The Senses Chapter 19 Section 3

Key Concepts How do your eyes enable you to see? How do you hear and maintain your sense of balance? How do your senses of smell and taste work together? How is your skin related to your sense of touch?

Key Terms Cornea Pupil Iris Lens Retina Nearsightedness Farsightedness Eardrum Cochlea Semicircular canal

Preview You waited in line to get on the ride and now its about time to begin. You grip the wheel as the bumper cars jerk into motion. The next thing you know, you are zipping around crazily and bump into cars driven by your friends. You can thrill to the motion of amusement park rides because of your senses.

Vision Your eyes respond to stimulus of light They convert that stimulus into impulses that your brain interprets, enabling you to see

How light enters our eye Light strikes the cornea-clear tissue that covers the front of the eye Then light passes through a fluid filled chamber behind the cornea and reaches the pupil Pupil is the opening through which light enters the eye The pupil changes size according to the light in the room/outside

(2) The pupil size is adjusted by muscles in the iris The iris is a circular structure that surrounds the pupil and regulates the amount of light entering the eye Iris also gives the eye its color

How light is focused Light that passes through the pupil strikes the lens Lens is a flexible structure that focuses light The lens of the eye bends the light rays, the image it produces is upside down and reversed Muscles that attach to the lens adjust its shape, producing an image that is in focus

How you see an image After passing through the lens the focused light rays pass through a transparent jellylike fluid Light rays strike the retina, the layer of receptor cells that lines the back of the eye The retina contain about 130 million receptor cells that respond to light

(2) Two types of receptors: Rods and cones Rod cells work best in dim light and enable your to see black, white, and shades of gray Cone cells work best in bright light and enable you to see colors

(3) When light strikes the rods and cones, nerve impulses travel to the cerebrum through the optic nerves One optic nerve comes from the left eye and the other one comes from the right eye In the cerebrum two things happen The brain turns the reversed image right side up It also combines the images from each eye to produce a single image

Correcting nearsightedness and farsightedness People with nearsightedness can see nearby objects clearly, but have trouble seeing objects far away. To correct nearsightedness eyeglasses with concave lenses are worn Concave lenses are thicker at the edges than it is in the center When light rays pass through a concave lens they are bent away from the center of the lens

(2) Concave lenses in glasses make light rays spread out before they reach the lens of the eye After the rays pass though the lens of the eye they focus on the retina rather than in front of it People with farsightedness can see distant objects clearly

(3) Nearby objects look blurry Eyeballs of people with farsightedness are too short The lens of the eye bends light from nearby objects so that the image does not focus properly on the retina If light could pass through the retina the image would come into sharp focus at a point behind the retina

Convex lenses are used to help correct farsightedness A convex lens is thicker in the middle than at the edges Convex lens makes the light rays bend toward each other before they reach the eye The lens of the eye bends the rays even more This bending makes the image focus exactly on the retina

Hearing and balance Your ears re the sense organs that respond to the stimulus of sound The ears convert the sound to nerve impulses that your brain interprets Sound is produced by vibrations Vibrations move outward from the source of the sound-ex. Water ripples

Outer ear Outer ear receives sound vibrations Outer ear is shaped like a funnel This funnel shape enables the outer ear to gather sound waves Sound vibrations then travel down the ear canal

Middle ear At the end of the ear canal, sound vibrations reach the eardrum The eardrum separates the outer ear from the middle ear Eardrum is a membrane that vibrates when sound strikes it Vibrations from the eardrum pass to the middle ear-which contains the three smallest bones in the body-hammer, anvil and stirrup

Inner ear Stirrup vibrates against a thin membrane that covers the opening of the inner ear The membrane channels the vibrations into fluid in the cochlea Cochlea is a snail shaped tube that is lined with receptor cells that respond to sound When the fluid in the cochlea vibrates it stimulates these receptors

Inner ear and balance Structures in your inner ear control your sense of balance Above the cochlea in your inner ear are the semicircular canals which are responsible for your sense of balance

Smell and Taste Senses smell and taste work closely together Both depend on chemicals in food or in the air The chemicals trigger responses in receptors in the nose and mouth Nerve impulses then travel to the brain where they are interpreted as smells or tastes

Touch Unlike vision, hearing, balance, smell and taste the sense of touch is not found in one specific place Sense of touch is found in all areas of your skin Skin contains different kinds of touch receptors that respond to a number of stimuli

(2) Receptors that respond to light touch are in the upper part of the dermis-let you feel textures of objects such as smooth glass and rough sandpaper Receptors deeper in the dermis pick up the feeling of pressure Dermis also contains receptors that respond to temperature and pain Pain is unpleasant but can be one of the body’s most important feelings because it alerts the body to possible danger