Unit 12
The Sense Organs
Question: Explain the structure and functions of the ear. Describe the physiology of hearing.
Answer:
Ear:
The ear is the organ of hearing and equilibrium. It helps in hearing sounds and maintaining body balance. It converts sound waves into nerve impulses and sends them to the brain.
Structure of Ear
The ear is divided into outer ear, middle ear and inner ear.
Outer Ear
• The outer ear consists of auricle or pinna, external auditory canal and tympanic membrane.
• The auricle is made of elastic cartilage covered by skin.
• It collects sound waves from the environment.
• The external auditory canal carries sound waves towards the tympanic membrane.
• The tympanic membrane is also called the eardrum.
• Earwax and fine hairs present in the canal protect the ear from dust and insects.
• The main function of the outer ear is to collect and conduct sound waves.
Middle Ear
• The middle ear is an air-filled cavity present in the temporal bone.
• It contains three auditory ossicles called malleus, incus and stapes.
• Malleus is attached to the tympanic membrane.
• These ossicles transmit sound vibrations from the tympanic membrane to the inner ear.
• The Eustachian tube connects the middle ear to the throat.
• It helps maintain equal air pressure on both sides of the tympanic membrane.
• The middle ear amplifies and conducts sound vibrations.
Inner Ear
• The inner ear is a fluid-filled structure situated deep inside the temporal bone.
• It consists of vestibule, semicircular canals and cochlea.
• The vestibule and semicircular canals maintain balance and equilibrium.
• The cochlea is the organ of hearing.
• The cochlea contains the organ of Corti with sensory hair cells.
• These hair cells convert sound vibrations into nerve impulses.
• The vestibulocochlear nerve carries impulses to the brain.
Functions of Ear
• Helps in hearing sounds.
• Maintains body balance and posture.
• Coordinates head and eye movements.
• Collects sound waves from the environment.
• Converts sound vibrations into nerve impulses.
• Sends hearing impulses to the brain for interpretation.
Physiology of Hearing
• Hearing is the process by which sound waves are converted into nerve impulses.
• Sound waves enter the external auditory canal.
• The sound waves strike the tympanic membrane and cause vibrations.
• Vibrations pass through the auditory ossicles of the middle ear.
• The stapes transfers vibrations to the cochlea of the inner ear.
• Vibrations in the cochlear fluid produce waves along the basilar membrane.
• Hair cells of the organ of Corti are stimulated.
• Hair cells convert mechanical vibrations into electrical nerve impulses.
• Nerve impulses travel through the cochlear nerve to the brainstem.
• Finally, impulses reach the auditory area of the cerebral cortex.
• The brain interprets these impulses as sound.
• Thus, hearing takes place and the person becomes aware of sound.
Question: Explain the structure and functions of the eye.
Answer:
Eye:
The eye is the organ of vision. It is a spherical structure situated in the eye socket called the orbit. The eye helps us to see objects, colors, shapes and movements. It receives light from the environment and converts it into nerve impulses which are sent to the brain for interpretation.
Structure of Eye
The eye is made up of three main layers and several important parts.
Sclera
• The sclera is the tough, white outer covering of the eye.
• It protects the inner structures of the eye.
• It helps maintain the shape of the eyeball.
• It is continuous in front with the cornea.
Cornea
• The cornea is the transparent curved front part of the eye.
• It allows light rays to enter the eye.
• It refracts or bends the light rays.
• It contains no blood vessels.
Choroid
• The choroid is the middle vascular layer of the eye.
• It contains blood vessels that nourish the eye tissues.
• It contains dark pigment which prevents reflection of light inside the eye.
Iris
• The iris is the colored muscular diaphragm of the eye.
• It lies in front of the lens.
• It controls the size of the pupil.
• It regulates the amount of light entering the eye.
Pupil
• The pupil is the black circular opening in the center of the iris.
• It allows light to pass into the eye.
• Its size changes according to the intensity of light.
Lens
• The lens is a transparent, elastic and biconvex structure.
• It is situated behind the iris.
• It focuses light rays on the retina.
• It changes shape during accommodation for near and distant vision.
Retina
• The retina is the inner sensory layer of the eye.
• It contains photoreceptor cells called rods and cones.
• Rods help in dim light vision.
• Cones help in bright light and color vision.
• The retina converts light rays into nerve impulses.
Fovea
• The fovea is the area of sharpest vision in the retina.
• It contains only cones.
• It provides clear and detailed vision.
Blind Spot
• The blind spot is the point where the optic nerve leaves the eye.
• It contains no rods or cones.
• No image is formed at this point.
Vitreous Humor
• The vitreous humor is a transparent gel-like substance filling the eye cavity.
• It maintains the shape of the eyeball.
• It supports the retina internally.
Functions of Eye
• Helps in vision and seeing objects.
• Detects light, color, shape and movement.
• Focuses light rays on the retina.
• Converts light into nerve impulses.
• Sends visual impulses to the brain through the optic nerve.
• Maintains coordination and clear vision.
Functioning of Vision
• Light rays enter the eye through the cornea.
• The cornea refracts the light rays.
• Light passes through the pupil controlled by the iris.
• The lens focuses the light on the retina.
• Photoreceptor cells in the retina convert light into nerve impulses.
• The optic nerve carries impulses to the brain.
• The brain interprets these impulses as vision.
Thus, the eye is an important sensory organ responsible for vision and interpretation of visual images.
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