damaged, no cure is possible.

New Words

单词 sensory cerebral cortex sensory receptor dendrite afferent embryo orbit dorsal eyelash epithelial membrane sclera choroid = chorioid pigment retina refraction cornea aqueous humor vitreous body crystalline lens biconvex presbyopia iris ciliary sphincter optic cranial ophthalmic trigeminal fovea centralis oculomotor trochlear abducens

音标和词性 ['sens?ri] adj. ['seribr?l] adj. ['k?:teks] n. ['sens?ri ri'sept?] n. 汉语释义 知觉的, 感觉的, 知觉器官的 大脑的, 理智的 外皮, （大脑）皮层 感觉感受器 树突, 树枝石, 树枝状结晶 输入的,传入的 胚胎 眼眶, 轨道 (动物）背的, 背部的,背侧的 睫毛 上皮的 [C] 薄膜, 膜状物 巩膜 脉络膜, 脉络膜的 色素，把 ... 加颜色, 变色 视网膜 折光, 折射 角膜 房水，眼房水 (眼睛的)玻璃体 (眼球的)水晶体 两面凸的 老花眼, 远视 虹膜 眼睫毛的, 睫状的 括约肌 视觉的 头盖的, 头盖形的 眼睛的,眼科的, 眼炎的 三叉的, 三叉神经的 视网膜中央凹 转动眼球的, 动眼神经的 软骨轮的, 滑车的, 轮状的 [解]外展神经, 外展肌肉 ['dendrait] n. ['?f?r?nt] adj. ['embri?u] n. ['?:bit] n. ['d?:s?l] adj. ['ail??] n. [?epi'θi:li?l] adj. ['membrein] n. ['sklir?] n. ['k?:r?id] n. adj. ['pigm?nt] n. v. ['retin?] n. [ri'fr?k??n] n. ['k?:ni?] n. ['eikwi?s 'hju:m?] n. ['vitri?s 'b?di] n. ['krist?lain lenz] n. [bai'k?nveks] adj. [?prezbi'?upi?] n. ['airis] n. ['sili?ri] adj. ['sfi?kt?] n. ['?ptik] adj. ['kreini?l] [?f'θ?lmik] adj. [trai'd?eminl] adj. [f?u'vi:? 'sentr?'?lis] [??kjul?'m?ut?] adj. ['tr?kli?] adj. (=abducens [?b'dju:s?nz] n.

nerve) lacrimal conjunctival sac nasolacrimal conjunctivitis cocci bacilli trachoma granular bacteria hygiene ophthalmia neonatorum gonococci sinus tonsillitis ophthalmologist eyestrain hyperopia myopia astigmatism curvature strabismus cataract glaucoma diabetes atrophy susceptible hemorrhage atherosclerosis equilibrium pinna auricle external auditory canal cerumen gland eczema tympanic membrane

['l?kriml] adj. [?k?nd???k'tɑiv? s?k] n. [n?'s?leikriml] adj. [k?n?d???kti'vaitis] n. ['k?ksai] n. [b?'silai] n. [tr?'k?um?] n. ['gr?njul?] adj. [b?k'ti?ri?] n. ['haid?i:n] n. [?f'θ?lmi?] n. ['g?n?u'k?k?s] n. ['sain?s] n. [?t?nsi'laitis] n. [??fθ?l'm?l?d?ist] n. ['ai,stren] n. ['haip?'r?upi?] n. [mai'?upi?] n. [?'stigm?tiz?m] n. ['k?:v?t??] n. [str?'bizm?s] n. ['k?t?r?kt] n. [gl?:'k?um?] n. [?dai?'bi:ti:z] n. ['?tr?fi] n.& v. [s?'sept?bl] adj. ['hem?rid?] n. [??θ?r?uskli'r?usis] n. [?i:kwi'libri?m] n. ['pin?] n. ['?:rikl] n. n. [si'ru:m?n] n. [gl?nd] n. ['eksim?] n. [tim'p?nik 'membrein] n. 泪腺的, 泪的 结膜囊 鼻泪的 [医]结膜炎 [微]球菌 杆状菌,芽孢杆菌 沙眼 粒状的，含颗粒的 (复数)细菌 卫生, 卫生学 新生儿眼炎 淋菌 湾, 穴, 窦, 静脉窦 扁桃体炎 眼科医师 眼睛疲劳 [医]远视 近视 散光 弯曲 斜视, 斜视眼 [医]白内障 青光眼 糖尿病 萎缩，衰退 易受影响的；过敏的 出血, 溢血 动脉硬化 平衡 [解]耳廓 外耳, 耳廓, 心耳 外耳道 耳垢, 耳屎 腺 湿疹 [解]鼓膜

meatus eustachian tube perforation pus exudate myringotomy ossicle eustachian mucous mastoid air cells temporal bone tympanic membrane malleus incus stapes oval window bony labyrinth vestibule cochlea semicircular canal perilymph endolymph vestibular nerve otitis media measles lymphoid otosclerosis quinine arsenic mercury taste buds glossopharyngeal gustatory olfactory epithelium anorexia polydipsia

[mi'eit?s] n. [ju:s'tei?j?n] n. [?p?:f?'rei??n] n. [p?s] n. ['eks??deit] n. [miri?'g?t?mi] n. ['?sikl] n. [ju:s'tei?i?n] n. ['mju:k?s] ['m?st?id] adj. n. [tim'p?nik] adj. ['m?li?s] n. ['i?k?s] n. ['steipi:z] n. ['?uv?l] n.& adj. ['b?uni 'l?b?rinθ] n. ['vestibju:l] n. ['k?kli?] n. [?semi's?:kjul? k?'n?l] n. ['perilimf] n. ['end?ulimf] n. [ves'tibjul? n?:v]n. [?u'taitis] n. ['mi:zlz] n. ['limf?id] adj. [??ut?skli'r?usis] n. ['kwainain] n. ['ɑ:s?nik] n. ['m?:kjuri] n. [teist b?dz] n. [?gl?s?uf?'rind?i?l] adj. ['g?st?t?ri] adj. [?l'f?kt?ri] adj. [?epi'θi:li?m] n. [??n?'reksi?] n. [?p?li'dipsi?] n. 道，口 [解]耳咽管, 咽鼓管 孔,穿孔 脓, 脓汁 渗出物,渗出液 [医]鼓膜切开术 小骨尤指听小骨 [解]欧氏管, 咽鼓管 黏液的, 似黏液的 乳突气房;乳突小房 [解]颞骨 解]鼓膜 锤骨 解]砧骨 [解]镫骨 卵圆窗 骨迷路 气门室,前庭 耳蜗 [解](耳的)半规管 [解]外淋巴 [解]内淋巴 前庭神经 中耳炎 麻疹 淋巴的, 淋巴样的 [医]耳硬化症 奎宁 砒霜，砷 水银，汞 味蕾 [解]舌与咽的, 舌咽的 味觉的 嗅觉的 上皮, 上皮细胞 厌食症 多饮,烦渴,大渴引饮

anesthetize subcutaneous tactile corpuscle dermis viscera free nerve ending referred pain gallbladder coronary analgesic phenacetin acetaminophen narcotic stupor anesthetic tendon proprioceptor spinal tabes dorsalis [?'nesθ?taiz] v. ['s?bkju:'teini?s] adj. ['t?ktail 'k?:p?sl] n. ['d?:mis] n. ['vis?r?] n. n. n. ['g?:l?bl?d?] n. ['k?r?n?ri] adj. [??n?l'd?i:sik] n.& adj. [fi'n?sitin] n. [?si't?min?f?n] n. [na:'k?tik] n.& adj. ['stju:p?] n. [?nis'θitiks] n. ['tend?n] n. [?pr?upri?'sept?] n. ['spain?l] adj. ['teibi:z d?'s?l?s] n. 使麻醉, 使麻木 皮下的 触觉小体 真皮 内脏,脏腑 游离神经末梢 关联痛,牵涉性痛 胆囊 冠状动脉或静脉的 镇痛剂,止痛的 非那西汀(解热镇痛剂的一种) [药]醋氨酚, 对乙酰氨基酚, 退热净 麻醉药, 镇静剂, 麻醉的 昏迷, 麻木, 不省人事 麻醉剂 腱 本体感受器 脊椎骨，脊骨的 脊髓痨 Idioms and Expressions

1. drain into 流入，排入

e.g. The lacrimal apparatus consists of the lacrimal gland and a series of ducts that drain the tears into the nasal cavity. 泪器包括泪腺和一系列引流泪液进入鼻腔的管道。 2. focus on 以?为焦点；集中于

e.g. Light is normally focused on the fovea when we look directly at an object. 当我们直视物体时，光通常集中在（视网膜）中央凹。

3. be detached from 与 ... 没有联系, 脱离 ...

e.g. A cone cell is similar except that the outer segment tapers to a point and the discs are not detached from the plasma membrane but are parallel infoldings of it.视锥细胞都是相似的，除了外层部分联系在一个点上，视管细胞不与血浆薄膜相连而与细胞的内褶平行。 4. by means of 用，凭借

e.g. Vision (sight) is the perception of objects in the environment by means of the light that they emit or reflect. 视觉（视力）就是通过环境中物体发出或反射的光而对物体的感知。 5. differ from 不同于, 与 ... 有区别

e.g. Olfactory signals differ from other sensory inputs in that they reach the cerebral cortex

without passing through the thalamus. 嗅觉信号不同于其他感官输入，就在于他们到达大脑皮层不用经过丘脑。

Notes

1. This refracting of the light rays makes it possible for light from a very large area to be focused upon a very small surface, the retina, where the receptors are located.

分析：该句中it 为形式宾语，真正的宾语是一个for-to结构的不定式，where引导非限制性定语从句，先行词是retina。

译文：光线的折射（折光系统）能使来自大面积的光聚焦在一个很小的表面，即感受器所在的视网膜上。

2. Tiny tubes carry the tears to the front surface of the eyeball, where they serve to constantly wash the sac that separates the front part of the eyeball from the larger back portion.

分析：该句中由where引导的非限制性定语从句修饰eyeball，where从句中又包含一个that引导的定语从句修饰sac。

译文：细小泪管将泪水带到眼球前表面，在那里泪水不断冲洗液囊。液囊将眼球的前部分和较大的后部分分开。

3. Removal of the lens may restore some vision, but the addition of a contact lens usually is required to achieve satisfactory visual acuity, as well as binocular vision which is desirable for driving a car, for example.

分析：该句中as well as表示同级比较，which引导限制性定语从句修饰binocular vision 。 译文：摘除眼球的水晶体可以恢复一定的视力，但通常需要戴隐形眼镜来达到满意的视觉灵敏度，就像驾车所需要的双目视力一样。

4. The waves of the endolymph are transmitted to the tiny hairlike receptors, which are thereby stimulated and which conduct nerve impulses through the nerve fibers to the brain for interpretation.

分析：该句有两个并列的由which引导的非限制性定语从句，修饰整个主句。

译文：进入内淋巴的声波被传达到细小的毛发状受体，从而刺激和引起神经冲动，经神经纤维到达大脑产生听觉。

Key Points

1. Senses: sight, hearing, taste, smell, pressure, heat, cold, pain, touch, position, balance, hunger, thirst. 2. Eye

(1) Parts and purposes.

① Protection: orbits, lids, eyelashes, tears, epithelial sac. ② Coats: sclera, choroid, retina.

③ Light path: cornea, aqueous humor, lens, vitreous body. ④ Muscles: intrinsic (iris, ciliary body); 6 extrinsic.

⑤ Nerves: optic (visual impulses from rods and cones of retina); ophthalmic (pain, touch, temperature impulses from eye and surrounding parts); 3 motor nerves.

⑥ Lacrimal apparatus: lacrimal gland produces tears which moisten conjunctiva.

(2) Disorders: infections (conjunctivitis, trachoma, ophthalmia neonatorum); defects

(hyperopia, myopia, astigmatism, strabismus). Causes of blindness: cataracts (lens loses transparency); glaucoma (excess pressure of eye fluid); diabetic retinal disease; injuries. 3. Ear

(1) Parts and purposes

① Divisions: external, middle, internal,

② External: pinna, auditory canal, tympanic membrane.

③ Middle: ossicles (malleus, incus, stapes) amplify sounds from tympanic membrane, transmit them to oval window. Eustachian tube connects to pharynx, equalizes pressure, pathway for infection.

④ Internal: bony labyrinth. Oval window, vestibule, cochlea, semicircular canals, all contain perilymph. Membranous canals (in semicircular canals), membranous cochlea (in cochlea) both filled with endolymph. Receptors in cochlear duct make up the organ of hearing.

⑤ Path for sound: eardrum vibrates, vibrations amplified by ossicles, transmitted to perilymph, to endolymph, to nerve receptors, to nerves, to brain.

⑥ Equilibrium: membranous canals connected with 2 sacs, 1 sac containing sensory nerves indicating position of head.

(2) Disorders: otitis media; deafness (causes: obstruction of auditory canal, blockage of eustachian tube, otosclerosis, infections, poisons).

Medical Focus

Hearing Damage and Deafness 听力损伤和耳聋 Two major types of deafness are conduction deafness and never deafness. In conduction deafness, the ossicles tend to fuse together, restricting their ability to magnify sound waves. Conduction deafness can be caused by a congenital defect, particularly when a pregnant woman contracts German measles (rubella) during the first trimester of pregnancy. (For this reason, every female should be immunized against rubella before the childbearing years.) conduction deafness can also be due to repeated infections or otosclerosis. With otosclerosis, the normal bone of the middle ear is replaced by vascular spongy bone. 耳聋分为两种类型：传导性耳聋和神经性耳聋。在传导性耳聋中，听小骨常会融合一

起，从而限制它们放大声波的能力。传导性耳聋可能是由先天性缺陷引起的，特别是当怀孕妇女在怀孕的前三个月中感染上风疹病毒。（因为这个原因，每个女性在分娩前都可接种风疹疫苗。）传导性耳聋也可能归因于重复性感染或耳硬化症，在耳硬化症中血管的松质骨代替了中耳的正常骨头。

Never deafness most often occurs when cilia on the receptors within the cochlea have worn away. Because this may happen with normal aging, older people are more likely to have trouble hearing. However, studies also suggest that age-associated hearing loss can be prevented if ears are protected from loud noises, staring even during infancy. Hospitals are now aware of the problem and are taking steps to ensure that neonatal intensive care units and nurseries are as quiet as possible.

神经性耳聋大多数通常发生在耳蜗内接收器上的纤毛出现衰退。由于这种情况可随年龄的增长而发生，老年人更有可能有听力问题。然而研究也表明如果耳朵免受噪音伤害，（噪音甚至开始于婴儿时期，）那么是有可能预防与年龄有关的听力损伤的。医院现在也意识到这个问题并采取措施确保新生儿童重症监护治疗病房和护理尽可能地安静。

Everyone should consider three aspects of noise to prevent hearing loss: (1) how loud is the noise, (2) how long is the noise heard, and (3) how close is the noise to the ear. Loudness is measured in decibels, and any level above 80 decibels could damage the hair cells of the organ of Corti. Exposure to intense sounds of short duration, such as a burst of gunfire, can result in an immediate hearing loss. Since the butt of a rifle offers some protection, hunters may have a significant hearing reduction in the ear opposite the shoulder they use for support while firing their gun. Because even listening to city traffic for extended periods can damage hearing, frequent attendance at rock concerts and constant listening to loud music from a stereo are obviously dangerous. Noisy indoor or outdoor equipment, such as rug-cleaning machine or a chain saw, is also troublesome. Even motorcycles and recreational vehicles, such as snowmobiles and motocross bikes, can contribute to a gradual hearing loss.

人人都应该考虑噪音的三个方面来预防听力的受损：（1）噪音有多大？（2）听到噪音的时间有多长？（3）噪音离而耳朵有多远？声音响度可用分贝测量，超过80分贝的声响可以损伤柯蒂氏器的纤毛细胞。听到短暂的强烈声响，比如突然的一声枪响，可能立即导致听力丧失。因为步枪托会提供一些保护，所以猎户开枪时用于支撑枪托的肩膀另一边的耳朵听到的声音就会减小。因为即使长时间听城市交通也会损伤听力，频繁地参加摇滚音乐会和不断地听立体声响中的大声音乐很显然是危险的。室内或户外的噪音设备，比如吸尘器或链锯也会引起麻烦。甚至摩托车和娱乐性车辆，比如摩托雪橇和越野自行车也会逐渐加剧听力损伤。

The first hint of a problem could be temporary hearing loss, a “full” feeling in the ears, muffled hearing, or tinnitus (ringing in the ears). If you have any of these symptoms, modify your listening habits immediately to prevent further damage. If exposure to noise is unavoidable, use specially designed noise-reduction ear-muffs or purchase earplugs made from a compressible, sponge-like material at a drugstore or sporting goods store. These earplugs are not

the same as those worn for swimming, and they should not be used interchangeably. 听力出现问题的第一个迹象可能是暂时性的听力损伤，耳朵里有“长鸣”感觉，耳朵听到的声音低沉而模糊，或耳鸣。如果你有这些症状中的任何一种，立刻调整你的听力习惯来防止进一步的听力损伤。如果处于噪音之中不可避免，就要使用特制的能减轻噪音的耳罩或在药店或体育用品店购买由可压缩的类似海绵材质的耳塞。这些耳塞和游泳时用的不一样，而且它们不能互换使用。 Finally, people need to be aware that some medicines are ototoxic (damaging to any of the elements of hearing or balance). Anticancer drugs—most notably, cisplatin—and certain antibiotics (for example, streptomycin, kanamycin, and gentamicin) make the ears especially susceptible to a hearing loss. People taking such medications should protect their ears from any excessive noises. 最后，我们需要记住有些药物是耳毒性的（即损害听力或平衡的成分）。抗癌药—尤其是顺铂—某些抗生素（比如链霉素，卡那徽素，和庆大霉素）会使得耳朵更容易损伤听力。服用这些药物的人们应该保护自己的耳朵免受过量噪音的影响。 Cochlear implants that directly stimulate the auditory never are available for persons with never deafness. However, they are costly, and people wearing these electronic devices report that the speech they hear is like that of a robot. 直接刺激听觉神经的耳蜗移植对患神经性耳聋的人很有用。但这些耳蜗移植价格昂贵，而且据佩戴这些电子仪器的人说他们听到的讲话音就像是机器人发出的。 Exercise

1. Find the right definition in Column B for each word in Column A. Put the corresponding letter in the space provided in Column A.

Column A Column B

1) cortex a. a doctor who studies and treats the diseases of the eye

2) cornea b. an infectious eye disease that causes pain and swelling in part of the eye

3) membrane c. any of the red or white cells found in blood

4) conjunctivitis d. the practice of keeping yourself and the things around you

clean in order to prevent diseases

5) anorexia e. a very thin layer of material that covers something

6) hygiene f. the outer layer of an organ in your body, especially your brain

7) ophthalmologist g. excessive or abnormal thirst

8) polydipsia h. a mental illness that makes someone stop eating

9) corpuscle i. the large organs inside the body, such as the heart, lungs and stomach

10) viscera j. the transparent protective layer covering the outer part of the

eye

2. Translate the five sentences below into Chinese or English.

(1) Injuries caused by foreign objects or by infection may result in scar formation in the cornea and a resulting area of opacity through which light rays cannot pass.

_________________________________________________________________ _________________________________________________________________ (2) Visual tests may not show that the condition exists unless drops which paralyze the ciliary muscles are used.

_________________________________________________________________ _________________________________________________________________ (3) Sometimes the areas of opacity can be seen through a pupil which becomes greatly enlarged because of the reduction in the amount of light that can reach the retina.

_________________________________________________________________ _________________________________________________________________ (4) 如果眼球过长或过短的人可能需要矫正视力的镜片，聚焦在视网膜上成像。 （focus…on）

_________________________________________________________________ _________________________________________________________________ (5) 视网膜可能脱离脉络膜进入玻璃体。（be detached from）

_________________________________________________________________ _________________________________________________________________ 3. Questions and Answers

(1) Name the 4 kinds of taste. Where are the taste receptors? (2) What is the difference between a general and a special sense?

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