Visual purple

Rods detect images in the dark because the cells contain a rose-red pigment called visual purple, or rhodopsin. When exposed to bright light, visual purple undergoes a chemical change in which it loses its color. This causes the rods to lose their sensitivity to light, thus enabling the eye to endure glaring light. Before the eye can see in the dark, visual purple must be re-formed in the retina. As more visual purple is produced, the eye's sensitivity to light increases. Thus when a person enters a darkened motion-picture theater his eyes do not contain much visual purple. As the visual purple is re-formed, the person can see better. In a short time his eyes' sensitivity to light is multiplied about 2,000 times. Visual purple can be produced only if the body has a sufficient quantity of vitamin A. Lack of vitamin A in the diet may lead to night blindness, or nyctalopia.

Post-text assignments

1 Describe the structure of the human eye using the picture in the text

2 Answer the questions:

1. What is the funtion of conjuctiva?

2. Is there a way to see how the pupils respond to light?

3. What is the lens?

4. What does the ciliary muscle do?

5. What is the retina?

6. What special cells does retina have?

7. What rods/cones can detect?

8. What is a visual purple? What is the function of visual purple?

9. What is the nyctalopia? What is the cause of this disorder?

3 Translate the following:

1. The outermost coat consists of the cornea and the sclera; the middle coat contains the main blood supply to the eye and consists of the choroid, the ciliary body, and the iris.

2. The cornea and the aqueous humor together make an outer lens that refracts, or bends, light and directs it toward the center of the eye.

3. When more or less light is needed to see better, the pupil becomes larger or smaller through the movement of the muscle in the iris.

4. The choroid is a layer of blood vessels and connective tissue squeezed between the sclera and the retina.

5. The lens is made of elongated cells that have no blood supply. These cells obtain nutrients from the surrounding fluids the aqueous humor in front and the vitreous body, a clear jelly, behind.

6. All of the structures needed to focus light onto the retina and to nourish it are housed in the eye, which is primarily a supporting shell for the retina.

7. There are three types of cones that receive color sensations. One type absorbs light best in wavelengths of blue-violet and another in wavelengths of green; a third is sensitive to wavelengths of yellow and red.

8. When exposed to bright light, visual purple undergoes a chemical change in which it loses its color. This causes the rods to lose their sensitivity to light, thus enabling the eye to endure glaring light.

Unit 18

Pre-text assignment

Learn the key words and phrases:

pinna, cerumen, tympanic membrane, ossicle, Eustachian tube, acoustic nerve

The Human Ear

The human ear is the most complex sensory system in the human body. Vision and smell are extraordinary senses, but they do not match in complexity the ear’s process of turning minute waves of sound pressure from air molecules banging against the eardrum into the neural signals that get sent to the brain and interpreted as sound.

With our sense of hearing we can hear the exquisite sounds of nature, enjoy all kinds of music, and understand the complexities of one or more spoken languages.

The ear is divided into four parts: the external or outer ear, the middle ear, the inner ear, and the neural ear. Each part serves an important and unique function in the process of hearing.

The external ear consists of the most visible ear structure, called the pinna or auricle, and the ear canal. This part of the ear acts as a sound collector to guide sound waves down the ear canal so that they impinge against the eardrum or tympanic membrane causing it to vibrate. The shape and features of the pinna help us in identifying where sounds are coming from. In locating the direction of sounds, we also depend on the fact that we have two ears so the brain can compare the sound arriving at one ear versus the other.

The pinna, of course, is useful to keep eyeglasses from sliding down our face and is often used to attach decorative jewelry.

Lying beneath the outermost part of the ear canal are glands that produce earwax or cerumen. The skin covering the ear canal is very thin, especially farther down the canal, near the tympanic membrane. Because of this, one should avoid pushing cotton swabs or anything else down the ear canal since abrasion of the skin can cause bleeding and increase the risk of injury or infection.

The middle ear lies between the tympanic membrane and the inner ear. The middle ear space is filled with air and within it are the three smallest bones in our body, called the ossicles. These are known as the malleus (hammer), incus (anvil) and stapes (stirrup).

The ossicles serve to mechanically amplify the pressure waves from the vibrating tympanic membrane to provide an efficient transfer of the sound energy into the fluid filled spaces of the inner ear.

In addition, the relative difference in the area of the tympanic membrane and the much smaller oval window lying beneath the stapes footplate allows for a concentration of the pressure wave much the way a spiked heel on a shoe concentrates force compared to a flat heel. These two mechanisms multiply the pressure wave at the tympanic membrane about 22 times.

Also in the middle ear is the opening of the Eustachian tube that allows for the equalization of air pressure on both side of the tympanic membrane. We all experience the opening and closing of the Eustachian tube when our ears pop going up or down in an elevator or airplane.

The inner ear consists of two functional systems, both encased in the temporal bone of the skull. One, called the vestibular system is responsible for our sense of balance and equilibrium. The three semi-circular canals, set at right angles to each other, respond to motion and the forces of gravity. The other system, the coiled tube called the cochlea, is the most extraordinary structure of the ear. It is here that sound waves, now propagating as waves in the fluid-filled spaces of the cochlea, get converted to the neural impulses that are sent to the brain.

In the cochlea, thousands of tiny structures called hair cells are set into motion by the moving fluid wave. The hair cells trigger the attached nerve fibers into activity. It is within the cochlea that the most common type of hearing loss, sensory hearing loss, occurs when exposure to loud noise or diseases cause the destruction of many hundreds of these delicate hair cells.

Once the sound waves, balance and equilibrium information is encoded into neural signals by the inner ear, the neural impulses are sent to the brain along a nerve fiber bundle called the vestibulo-cochlear nerve, sometimes called the acoustic nerve. This nerve is numbered eight of the twelve cranial nerves that enter the brainstem. Other cranial nerves include the optic nerve, the olfactory nerve, the facial nerve and more.

In the brain’s central auditory cortex, the neural impulses are interpreted as sound based on our learning and cognitive processes associated with our auditory experiences.

Post-text assignments

1 Describe the structure of the human ear using the picture in the text

2 Answer the questions:

1. What is called the pinna?

2. What are the ossicles? What is the function of the ossicles?

3. What is the function of the Eustachian tube?

4. What are the systems of inner ear?

5. What is the purpose of the vetibular system?

6. What is the cochlea?

3 Translate the following:

1. Vision and smell are extraordinary senses, but they do not match in complexity the ear’s process of turning minute waves of sound pressure from air molecules banging against the eardrum into the neural signals that get sent to the brain and interpreted as sound.

2. Lying beneath the outermost part of the ear canal are glands that produce earwax or cerumen.

3. The relative difference in the area of the tympanic membrane and the much smaller oval window lying beneath the stapes footplate allows for a concentration of the pressure wave much the way a spiked heel on a shoe concentrates force compared to a flat heel.

4. It is within the cochlea that the most common type of hearing loss, sensory hearing loss, occurs when exposure to loud noise or diseases cause the destruction of many hundreds of these delicate hair cells.

5. Once the sound waves, balance and equilibrium information is encoded into neural signals by the inner ear, the neural impulses are sent to the brain along a nerve fiber bundle called the vestibulo-cochlear nerve, sometimes called the acoustic nerve

6. In the brain’s central auditory cortex, the neural impulses are interpreted as sound based on our learning and cognitive processes associated with our auditory experiences.