Human Anatomy and Physiology: Sense Organs PDF

# Unit 3: Human Anatomy and Physiology ## 11 Sense Organs ### In the Chapter - **Syllabus**: Sense organs - Eye: Structure, functions, defects and corrective measures; Ear: Parts and functions of the ear. - **Scope of Syllabus**: - External and internal structure and functions of the human...

# Unit 3: Human Anatomy and Physiology ## 11 Sense Organs ### In the Chapter - **Syllabus**: Sense organs - Eye: Structure, functions, defects and corrective measures; Ear: Parts and functions of the ear. - **Scope of Syllabus**: - External and internal structure and functions of the human eye and ear and their various parts. - A brief idea of stereoscopic vision, adaptation and accommodation of eye. - Defects of the eye (myopia, hyperopia/hypermetropia, presbyopia, astigmatism and cataract) and corrective measures (diagrams included for myopia and hyperopia only). - The course of perception of sound in the human ear. - Role of ear in maintaining balance of the body The sense organs enable us to be aware of the conditions of our external as well as internal environment. The major sense organs in our body are the eyes, ears, tongue, nose and skin which are sensitive to light, sound, chemical substances (responsible for taste and smell) and touch respectively. In addition, there are also the senses of balance, body movements, hunger, thirst and pain, etc. The actual sensation is perceived by the sensory cells located in these organs - such cells are categorized as **receptors**. ### Receptors **Receptor** is any specialized tissue or cell sensitive to a specific stimulus. - **Photoreceptors** - are rods and cones of the retina of eye. These are sensitive to light and are responsible for vision. - **Phonoreceptors** - are receptors present in the inner ear. These are sensitive to sound and are responsible for hearing and balance. - **Chemoreceptors** - are receptors of the tongue and nose. They are sensitive to chemical substances and are responsible for taste and smell. - **Thermoreceptors** - are temperature-sensing receptors present in the skin. They are sensitive to relative changes in the temperatures and are responsible for our body feeling hot or cold. - **Mechanoreceptors** - are receptors present in the skin. They are sensitive to different mechanical stimuli such as touch, pressure, vibrations, etc. ### 11.1 The Eyes **Orbits**: The two eyes are located in deep sockets or orbits on the front side of the head) Each eye is in the form of a ball and can be rotated with the help of six muscles. **Eyelids**: The upper (movable) and the lower eyelids protect the outer (front) surface of the eyes and can shut out light. Each eyelid carries outwardly curved eyelashes which prevent falling of larger particles into the eye. **Eyebrows**: Although virtually not a part of the eye, these are also protective; they prevent the rain drops or the trickling perspiration from getting into the eyes. **Tear glands (lacrimal glands)** (Fig 11.1): They are located at the upper sideward portion of the orbit. Six to twelve ducts of the gland pour the secretion over the front surface. - The movements of the eyelids (blinking) spread the liquid which mainly serves as a lubricant. - The tears also keep the front surface of the eye clean by washing away dust particles. - They also have an antiseptic property due to the enzyme lysozyme which kills the germs. **Tear ducts**: These ducts drain off the liquid into a sac lying at the inner angle of the eye. A nasolacrimal duct conducts the secretion into the nasal cavity. All of us have sometimes experienced that medicines dropped in the eyes come into the nose and even into the throat. This happens through the above-mentioned duct. Due to irritation or in certain emotional states, the tear glands pour out a lot of liquid which "waters the eyes" or overflows as "tears". You may shed tears both in grief and in extreme joy. ### Functions of Tears 1. Lubricate the surface of the eye 2. Wash away dust particles 3. Help in killing germs 4. Communicate emotions ### Conjunctiva It is a thin membrane covering the entire front part of the eye (Fig. 11.2). It is continuous with the inner lining of the eyelids. Over the cornea, it is reduced to a single layer of transparent epithelium. You must have often heard of a very common eye disease "conjunctivitis" in which this outermost layer turns red due to a viral infection. The bony orbit socket, eyebrows, tear glands and conjunctiva serve for eye protection in their own ways. ### Structure of the Eyeball The wall of the eyeball is composed of three concentric layers: (1) outer sclerotic, (2) middle choroid, and (3) inner retina. 1. **The sclerotic layer (or sclera)** is made of tough fibrous tissues and is white in colour. The white portion on the front of the eye is the sclerotic layer, itself visible through the conjunctiva. It bulges out and becomes transparent in the front region where it covers the coloured part of the eye, this part is called the cornea. Sometimes, the cornea of some patients turns opaque (white) and non-functional. In such cases, the defective cornea can be replaced by a healthy cornea from a donated eye. **Donation of eye** Cornea remains alive up to nearly 40 hours after the death of a person. If donated, the eye can be removed soon (within about 4 hours) after death. It is stored in an eye bank at a very low temperature in a suitable medium such as blood plasma or certain other sterile (germ-free) liquid medium. For grafting, the cornea part is taken out from the eye and is fixed in place of the defective cornea and the vision is restored. 2. **The choroid layer** is richly supplied with blood vessels for providing nourishment to the eye. It contains a dark black pigment (melanin) which prevents light rays from reflecting and scattering inside the eye. In the front of the eye, the choroid expands to form the ciliary body (containing circular muscles). The smooth muscles in the ciliary body alter the shape of the lens. The iris (Latin iris: rainbow) is also an extension of the choroid, partially covering the lens and leaving a circular opening in the centre, the pupil. The blue, brown or black colour of the eye refers to the colour of the iris. ["PUPIL" name has been derived from the Latin word "pupa" meaning "doll", which in this context refers to the tiny image of oneself seen reflected in another's eye]. The iris contains radial muscles to widen and circular muscles to constrict the pupil. This adjustment of the size of the pupil regulates the amount of light entering the eye. You can easily observe this by throwing a torch light into the eyes while looking in a mirror. In dim or dark light, the pupil is dilated, while in bright light, it is constricted (Fig. 11.5). The pattern and arrangement of the iris muscles is unique for every individual and therefore these are also a source of an individual's identification. 3. **The retina** or the innermost layer is sensitive to light. It contains two types of sense cells called rods and cones. **Table 11.1: Comparison of rods and cones** | Rods | Cones | |---|---| | More numerous | Less numerous | | Mostly at the periphery of retina | Mostly located in the centre of retina | | Very sensitive to low levels of illumination | Only stimulated by bright light | | One type of rods only, stimulated by most wavelengths of visible light except red. | Three types of cones, each selectively responsive to different wavelengths, therefore, allowing colour perception. | | Rapid regeneration of light-sensitive pigment, therefore, can perceive flickering light well. | Slower regeneration of light-sensitive pigment, therefore, less responsive to flickering light. | - The rod cells (inner ends rod-like) are sensitive to dim light but do not respond to colour. They contain the pigment rhodopsin or visual purple. The rod cells are distributed almost throughout the retina. - The cones (inner ends conical) are sensitive to bright light and are responsible for colour vision. They contain the pigment iodopsin or visual violet. The cone cells are mostly confined to the yellow spot. ### Yellow Spot - The area of best vision. The distribution of rods and cones is not uniform. A particular spot called the macula lutea (macula : spot; luteum: yellow) or simply yellow spot lies at the back of the eye almost at the centre on the horizontal axis of the eyeball. This spot contains the maximum number of sensory cells and particularly the cones. As a result, this is the region of brightest vision and also of the colour vision. The rest of the retina has fewer cones and more rods. Yellow spot is the place of best vision of the normal eye. This is the reason why you move your eyes from word to word as you read a line through a printed page. ### Blind Spot - The area of no vision Lateral to the yellow spot on the nasal side is the blind spot. There are no sensory cells here and, therefore, this is the point of no vision. This is the point at which the nerve fibres from all the sensory cells of the retina converge and bundle together to leave the eyeball in the form of the optic nerve) ### Experience your own blind spot Look at the drawing below (Fig. 9. 12). Close your right eye and hold the page at an arm's length and look at the circle with your left eye. You can see both the circle and the square from this distance. Now bring the page slowly towards yourself with the left eye still fixed on the circle. When the drawing is about 15cm away from the eye, the square disappears. Repeat this experiment with the left eye closed and the right eye focused on the square. As you bring the drawing closer this time, the circle will disappear. ### Lens The lens is a transparent, flexible, biconvex crystalline body located just behind the pupil. It contains transparent lens fibres (long thin cells which have lost their nuclei). The lens is collectively held in position by fibres called the suspensory ligament, which attaches it to the ciliary body. The ciliary body lies at the junction of the choroid and the iris, and is itself a part of the choroid. The ciliary body contains muscles which on contraction and relaxation, change the shape of the lens (being somewhat elastic) for viewing objects at different distances. ### Two Chambers of the Eye - Aqueous and Vitreous Chambers The lens, together with its suspending structures, divides the inner cavity of the eyeball into two chambers: aqueous chamber in front of the lens and vitreous chamber behind the lens (Fig. 11.2). 1. **Aqueous chamber** is the front chamber between the lens and the cornea. It is filled with a clear watery liquid called aqueous humour (aqueous: watery; humour: fluid). The aqueous humour serves in two ways: - Keeps the lens moist and protects it from physical shock, - It refracts light. 2. **Vitreous chamber** is the larger cavity of the eyeball behind the lens. It is filled with a transparent jelly-like thicker fluid called vitreous humour (vitreous: glassy; humour: fluid). The vitreous humour serves two functions: - It helps in keeping the shape of the eyeball, - It protects the retina and its nerve endings. ### How Do We See? The four major steps in seeing an object are as follows: 1. **Entry of light rays**: Light rays from the object enter the eyes through the transparent structures (conjunctiva, cornea, aqueous humour, lens, vitreous humour). 2. **Focusing of image**: First, the curvature of the cornea converges the light rays to some extent and the lens converges them further to form an image on the retina. The image on the retina is inverted and real. 3. **Transmission of nerve impulses from retina to brain**: The light energy of the image produces chemical changes in the sensitive cells (rods and cones). These changes generate nerve impulses which travel through the optic nerve and reach the visual area of the cerebrum, where they give the sensation of sight. 4. **Interpretation by the brain**: Our brain interprets the image in many ways, e.g., it "sees" the objects upright even if the image formed in the eye is inverted. ### Accommodation (viewing objects in sharp focus) To see an object clearly, its image should be in sharp focus in each eye). The process of focusing the eye to see objects at different distances is called accommodation. This is mainly brought about by a change in the curvature of the elastic lens making it thinner or fatter. - For distant vision, the lens is more flattened or thinner. - For near vision (nearer than 6 metres), the lens becomes more convex or rounded. These changes in the shape of the lens is brought about by the ciliary muscles. In the normal condition (ciliary muscles relaxed), the lens remains stretched by the suspensory ligaments and it is less convex, suited for viewing distant objects (Fig. 11.4A). When we look at nearby objects, the ciliary muscles (which are circular) contract and tend to pull the ciliary body slightly forward). This releases the tension on the suspensory ligament making it loose and the lens, on account of its elasticity, becomes thicker and more rounded or convex (Fig. 11.4.B). ### Light and dark adaptation When you pass from a brightly lit area to a dark room (such as a cinema hall), you experience difficulty in seeing objects for a short while. Slowly, your vision is improved. This improvement is called dark adaptation. This change is due to - regeneration of the visual purple or rhodopsin, the pigment of the rods, which was earlier broken down due to bright light, and - dilation of the pupil permitting more light to enter the eyes (Fig. 11.5). When a person with dark adapted eyes moves to a brightly lit area, as in coming out of a cinema hall after the noon show, he experiences a dazzling effect for a short period. After a few seconds, he comes back to normal viewing through light adaptation. The adaptation is due to reverse of the previous changes, ie., - the visual purple of the rods is bleached, reducing their sensitivity, and - the pupil constricts (gets narrower), to reduce the amount of light entering the eyes. The partial closure of the eyelids in dazzling light also serves the same purpose. ### Colour Vision Colour vision is possible only through cones of the retina which are stimulated only in bright light. You cannot make out the red, violet or purple flowers in a garden on a moonlit night, because then only the rods function and not the cones. ### 11.2 Common Defects of the Eye 1. **Near or short-sightedness (Myopia)** is a condition in which the near objects can be seen clearly while the distant objects appear blurred. In it, the image of distant objects is formed in front of the retina (Fig. 11.6). Reasons for myopia: The two possible reasons are - the eye ball is lengthened from front to back OR - the lens is too curved (even both reasons may occur together). **Table 11.2: Differences between light and dark adaptation** | Light adaptation | Dark adaptation | |---|---| | It is the adjustment of eyes when people move from a dimly lit area to a brightly lit one. | It is the adjustment of eyes when people move from a brightly lit area to a dimly lit one. | | Cones become inactive and disintegration of iodopsin starts. | Rods become inactive and disintegration of rhodopsin starts. | | Cones become active and start synthesizing iodopsin. | Rods become active and start synthesizing rhodopsin. | | Pupil constricts and allows less amount of light to enter. | Pupil dilates and allows more amount of light to enter. | | Eyelids close partially to minimize the amount of light entering the eyes. | Eyelids open up more to maximise the amount of light entering the eyes. | **Correction of myopia**: This defect can be corrected by suitable concave (diverging) lens which causes the light rays to diverge before they strike the lens of the eye. Most of your classmates using spectacles may be suffering from myopia (power of glasses used is mentioned in minus "-"). 2. **Far or long-sightedness (Hyperopia, old term- Hypermetropia)** is a condition in which there is a difficulty in seeing near objects. In it, the image of near object falls behind the retina. Reasons for hyeropia: This defect results on account of either shortening of the eyeball from front to back or the lens is too flat. **Correction of hyeropia**: A convex (converging) lens is required to correct it (Fig. 11.7) (power of the glasses used is mentioned in plus "+"). 3. **Astigmatism** is a defect in which some parts of the object are seen in focus while others are blurred (Fig. 11.8). (It arises due to the uneven curvature of the corneal This is corrected by cylindrical lenses 4. **Presbyopia** is a condition affecting older people who cannot see near objects clearly. Their lens loses flexibility resulting in a kind of far-sightedness. This again is corrected by a convex lens. 5. **Cataract** is a condition in which the lens turns opaque and the vision is cut down even to total blindness. It can be corrected by surgically removing the lens, and by using spectacles with highly convex lenses, compensating for the missing lens, or in a newer technique, a small plastic lens is implanted behind or in front of the iris. 6. **Night-blindness** is a condition in which a person feels difficulty in seeing in dim light as during the night. This is due to non-formation of the pigment visual purple of the rods. Only rods function in dim light and in the absence of the pigment, they cannot function. This is usually due to the deficiency of vitamin A which is required for the synthesis of the pigment. 7. **Colour blindness** is a condition in which some people by birth cannot discriminate between certain colours such as red and green. This is due to a genetic defect. Mostly males suffer from this defect, whereas it rarely occurs in females. 8. **Corneal opacities**: The cornea of some patients gets scarred and turns opaque (white) and non-functional. Such defects can cause anything from minor irritation to vision problems and even blindness. In such cases, the defective cornea can be replaced by a healthy cornea from a donated eye. 9. **Squint**: In this defect, the two eyes somewhat converge leading to what is called "cross eye". An opposite condition appears when they diverge and is called the "wide eye". Both conditions may cause double vision or diplopia. Surgery and suitable exercise can correct these defects. ### Stereoscopic (binocular) vision. All monkeys/apes and particularly humans can perceive depth or the relative distance of the objects. This is due to simultaneous focusing of an object in both eyes, and their images by a kind of "overlapping" in the brain gives the three dimensional effect. Try one activity: - Hold a pencil horizontally with its point facing inward at about arm's length. Close one eye and try to touch the point of the pencil with the point of another pencil in your other hand, starting from a position with the arm at the side of your body. You cannot do it speedily but with both the eyes open, you can do so more easily and quickly. Find out which eye is used more. Hold a pencil at arm's length with its point in line with some distant object with both eyes open. First, close one eye and open it and then close the other and open it. With one, the pencil will seem to jump sideways and not with the other. This shows that the other eye was used to line up the pencil and that is the one you use more. Is it not surprising? The size of our eyes does not change in our life time as we grow. ### Are you colour blind? Try to read the numbers printed alongside, in the three coloured circles. If you can read these numbers in 1, 2 and 3, you are normal, otherwise colour blind, as for example in circle 4. ### Experience an after-image Fix your gaze on the spot at the centre of the drawing on the left for half a minute or more. Then look at the spot in the centre of the drawing on the right. Do you visualise a complete hand? ### After-images — the basis of motion pictures If one looks at a bright object for a moment and then closes the eyes, the sensation of light persists for a short period. In the same way, if one looks at a brightly coloured object and then looks at a dark surface, an image of the object in the same colour will persist. This is known as persistence image or the after-image. It lasts for about one-tenth of a second). This is the principle on which the technique of motion pictures is based. In a movie, pictures are projected on a screen at the rate of about 24 pictures per second, but we cannot see the individual frames on account of the after-images in our eyes. The life-like continuous movement on the screen is an illusion. Television too is similar, where the scanning beam of a picture frame of the TV camera moves so rapidly on the viewing screen of the TV set that our eyes cannot keep pace with it. Out of numerous other optical illusions, two are shown at the end of this chapter. ### Can you now answer? You saw a dream — Did the eyes see it? Is it the eyes that see or the brain through the eyes? ### Progress Check 1. State the functions of the following: - Eyelids - Eyelashes - Tears - Iris - Ciliary muscles 2. Write in proper sequence the names of all the parts of the human eye through which the light rays coming from an object pass before they form an image on the retina. 3. Name the following: - Place of best vision in the retina of the eye - Place of no vision in the retina of the eye - Kind of retinal cells sensitive to dim light - The circular opening enclosed by iris. - The fibres which collectively hold the lens in position - Capacity of the eye to focus at different distances - The kind of lens required to correct near sightedness - The layer of the wall of the eye-ball that corresponds to the black lining of the box of a camera 4. Give the reason for the following: - Medicines dropped in the eye flow down into the nose - A person from bright sunlight outside enters a poorly lit room and feels blinded for a short while. ### 11.3 The Ear - Organ For Hearing and Balance The human ear (Fig 11.9) is concerned with two functions, hearing and body balance. It has three main divisions: (i) outer ear, (ii) middle ear and (iii) inner ear. 1. **The outer ear** consists of the projecting part pinna (also called "auricle") and the passage auditory canal leading to the ear drum (or tympanum). 2. **The middle ear** contains three tiny bones malleus, incus and stapes or hammer, anvil and stirrup in popular terms and a eustachian tube which connects the cavity of the middle ear with the throat. The three bones are collectively called the ear ossicles (osseus bone, ossicle: little bone)). The handle of the hammer bone is attached to the inner surface of the ear drum. Its opposite end is connected to the anvil which, in turn, is joined to the stirrup. The flat part of the stirrup fits on the so-called oval window, a membrane-covered opening leading to the inner ear. A second opening, the round window, also covered by a thin membrane, connects the middle and the inner ear. 3. **The inner ear or membranous labyrinth** has three parts - the cochlea, semicircular canals and the vestibule (Fig. 11.10A). The cochlea is spiral-shaped and looks like a snail shell. It has two and a half turns. Its inner winding cavity is divided into three parallel canals separated by membranes (Fig. 11.10C, 1, 2, 3). The median (cochlear) canal (2) is filled with a fluid called endolymph and the other two (1 and 3) with perilymph. The middle canal contains areas possessing sensory cells, spiral organ called organ of Corti for hearing. The nerve fibres arising from these cells join the auditory nerve. The sensory cells lie on the basilar membrane. **Parts of Ear** | Outer ear | Middle ear | Inner ear | |---|---|---| | Pinna | Ear ossicles | Semi-circular canals | | Auditory canal | - Malleus (hammer) | Utriculus | | Ear drum | - Incus (anvil) | Sacculus | | | - Stapes (stirrup) | Cochlea | | | Oval window | | | | Round window | | | | Inner opening of Eustachian tube | | The other part of the inner ear is a set of three semi-circular canals which are arranged at right angles to each other in three different planes so that one is horizontal and the other two are vertical. One end of each canal is widened to form an ampulla which contains sensory cells for dynamic balance while the body is in motion and nerve fibres from them join the auditory nerve. The short stem joining the bases of semicircular canals to the cochlea shows two parts - a utriculus and a sacculus, collectively termed as vestibule. These parts also contain sensory cells for static balance when the body is stationary as in standing. ### Functions of the Ear The internal ear is involved in two sensory functions: hearing and body balance. #### A. Hearing - The pinna collects the sound waves and conducts them through the external auditory canal. They finally strike on the ear drum which is set into vibration. - The eustachian tube equalises the air pressure on either side of the ear drum allowing it to vibrate freely. - The vibrating ear drum also sets the three ossicles into vibration. - The vibration of the last ossicle (stirrup) is amplified due to lever-like action of the first two ossicles. - The vibrating stirrup transmits the vibration to the membrane of the oval window which in turn sets the fluid contained in the cochlear canals also into vibration (see arrow marks in Fig. 11.10A). - The vibrating movements of the fluid stimulate the hair-like processes of the sensory cells of the cochlea (in spiral organ) and the impulses are transmitted to the brain via the auditory nerve. - The different areas of the cochlear canal are suited to sounds of different pitches. Most of the sounds we hear are combinations of vibrations at many different rates of speed, te., of different pitches. We cannot pick up vibrations of all frequencies. Our sensory endings can receive only those from 20 to 20,000 Hertz, but the most keenly heard sounds are those at frequencies between 1000 and 4000 Hz. The dogs can perceive sounds of even higher frequencies. #### B. Balancing As the head is turned in different directions, the fluid inside the semicircular canals is also shaken. The moving fluid in the canals pushes against sensory hair cells sending the nerve impulse through the nerve fibres attached to them, to the brain via the auditory nerve. The sensory cells in the semicircular canals are concerned with dynamic equilibrium te.. while the body is in motion. Similar sensory patches are also located in the utriculus and sacculus which register the static (positional) balance with respect to gravity. If you spin round and round, the fluid in the semicircular canals continues to spin for a short time even after you stop, and you feel dizzy and at the same time, your eyes perform to-and-fro movements caused due to stimulation of semi-circular canals. Sea-sickness, air-sickness, and car sickness are often due to these unusual sensations of equilibrium.