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BIOLOGY Are eyes enough to enjoy this natural splendour ...? Didn't you notice the picture drawn by Raju and its caption? What is your opinion about the caption? You too might have got the opportunity to enjoy such scenes from nature. What have your blissful experiences in such situations been? — Colourful scenes — The chirping of birds — The gentle pat of the breeze — The intoxicating fragrance of flowers — The honey-sweetness of mango 7 BIOLOGY Though these are different experiences, can't they be felt at the same time? Can’t we hear, touch, smell and taste while seeing? Let us examine the parts of the eye. Make a list of those you know. Have you ever thought how this happens? — You know that we get information about the changes in the environment through the sense organs. What are the sense organs we have? What is the function of each? Prepare a note on it. Don't you want to know how these parts are arranged in the eye? Collect the eye of an animal from the butcher shop. Identify its morphological characteristics by observing and feeling it. Prepare a note based on the indicators. ..................................................................... — Pupil — Retina ..................................................................... Eye Eyes enable us to see things. How are they protected? List the various means for the protection of eyes. — Position of the eyes – sockets in the skull — Tears — — Lysozyme, the enzyme present in tears destroys germs that enter the eyes to a certain extent. Indicators — Colour — Shape — Firmness With the help of your teacher take a cross section of the eye that has been collected by you. Making use of the features you observed and the following description write a note on the structure of the eye. Fill up the blanks in Figure 1.1. Eye – The Window opening to Nature The eye ball has three layers. The outermost layer is the sclera. It imparts firmness to the eye ball. The transparent front portion of the sclera is the cornea. The anterior part of the eye except the cornea is protected by a membrane called conjunctiva. The middle layer, choroid contains many blood capillaries. The tissues in the eye receive oxygen and nutrients from the blood that flows through these capillaries. In the anterior part of the choroid behind the cornea there is a circular dark screen named iris. Pupil is the aperture at the centre of the iris. The pupil contracts when the intensity of light increases and dilates when it decreases. In this way it regulates the amount of light entering the eye. This is facilitated by the muscles in the iris. The convex lens is placed behind the pupil. It is connected to the ciliary muscles by ligaments. 8 BIOLOGY Retina is the innermost layer of the eye where the image is formed. Cells that are stimulated by light are seen here. The part of the retina with greatest vision is called the yellow spot and that with no vision is called the blind spot. The optic nerve carrying impulses to the brain starts from the blind spot. Inside the eye there are two chambers. The chamber between the lens and the cornea is called aqueous chamber. It is filled with a watery fluid, called aqueous humour. This fluid supplies nutrients and oxygen to the cells of the cornea and the lens. The aqueous humour formed from the blood is reabsorbed into the blood itself. The large chamber seen between the lens and the retina is the vitreous chamber. It is filled with the jelly like vitreous humour which helps to maintain the shape of the eyeball. ciliary muscles sclera ? ? iris ? ? ? ? vitreous chamber yellow spot ligaments ? optic nerve Fig 1.1 Structure of the eye Couldn't you identify the parts you had seen in the cross section of the eye in the figure? Now illustrate each part of the eye, their peculiarities and functions in an interrelated manner in your Science diary. A model is given below. Cornea peculiarity Transparent anterior part function Allows light rays to enter Outermost layer 9 BIOLOGY You have understood the structure and function of the different parts of the eye. Of them the peculiarities of the lens and the retina deserve special mention. Lens and Vision You have seen the position and peculiarities of the lens of the eye. In order to understand how the lens helps to view near and distant objects, analyse Fig.1.2 A, B and the following description and prepare notes. ciliary muscles ligaments lens Fig - 1.2. A B Viewing distant objects Viewing near objects — Ciliary muscles relax — Ciliary muscles contract — Ligaments contract — Ligaments relax — Curvature of lens decreases — Curvature of lens increases — Focal length increases — Focal length decreases Formation of Image How is the image formed in the retina? Analyse Figure 1.3. Note down the peculiarities of the image. image object Fig - 1.3. Formation of image in the retina. Characteristics of the Image — Real — — 10 BIOLOGY Is vision possible merely by the formation of the image in the retina? What happens in the retina after the image is formed? To know this, the structure of the retina is to be learnt first. Read the description given below. Analyse it with the help of indicators. On Exposure to Light rod cell cone cell Fig - 1.4. Structure of the retina There are two types of receptors in the retina which are stimulated by light rays. They are rod cells and cone cells. Rod cells are stimulated by dim light and cone cells by intense light. This is because of the peculiarities in their pigments. Rod cells contain the pigment rhodopsin and cone cells contain the pigment photopsin. Retinal formed from vitamin A and opsin the protein molecule are their basic components. In the presence of light these pigments dissociate to form retinal and opsin. It is this chemical change that generates nerve impulses. Retinal and opsin can again combine together to form pigments. Retinal and opsin formed by the dissociation of rhodopsin do not recombine in intense light. Hence in intense light rhodopsin becomes inactive. Photopsin dissociates in intense light. Hence bright light is required to see the world of colours. Vision in dim light is decreased by vitamin A deficiency. This condition is called night blindness. In man, there are three types of cone cells which are stimulated to the maximum by red, green and yellow rays. Colour of the object seen by us depends upon the type of cone cells stimulated by light rays from the object. If all the three types of cone cells are stimulated at the same rate we see white colour and if none of these three is stimulated we see black colour. Many other mammals such as dog and cat have only two types of cone cells. They see the entire world as a blend of two colours. Some human beings may lack one of these types of cones due to certain hereditary defects. As a result they are unable to see certain colours. This is called colour blindness. Many animals including birds 11 BIOLOGY have four or more types of cones. The world they see is more colourful than ours. In man rod cells are few and cone cells are more in the yellow spot. Rod cells and cone cells are absent in the blind spot. Animals like cat and owl have more rod cells than man. So they have greater power of vision at night. Since the owl has no cone cells it cannot see in intense light. Indicators « What are the changes taking place in the rod cells in dim light and intense light? « How is vitamin A related to vision? « How do rod cells differ from cone cells physiologically? « What is the difference between night blindness and colour blindness? « What is the reason for the absence of vision at the blind spot? Let us try an activity to examine the power of vision in the blind spot? Spotting the Blind spot Hold the given figure one arm away in front of the eye. Shut the right eye and look at the circle with the left eye. Slowly bring the figure towards the eye. At a particular position, doesn't the square disappear? When it is brought closer to the eye it reappears. Repeat the experiment with the left eye closed and viewing the square with the right eye. What happens to the circle? This happens when the image of the object falls on the blind spot in the retina. How does the information received by the sense organs reach the brain? Look at the following facts related to this. — Brain, spinal cord, nerves and sensory receptors constitute the nervous system. — It is the nervous system that controls and co-ordinates the functions of the body. Now analyse the following figures and description based on indicators and record your inferences. — The basic unit of the nervous system is neuron. ..................................................................... 12 BIOLOGY Neuron and Nerve Neuron is the basic unit of nervous system and are seen more in the brain and the spinal cord. There are millions of neurons in the human body. They are not capable of division. A neuron has three important parts- cyton or cell body, axon and dendron. Axon and dendron are developed from the cyton. Dendrites are branches of the dendron. Branches of the axon are called axonites. The axon in certain neurons is covered with a sheath made up of the lipid, myelin. It has a glistening white colour. The functions of the myelin sheath are to provide nutrients and protection to the nerve cells and to enhance the transmission of nerve impulses by acting as an insulator. The swollen tips of the axonites are called synaptic knobs. dendrite dendron axon myelin sheath axonite nucleus synaptic knob cyton Fig - 1.5 Structure of a neuron A group of nerve fibres or axons covered by connective tissue constitutes a nerve. Nerves are of three types; sensory, motor and mixed nerves. Sensory nerves are composed of nerve fibres carrying impulses from the sense organs to the brain Fig - 1.6. Nerve and spinal cord. Nerves consisting of nerve fibres that transmit messages from the brain and spinal cord to different organs and muscles constitute motor nerves. Nerves composed of both sensory and motor nerve fibres are mixed nerves. axon A cluster of cell bodies found in certain nerves which appears like a tiny globular swelling is called nerve ganglion. Indicators — Difference between axon and dendron — Three types of nerves — Ganglion Nerves carry the messages related to the responses that are formed as a result of changes taking place in the surroundings and in the body. Read the description given below and prepare a note on how information about the surroundings is transmitted through the receptors. ..................................................................... 13 BIOLOGY Stimulus, Receptors, Impulse, Response Nerve cells or receptors that are capable of receiving stimuli from within the body and external environment are located in sense organs and in other different organs. Receptors are modified neurons. They are of different types. Rods and cones in the eye, sound receptors in the ear, taste receptors on the tongue, olfactory receptors in the nose and receptors on the skin are examples. As in all cells, opposite electric charges exist on either side of the plasma membrane of the receptor cells also. Changes in the internal and external environment of the body cause variations in the electrical equilibrium existing on either side of the plasma membrane. Such changes are known as stimuli. eg:- light, heat, cold etc. The flow of electric charges resulting from variations in equilibrium are called impulses. When such impulses reach the brain or the spinal cord through nerves corresponding changes take place in the body. These changes are termed responses. You have understood that when images are formed in the retina, the receptors located there are stimulated. As a result impulses are formed. These impulses sometimes have to pass through more than one neuron to reach the brain. How are neurons interconnected? Complete the flow chart analysing the given description and Figure 1.7. Synapse Synapse is the junction between two neurons. Impulses transmitted from one neuron to another pass through the synapse. When impulses reach the synaptic knob neurotransmitter (eg: acetyl choline) is secreted into the synaptic cleft. It transforms nerve impulses to chemical impulses and transmits them on to the adjacent neuron. dendrite synaptic knob dendron synaptic cleft axon synapse Fig - 1.7. Synapse between neurons Dendrite Cyton Axon ASpØ \yqtdmWns‚ sU≥ss{U‰v 14 BIOLOGY Is the synapse seen only at the point where two neurons meet? Analyse Figure 1.8 and form inferences. ..................................................................................................................... synaptic cleft muscle cell synaptic knob Fig - 1.8. Synapse between a nerve cell and a muscle cell Hope you have understood how the information of the formation of the image in the retina reaches the brain? Observe Figure 1.9 and write down the neural activities related to this sequentially. brain optic nerve image centre of vision optic nerve Fig - 1.9. Impulses from the eye to the brain — Photoreceptors are stimulated — ................................................................... — How is the experience of vision made possible when impulses reach the brain? To understand this, much more is to be learnt about the brain which is the centre of the nervous system. Analyse Figures 1.10 and 1.11 and descriptions based on the indicators and prepare notes. ................................................................... 15 BIOLOGY movement touch cerebrum thought grey matter white matter thalamus hypothalamus vision cerebellum pituitary gland medulla oblongata spinal cord Fig - 1.10. Structure of the brain speech taste smell hearing maintenance of equilibrium Fig - 1.11. Parts of the brain that control various activities in man Brain – The Centre of Wonders The brain is the most complex organ in the human body. It is the centre of all the peculiar characteristics which make man unique. The brain is protected inside a hard case called skull. The three layered covering of the brain is called meninges. The cerebrospinal fluid (CSF) fills between the inner layers. The CSF which is formed from the blood is reabsorbed into the blood. CSF helps to provide nutrients and oxygen to the tissues in the brain, and also ensures the protection of the brain. CSF is also contained in the cerebral ventricles, the cavities in the brain. Cerebrum is the largest part of the brain. Cytons (cell bodies) of neurons are seen crowded together in this part. The outer part of the cerebrum is grey coloured due to the absence of myelin. This is called grey matter. The folds and grooves in the grey matter are adaptations to accommodate more number of neurons. The interior of the cerebrum is called white matter. Nerve fibres covered with white coloured myelin are seen in this part. The cerebrum is the centre of qualities like intelligence, memory, thought and imagination. Senses like sight, hearing and taste are made possible by this part. Impulses from sense organs reach specific centers of the cerebrum through nerves. Here impulses are interpreted and integrated to give us precise sensations. The cerebrum controls all voluntary movements. Cerebellum, the part that lies below the cerebrum, is the second largest part of the brain. Here too, grooves and folds are present. The cerebellum maintains the equilibrium of the body by coordinating muscular activities. Medulla oblongata is a stem-like part seen close to the cerebellum below the cerebrum. Medulla oblongata controls involuntary actions like breathing, heartbeat, etc. It continues as the spinal cord which extends through the vertebral column till its posterior end. In the medulla oblongata and spinal cord, white matter is seen outside and grey matter inside. The thalamus seen at the interior of the brain is the relay station of impulses to and from the cerebrum. Impulses reaching the thalamus are analysed before being relayed. At the base of the thalamus is the hypothalamus which plays an important role in maintaining homeostasis. 16 BIOLOGY Indicators « How is the brain protected? « What are the main parts of the brain? « How do the cerebrum and medulla oblongata differ in structure and function? « What is the role of the brain in providing sensory experiences? « Which part of the brain enabled Raju to draw the scenery? Don't you doubt why two eyes are needed to see? One can see even with a single eye. Try the given activity. Remove the cap of a pen and hand it over to your friend. Ask your friend to replace the cap on the pen in your hand, with one eye closed, standing at a distance of atleast half a metre. What happens? Find the reason and write it down. ..................................................................... Man has the power to focus both eyes at the same time on objects in front of him. This is binocular vision. As a result, a three dimensional vision, giving a correct understanding of the distance from the object, thickness of the object etc. is possible. Behind Hearing You have understood how vision is experienced. Is it in the same manner that hearing also is effected? In order to understand the structure and function of the ear, analyse the following figure and description with the help of indicators. External ear Internal ear Ear drum or tympanum Membrane at the innermost end of the ear canal. Vibrate in accordance with sound waves. Middle Ear semicircular canals auditory nerve Cochlea Part with three fluidfilled sacs. Auditory receptors are situated in this part. Pinna Directs the sound waves towards the ear canal. Ear canal Directs the sound waves to the ear drum. Small hairs and ear wax seen in this area prevent dust and pathogens. incus malleus stapes Ear ossicles Direct the vibrations that occur in the ear drum towards the oval window. Eustachian tube Oval window Vibrates in accordance with the vibration of stapes. Fig - 1.12. Structure of the ear Connects middle ear with the pharynx. Regulates the pressure in the middle ear. 17 BIOLOGY Cochlea and Hearing The part of the inner ear appears like the shell of a snail is the cochlea. It has three internal chambers. The middle chamber is filled with a fluid called endolymph and the other two chambers with the fluid perilymph. Auditory receptors are seen in the Organ of Corti located in the middle chamber. Vibration of the oval window causes vibration in the perilymph. This is transmitted to the endolymph and the auditory receptors are stimulated. Impulses thus formed reach the auditory centre of the brain through the auditory nerve. Indicators — Parts of the external ear, middle ear, and internal ear and the functions of each. — The way through which the vibrations of the tympanic membrane reach the interior of cochlea. — The mode of formation of impulses. — Part to which impulses reach through the auditory nerve. Discuss how the sensation of hearing is enabled. Based on your inferences fill up the flow chart. Sound wave Pinna Cochlea Auditory centre of the brain Is hearing the only function of the ear? Look at the following picture. 18 Don’t turn round and round my child... you will feel giddy! BIOLOGY Why do you feel giddy when you turn round and round? Based on indicators analyse the given description, Figure 1.13 and the flow chart and form inferences. ..................................................................................................................... Ear and Equilibrium of the Body The semicircular canals and vestibule that are parts of the inner ear help to maintain the equilibrium of the body. These parts are filled with endolymph. Semicircular canals – They are three in number, seen adjacent to the utricle. Their swollen end is called ampulla. Receptors in the ampulla are stimulated in accordance with the movement of the head. Vestibule – It lies adjacent to the semicircular canals. It has two sacs – utricle and saccule. Receptors in these are stimulated according to the movements of the head. semicircular canals ampulla utricle saccule } vestibule cochlea Fig - 1.13. Parts of the inner ear Movement of the head and other parts of the body Creates movements in the endolymph of semicircular canals and vestibule Cerebellum Receptors are stimulated Impulse Auditory nerve Coordinates muscular activities and maintains equilibrium of the body Indicators « Where are the receptors related to equilibrium of body located? « What are the changes brought about by body movements in the parts related to equilibrium? « Why is giddiness felt when you turn round and round? 19 BIOLOGY Till now we have been discussing the structure and function of the eye and the ear, and the control of the brain over their functions. How are the other senses experienced? The Sense of Taste The taste buds located on the tongue, cheek and throat enable us to detect taste. Primary tastes such as sweetness, bitterness, sourness and saltiness can be detected by different taste buds. Analyse Figure 1.14 and understand how taste is detected and complete the flow chart. bitterness taste receptors impulses to the centre of taste in the brain sweetness taste buds sourness saltiness minute pore to taste bud Fig - 1.14. Taste buds in the tongue Food particles dissolve in the saliva Taste receptors in the taste buds are stimulated Nerve The sense of taste To detect Smell Olfactory receptors seen on the walls of the nasal cavity are stimulated by olfactory particles which enter the nose through air. Analyse the illustration given below and enlist the activities related to the formation of olfactory sensation sequentially. olfactory receptor Olfactory Nerve Impulses reach the olfactory centre of the brain. olfactory receptor mucus Fig - 1.15. Olfactory receptors 20 BIOLOGY — Olfactory particles enter the nose along with air. — Olfactory particles dissolve in the mucus and reach the olfactory receptors. — Olfactory receptors are stimulated and impulses are formed. — …………………….................................................................. — Skin – the Largest Sense Organ We recognise several stimuli through the skin. What are they? Analyse Figure 1.16 and identify the receptors in the skin. pain heat touch cold pressure Fig - 1.16. Receptors in the skin When receptors are stimulated impulses are formed. Impulses reach the brain through respective nerves and become senses. The largest number of receptors is seen in fingers and palm. You have now understood the significant role of the sense organs in recognizing changes in our surroundings and responding to them specifically. The sense organs may have disorders or diseases due to many reasons. Myopia (short sight), hypermetropia (long sight) etc. are eye disorders. You might have studied about it. Find the causes and remedies for these and complete the following table. 21 BIOLOGY Condition Cause Remedy Hypermetropia Myopia this. Since the eye lens becomes opaque, the power of vision gradually diminishes. Surgical replacement of the lens is the remedial measure. Note the indications given about other disorders of the eye and the ear. Collect more information about them and record it in the Science diary. — — Squint eye: The condition where both eyes are not able to focus on the same object. This defect can be rectified through surgery. Glaucoma: As reabsorption of aqueous humour is hindered, pressure increases in the eye. The optic nerve and photoreceptors in the retina may get damaged due to glaucoma. Early treatment may prevent the onset of blindness. — Presbyopia: This visual disorder is usually seen in middle-aged persons. This is due to the loss of elasticity of the lens. In this condition nearby objects are not clearly seen. Use of convex lens is the remedial measure. — Cataract: Aged people are affected by 22 — Deafness: This is the condition in which hearing ability is lost. It can be caused by several reasons. Many habits of modern life adversely affect the health of our sense organs. Find such habits and their ill effects and organize a seminar on the topic. Not only man, all living beings have sensory experiences. However, man can transform mere sensations into blissful experiences. While enjoying a scenery what we experience is an integrated blend of sight, hearing, touch, etc., a fresh experience beyond absolute sensation. This experience is rendered by the brain. The painting by Raju is the outcome of such an experience. It is the brain that provides us the ability to enjoy nature. BIOLOGY Follow up Activities 1) An ordinary situation is given below. Analyse it and answer the questions. — Two friends meet after a long time and have a chat. (a) What are the sensory functions performed here? (b) What are the nervous activities that enabled them to recognize each other? (c) How do the ear and the brain help in their communication? Analyse it. (d) Write an example of another situation in which sense organs and brain function in collaboration. 2) Enlist the nervous functions that take place in relation to the following situations sequentially. (a) Sweetness is experienced on chewing toffee. (b) Enjoying the fragrance of flowers. (c ) The circus performer does not fall while walking over a wire. 3) Correct the mistakes, if any, in the flow chart and redraw. Stimulus Dendrite Synapse Axonite Cyton Axon Adjacent Neuron Dendron 4) Complete the table Sense organs Receptors Stimuli Experience Eye Rod Cells Cone cells Light Vision Tastes like saltiness, sourness, sweetness, bitterness Taste Ear Nose Tongue Skin — Touch — Pain — — 23