Sense Organs PDF

Unt 3:Human Anatomy and Physiology Sense Organs (iii) Chemoreceptors - are receptors of...

Unt 3:Human Anatomy and Physiology Sense Organs (iii) Chemoreceptors - are receptors of thetongue They nose, are sensitive to chemical In the Chapter (iv) are responsible for taste and smell. Thermoreceptors - are substances Sylabus : Sense organs -Eye: Sructure, functions, delects and corrective measures; Ear : Parts and temperat receptors present in the skin. to relative changes in They ure-are -sensin unctions of the ear. the temperatures sensi andta Soope of Syllabus : responsible for our body feeling hot or cold (v) External and Internal structure and functions of the human eye and ear and their various parts. Mechanoreceptors skin. - are They are sensitive toreceptors present in Abriel idea of stereoscopic vision, adaptation and different mechanie stimuli such as touch, pressure, accommodation of eye. vibrations, ete Defects of the eye (myopia, hyperopia/ 11.1 THE EYES el te sLyalis hypermetropia, presbyopia, astigmatism and cataract) and corrective measures (diagrams Orbits : The two eyes are located in deep sodke included for myopia and hyperopia only). or orbits on the front side of the The course of perception of sound in human ear. the form of a ball and can be head. Each eve ic Role of ear in maintaining balance of the body. six muscles. rotated with the he The sense organs enable us to be aware of the Eyelids : The upper (movable) and the lowe eyelids protect the outer and can shut out light. Each(front) conditions of our external as well as internal surface of the e environment. eyelid carries outward The major sense organs in our curved eyelashes which prevent falling of large ears, tongue, nose and skin whichbody are the eyes, particles into the eye. are sensitive to Eyebrows : Although virtually not a part of the eg light, sound, chemical and smell) and touch substances (responsible for taste these are also protective; they prevent the rain dropsr respectively. In addition, there the trickling are also the senses of balance, hunger, thirst and pain, etc. The body movements, perspiration from getting into the eys Tear glands (lacrimal glands) actual sensation is are located at the upper (Fig. 11.1) :The perceived by the sensory cells located in these sideward portion of the orbil organs such cells are Six to twelve ducts of the categorised as receptors. over the front gland pour the secretion RECEPTORS surface. Receptor is any specialised tissue or cell sensitive to a specific stimulus. (i) Photoreceptors - are rods and cones of the retina LACRIMAL of eye. These are sensitive to light and are LACRIMAL SAC GLAND responsible for vision. DUCT OF (ii) Phonoreceptors - are LACRIMAL inner ear. These are receptors present in the ACRIMAL DUCT GLAND sensitive to sound and are responsible for hearing and balance. NASOLACRIDUCT MAL Fig. 11.1 142 :Lacrimal (tear) apparatus of the human ey CONCISE BIOLOGY  moements of the eyelids (blinking) The which mainly serves as a SUSPENSORY gres the liquid LIGAMENT SCLERA labricant CONJUNCTIVA RETINA tearsalso keep the front surface of the CILIARY BODY The away dust particles. CHOROID clean by washing CORNEA ee also have an They antiseptic property due IRIS VITREQUs CHAMBER YELLOW SPOT (MACULA LUTEA) enzyme Iysozyme which kills the to the PUPIL OPTIC NERVE germs. LENS :These ducts drain off the liquid Tearducts CILIARY sac lvingatthe inner angle of the eye. A MUSCLES into a nasalacrimalduct conducts the secretion into BLIND SPOT nasal cavity. AIl of us have sometimes the eperiencedthat medicines dropped in the eyes the nose and even into the throat. SUSPENSORY comeinto happens through the above-mentioned This CONJUNCTIVA LIGAMENT LENS irritation or in certain emotional duct. Due to glands pour out a lot of liquid CORNEA RETINA states, the tear or coverflows as "tears" which"waters eyes" both the PUPIL Jou may shed tears in grief and in AQUEOUS HUMOUR OPTIC NERVE extreme joy. IRIS FUNCTIONS OF TEARS CIRCULAR CILIARY MUSCLES BLIND SPOT 1. Lubricate the surface of the eye CHOROID SCLERA 2 Wash away dust particles VITREOUS HUMOUR 3. Help in killing germs Fig. 11.2:Cross section of the eye, in colour and 4-Communicate emotions black and white. Conjunctiva : It is a thin membrane covering the itself visible through the conjunctiva. It bulges out entire front part of the eye (Fig l1.2). It is continuous and becomes transparent in the front region where with the inner lining of the eyelids. Over the cornea, it covers the coloured part of the eye; this part is itis reduced to a single layer of transparent called the cornea. epithelium. You must have often heard of a very Sometimes, the cornea of some patients turns common eye disease "conjunctivitis" in which this opaque (white) and non-functional. In such cases, outermost layer turns red due to a viral infection. the defective cornea can be replaced by a healthy The bony orbit socket, eyebrows, tear glands and cornea from a donated eye. conjunctiva serve for eye protection in their own ways. Donation of eye STRUCTURE OF THE EYEBALL (Fig. 11.2) Cornea remains alive up to nearly 40 hours after the death of a person. If donated, the eye can be removed The wall of the eyeball is composed of three soon (within about 4 hours) after death. It is stored in concentric layers : (1) outer sclerotic, (2) middle an eye bank at a very low temperature in a suitable choroid, and (3) inner retina. medium such as blood plasma or certain other sterile 0) The sclerotic layer (or sclera) is made of tough (germ-free) liquid medium. For grafting, the cornea fibrous tissues and is white in colour. The white part is taken out from the eye and is fixed in place of the defective cornea and the vision is restored. portion on the front of the eye is the sclerotic layer, 143  Acconding to a recent report, AlIMS (New Delhi) The rod cells has developed the technique of slicing the cornea into (inner to dim light but ends do not rod-like) respond orto are colourse. nstTheyive 2-3 lavers that can be gratted on as many detective contain the pigment rhodopsin The rod cells are the retina. distributed almostvisual purple (2) The choroid layer is richly supplied with blood vessels for provid1ng nourishment to the eye. It The cones (inner ends to bright light and are conical) are throughou contains a dark black pigment (melanin) which prevents light rays from reflecting and scattering vision. They contain theresponsible for colour visual violet. The cone pigment iodopsin sensitive inside the eye. In the front of the eye, the choroid cells are expands to form the ciliary body (containing circular to the yellow spot. mostly confinel muscles). The smooth muscles in the ciliary body YELLOW SPOT - The area of best vision alter the shape of the lens. The distribution of rods and cones is not unifo The iris (Latin iris: rainbow) is also an extension Aparticular spot called the macula of the choroid, partially covering the lens and leaving spot; uteum : yellow) or lutea (macula the back of the eye almost at the simply yellow spot lies a circular opening in the centre, the pupil. The blue, horizontal axis of the eyeball. This spotcentre on tha brown or black colour of the eye refers to the colour contains of the iris. [PUPIL name has been derived from maximum number of sensory cells and particulath.the the Latin word"pupa" meaning "doll, which in this the cones. As a result, this is the region of brightes vision and also of the colour vision. The rest of the context refers to the tiny image of oneself seen retina has fewer cones and more rods. reflected in another's eye). The iris contains radial muscles to widen and circular muscles to constrict Yellow spot is the place of best vision of the nornal the pupil. This adjustment of the size of the pupil eye. This is the eason why you move your eyes from regulates the amount of light entering the eye. You word to word as you read a line through a printed can easily observe this by throwing a torch light into page. the eyes while looking in a mirror. In dim or dark light, the pupil is dilated, while in bright light, it is BLIND SPOT - The area of no vision constricted (Fig. 11.5). The pattern and arrangement Lateral to the yellow spot on the nasal side s of the iris muscles is unique for every individual and the blind spot. There are no sensory cells here therefore these are also a source of an individual's and, therefore, this is the point of no vision. This identification. is the point at which the nerve fibres from all (3) The retina or the innermost layer is sensitive to the sensory cells of the retina converge and bundle together to leave the eyeball in the form of the optic light. It contains two types of sense cells called rods nerve. and cones. Table 11.1 :Comparison of rods and cones Cones Rods 1. More numerous. 1. Less numerous. 2. Mostly located in the centre of retina. 2. Mostly at the periphery of retina. Very sensitive to low levels of illumination. 3. Only stimulated by bright light. 3. selectively responsive to difterent 4. One type of rods only, stimulated by most 4. Three types of cones, each wavelengths of visible light except red. perception. wavelengths, therefore, allowing colourpigment 5. Slower regeneration of light-sensitive 5 Rapid regeneration of light-sensitive pigment, therefore, less responsive to flickering light. therefore, can perceive flickering light well. CONCISE BIOTDGY-3 144  EXPERIENCE YOUR OWN BLIND SPOT (2)Vitreous chamber is the larger cavity of the drawing below (Fig. 9. 12). Close:your right eyeball behind the lens. It is filled with a Lookatthe length and look at the andholdt the page at an arms both the circle and transparent jelly-like thícker fluid called vitreous oe with your left eye. You can see humour (vitreous : glassy: humour: fluid). squarefromthiss distance. Now bring the page slowly cinle " The vitreous humour serves two the yourself with the left eye still fixed on the circle. functions: owards drawingis about 15cm away from the eye, the (î) It helps in keeping the shape of the Whenthe syuare disappears eyeball, experiment with the left eye closed and (ii) It protects the retina and its nerve Repeatthis eye focused on thecircle the rigihtcloser square. As you bring the endings. drawing this time, the will disappear. 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 Fia. 11.3 :A chart for experiencing the blind spot enter the eyes through the structures (conjunctiva, cornea, transparent humour, lens, vitreous humour). aqueous LENS The lens is a transparent, flexible, biconvex (2) Focusing of image : First, the curvature of the crystalline body located just behind the pupil. It cornea converges the light rays to some extent contains transparent lens fibres (long thin cells which and the lens converges them further to form have lost their nuclei). The lens is collectively held an image on the retina. The image on the in position by fibres called the suspensory ligament, retina is inverted and real. which ataches it to the ciliary body. The ciliary body (3) Transmission of nerve impulses from retina lies at the junction of the choroid and the iris, and is to brain :The light energy of the iself apart of the choroid. The ciliary body contains produces chemical changes in the sensitiveimage cells muscles which on contraction and relaxation, change (rods and cones). These changes generate nerve the shape of the lens (being somewhat elastic) for impulses which travel through the viewing objects at different distances. and reach the visual area of the optic nerve cerebrum, where they give the sensation of sight. TWO CHAMBERS OF THE EYE (4) Interpretation by the brain : Our brain AQUEOUS AND VITREOUS CHAMBERS The lens, together with its suspending structures, interprets the image in many ways, e.g.. it sees the objects upright even if the image divides the inner cavity of the eyeball into two formed in the eyc is inverted. chambers: aqueous chamber in front of the lens and vitreous chamber behind the lens (Fig lL2). Accommodation (viewing objects in sharp focus). To see an object clearly, its inage should be () Aqueous chamber is the front chamber between in sharp focus in cach eye. The process of the lens and the cornea. It is filled focusing the eye to see objects at different distances is called with a clear watery liquid called aqueous humour accommodation. This is mainly brought about by a (aqueous : watery; humour: fluid). change in the curvature of the elastic lens making it The aqueous humour serves in two thinner or fatter. ways () Keeps the lens moist and For distant vision, the lens is mare flattiened from physical shock protects it or thinner. () h refracts light. For near vision (nearer than 6 metres), the lens becomes more covex or rounded These 145  CLUARY MUSCLES RELAXED SUSPENSORY LIGANMENT TIGHr LENS PULLED THIN. LENS MADE FOR NEAR THCKER 0BJECT LIGHT FROM LENS MADE FOR DISTANT THNVER DISTANT OBJECT 0BJECT UGHT CHANGING SHAPE FOCUSEO CILIARY MUSCLES CONTRACT ON PETA OF THE LENS LENS GETS THICKER ofthe lens is brought changes in the shape muscles. about by the ciliary (ciliary muscles in the normal conditionstretched by the LIGHT FROM NEARBY OBJECT remains relaxed). the lens convex, suited sUspensory lgaments and it is less TENSION IN SUSPENSORY R LIGHT 1l.4A). for viewing distant objects (Fig. LIGAMENT RELAXED FOCUSED ON RETNA When we look at nearby objects, the ciliary tend Fig. 11.4 :Accommodation of the eye: A-for distant obiect muscles (which are circular) contract and B-for near object forward. This to pull the ciliary body slightly relcases the tension on the suspensory ligament IRIS making it loose and the lens, on account of its IRIS clasticity, becomes thicker and more rounded or CIRCULAR MUSCLES convex (Fig. 11.4.B). RELAX Light and dark adaptation wi1 PUPIL RADIAL MUSCLES When you pass from a brightly lit area to a DIM LIGHT (dilated) CONTRACT dark room (such as a cinema hall), you THE PUPIL IN DIM LIGHT DILATED experience difficulty in seeing objects for ashort IRIS A IRIS while. Slowly, your vision is improved. This improvement is called dark adaptation. This CIRCULAR MUSCLES change is due to CONTRACT (a) regeneration of the visual purple or RADIAL rhodopsin, the pigment of the rods, which PUPIL MUSCLES was earlier broken down due to bright (constricted) THE PUPIL IN BRIGHT RELAX BRIGHT LIGHT light, and LIGHT CONSTRICTED (b) dilation of the pupil permitting more light to enter the eyes (Fig, 11.5). B When a person with dark adapted eyes moves to abrighty lit area, as in coming out of acinema hall after the noon show, he experiences a Fig. 11.5 : A-Size of pupil in dim lght and bright ight. B-One can ai dazzling effect for a short period. After a few Well experience the pupil reflex by covering one eye, the pupil of e other eye opens up as a result of reflex linking the two eyes 146 CONCISE BIOLOGY  he comes back to normal viewing through Table 1L.2 : Differences between light and bdaptation, The adaptation is VnNs due to reverse light changes,te. dark adaptation pev7OUs Light adaptation the visoal purple of the rods is bleached, Dark adaptation a the and rocing their sensitivity, 1. Itis the adjustment of It is the adjustment of constricts (gets narrower), to reduce eyes when people move eyes when people move hepupil from a dimly lit area to a from a brightly lit area anount of light entering the cyes. the epartialclosure offthe eyelids in dazling light brightly lit one. to a dimly lit one. The the same purpose. 2. Rods become inactive Cones become inactive also serves and disintegration of and disintegration of ColourVision rhodopsin starts. iodopsin starts. possible only through cones of 3. Cones Colour Vision is become active Rods become active and retinawhich are stimulated only in bright light. and start synthesizing start synthesizing the makeoutthe red, violettor purpleflowers iodopsin. thodopsin. Nou cannot 4. Pupil constricts and garden on a moonlit night, because then only Pupil dilates and allows ina function and not the cones. allows less amount of more amount of light therods light to enter. to enter. COMMON DEFECTS OF THE EYE 112 oye ba cha Eylids close partially to Eyelids open up more Near or short-sighte dnessi (Myopia) is a minimize the amount of to maximise the amount conditionin which the near objects can be seen light entering the eyes. of light entering the eyes. dearly while the distant objects appear blurred. In it the image of distant objects is formed in Correction of myopia: This defect can be corrected front of the retina (Fig. 11.6). Reasons for by suitable concave (diverging) lens which causes the myopia : The two possible reasons are (i) the light rays to diverge before they strike the lens of ee ball is lengthened from front to back OR the eye. Most of your classmates using spectacles m the lens is too curved (even both reasons may be suffering from myopia (power of glasses used may occur together). is mentioned in minus ""). HYPEROPIA MYOPIA (Long-sightedness) (Short-sightedness) The image is formed behind the retina. The image is formed in front of the retina. CONVEX LENS CONCAVE LENS, Aconcave lens diverges the light rays. As a result, A convex lens converges the light rays As a the image is now formed on the retina result, the image is now formed on the retina Hig. 11.§ Short-sightedness (Myopia) and its correction Fi.11.7:Long-sightedness (Hyperopia) and its coection by a CONCAVE LENS by aCONVEX LENS diA 147 Sense Organs  7. Colour blindness is a people by birth cannotcondition sightedness (Hyperepia, old term in 2Pr o long is a condition in which there is Hiperetopia) objects. In it, the image adficaky in seeing ncar the retina Reasons ne obict falls behindresults on account of certain colours such as red due to a genetic defect. this defect, whereas it diMostslcyandrimmalingraetseewhen.hcbhetwThiesoa for hyeropia : This defect rarely occurs insul er Iro eyeball from front to 8. Corneal opacities: The cornea of female elther shortening of the back or the lens is too flat Correction of hyeropia :A convex (converging) gets scarred and turns functional. Such defects opaqie some patign (Whitéy can cause of the glasses used is mentioned in plus (power lens is required to correct it (Fig. 11.7) "+"). minor irritation to vision blindness. In such cases, the problems Irto anylhin g and defective even 3. Astigmatism is a defect in which some parts of the object are seen in focus while others are be replaced by a healthy cornea Irom a cormea cat blurrod (Fig lL8). It arises due to the uneven eye. 9. Squint : In this defect, the two donatel curvature of the cornea. This is corrected by converge leading to what is calledeyes Cylindrical lenses. An opposite condition appears when "crsOosmewha eye. 4. Presbyopia is a condition affecting older people and is called the wide th ey diverg who cannot see near objects clearly. Their lens may cause douuble vision oreye." Both diplopia. conditioandn, loses flexibility resulting in a kind of far sightedness, This again is corrected by a convex suitable exercise can correctthese Surgery defects lens. Stereoscopic (binocular) vision. All monkeys/apes and Fig. 11.8 : Do you suffer from astigmatism? Shut one eye and perceive depth or the This is due to relativeparticularly distance humans ca of the object look at this picture. If all the lines appear equaly dark, you simultaneous focusing of an obiect both eyes, and their do not But if some sets of lines in the images by a kind of "overlabnin brain the three appear dark and those at right angles to them appear lighter, then you do. Try the same with one activity : - Hold a pencil 8a dimensional effect. Th the other eye horizontally inward at about arn's length. with its point faciny 5. Close one eye and try t touch the point of the pencil with Cataract is a condition in which the lens the point of anothe opaque and the vision is cut down even to turns pencil in your other hand, s blindness. It can be total the arm at the side of starting from a position wih corrected by surgically your body. You cannot removing the lens, and by using spectacles speedily but with both the eyes open, you can do so do it e with easily and quickly. mo highly convex lenses, compensating the missing lens, or in a newer for Find out which eye is used plastic lens is implanted behindtechnique, or in a small Hold a pencil at arm's lengthmore. iris. front of the its point in line with with 6. some distant Nightdificulty feels -blindnessinisseeing a condition in in which a person object with both eyes open. cdose one eye and open it First, the night. This is, dug dim light as during close the other and and then to open pigment visual purple ofnon-format the rods. ion of the one, the pencil will seem to it. With function in dim light and in the Only rods sideways and not with the jump pigment, they absence of the This shows that the other. due to the cannot function. This is usually used to line other eye was required for thedeficiency of symthesis ofvitamin the A which is is the one up the pencil and that you use more. pigment. Is it not change insurprising ? The our life time as size of our eyes does not 148) we grow. CONCISE BIOLOGY-Y  EXPERIENCE AN AFTER IMAGE 2 COLOUR BLIND ARE YOU alongside, in the numbers printed readthe read these numbers colured circles. nomal, l you can otherwise colour blind, T hee you are nt2and3, circle exNample in e for drawing on the Fix your gaze on the spot at the centre of the the spot in left for half a minute or more. Then look at thevisualisea centre of the drawing on the right. Do you complete hand ? Progress Check 1. State the functions of the following: After-images the basis of motion pictures (i) Eyelids (i) Eyelashes (iv) Iris object for a moment and (ii) Tears If one looks at a bright sensation of light persists (v) Ciliary muscles the eyes, the 2. Write in proper sequence the names which of all then closes same way, if one looks at a eye through human for a short period. In the dark the parts of the and then looks at a brightly coloured object object the light rays coming from on an object pass the retina. in the same colour before they form an image surface, an image of the persistence image or 3. Name the following : wll persist. This is known as about one-tenth of a (i) Place of best vision in the retina of the the after-image. It lasts for technique eye second. This is the principle on which the (iü) Place of no vision in the retina of the based. of mation pictures is at eye (iiü) Kind of retinal cells sensitive to dim light In a movie, pictures are projected on a screen but we (iv) The circular opening enclosed by iris the rate of about 24 pictures per second, (v) The fibres which collectively hold the account of the cannot see the individual frames on lens in position continuous (vi) Capacity of the eye to focus at different after-images in our eyes. The life-like Television movement on the screen is an illusion. distances (vii) The kind of lens required to correct near too is similar, where the scanning beam of a sightedness picture frame of the TV camera moves so rapidly on (vii) The layer of the wall of the eye-ball that the viewing screen of the TV set that our eyes cannot corresponds to the black lining of the box keep pace with it. Out of numerous other optical of a camera illusions, two are shown at the end of this chapter. 4. Give the reason for the following : () Medicines dropped in the eye flow down Can you now answer ? into the nose You saw a dream - Did the eyes see it ? (ii) A person from bright sunlight outside enters a poorly lit room and feels blinded Is it the eyes that see or the brain through the for a short while. eyes ? 149 Sense Organs  HEARING SEMICIRCULAR CANALS EARORGAN FOR 113 THE N AND BALANCE is concerned with (wo balancemy AMPULLA A (Fig. I1.9) The human ear body balance. It has three functions hearing and ear, (i) middle ear and main divisions : () outer AUDITORY IERE () inner car. outer ear consists of the projecting part EAR DRUM () The pinna (also called "auricle") and the passage ear drum (or EAR OSSICLES UTRICULUS auditory canal leading to the MALLEUS, INCUS, STAPES SACCULUS tympanum). three tiny bones (1) The middle ear contains hammer anvil letk d COCHLEA malleus, incus and stapes or eustachian and stirrup n popi terms andyofathe'middle S B 1(VESTIBULAR CANAL) tube Whichtorinects the cavity are oheuld ear with the throat. The three bones 2 (MEDIAN CANAL) bone, collectively called the ear ossicles (osseus: hammer ossicle :little bone). The handle of the ear -3 (TYMPANIC CANAL) surface of the bone is attached to the inner the drum. Its opposite end is connected to VESTIBULAR CANAL anvil which, in turn, is joined to the stirrup. so-called MEDIAN CANAL The flat part of the stirrup fits on the oval window, a membrane-covered opening TECTORIAL MEMBRANE leading to the inner ear. A second opening, the round window, also covered by a thin ORGAN OF CORTI (NCLUDES SENSORY 3 membrane, connects the middle and the inner HAIR CELLS) AUDITORY ear. TYMPANIC NERVE FBRES BASILAR (iii) The inner ear or membranous labyrinth has CANAL MEMBRANE three parts the cochlea, semicircular canals Fig. 11.10 A-The course of perception of sound in human ex and the vestibule (Fig. 11.10A), The cochlea 8. Cochlea, and its inner cavities, is spiral-shaped and looks like a snail shell. It CInner cavity of cochlea showing three parallelcanals OUTER EAR MIDDLE EAR INNER EAR PifbN EAR CANAL HAMMER SEMICIRCULAR VESTIBULAR PORTION OF CANALS AUDITORY NERVE ANVIL -COCHLEAR PORTION Avl OF AUDITORY NERVE Oval windoe COCHLEA Harnmer SOUND WAVES Ear drum Round window EAR DRUM sfRAUP A EUSTACHAN TUBE B Fig. 11.9:A-The principal parts of human ear. B-Close-up view of middle ea 150 CONCISE BOIOG-3  twe and a half turns, Its inner winding The vibrating ear drum also sets the three has divided into three parallel canals cevityis ossicles into vibration. 1by membranes (Fig. 11.l0C, 1, 2, 3). separated canal (2) is illed with The vibration of the last ossicle (stirrup) is median (cochlear) amplified due to lever-like action of the first two The called endolymph and the other uid with perilymph. The middle canal two ossicles. an 3) The vibrating stirrup transmits the vibration to Contains areas possessing sensory cells, spiral the called organ of Corti for hearing. The membrane of the oval window which in ogan arising from these cells join the turn sets the fluid contained in the cochlear nerve ibres canals also into vibration (see arrow marks in auditery nerve. The sensory cells lie on the Fig. 11.10A). hasilar membrane, PARTS OF EAR The vibrating movements of the fluid stimulate Inner ear the hair-like processes of the sensory cells of the Middie ear Dutercar Ear ossicles cochlea (in spiral organ) and the impulses are Pinna Auditory -Malleus (hammer) Semi-circular transmitted to the brain via the auditory nerve. canal Incus (anvil) canals The different areas of the cochlear canal are suited Bar drum Stapes (stirrup) Utriculus to sounds of different pitches. Most of the sounds Oval window Sacculus we hear are combinations of vibrations at Round wvindow - Cochlea many Inner opening of different rates of speed, i.e., of different pitches. We Eustachian tube cannot pick up vibrations of all frequencies. Our sensory endings can receive only those from The other part of the inner ear is a set of three 20 to 20,000 Hertz, but the most keenly heard sounds semi-circular canals which are arranged at right are those at frequencies between 1000 and 4000 Hz. anglesto each other in three different planes so that The dogs can perceive sounds of even higher one is horizontal and the other two are vertical, One frequencies. end of each canal is widened to formn an ampulla B. BALANCING which contains sensory cells for dynamic balance while the body is in motion and nerve fibres from As the head is turned in different directions, the them join the auditory nerve. fluid inside the semicircular canals is also shaken. The short stem joining the bases of semicircular The moving fluid in the canals pushes against sensory canals to the cochlea shows two parts a utriculus hair cells sending the nerve impulse through the and a sacculus, collectively termed as vestibule. nerve fibres attached to them, to the brain via the These parts also contain sensory cells for static auditory nerve. The sensory cells in the semicircular balance when the body is stationary as in standing. canals are concerned with dynamic equilibrium i.e., while the body is in motion. Similar sensory patches FUNCTIONS OF THE EAR: The internal ear are also located in the utriculus and sacculus which is involved in two sensory functions : hearing and body balance. register the static (positional) balance with respect to gravity. A. HEARING If you spin round and round, the fluid in the The pinna collects the sound waves and conducts semicircular canals continues to spin for a short time them through the external auditory canal. They even after you stop, and you feel dizzy and at the tinally strike on the ear drum which is set into same timne, your eyes perform to-and-fro movements vibration. caused due to stinulation of semi-circular canals. - The eustachian tube equalises the air Sea-sickness, air-sickness, and car-sickness are often on either side of the ear drum pressure allowing it to due to these unusual sensations of equilibriun. vibrate freely. Sonse Organs 151