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38.3 Sense of Vision -- Part 3
The inner layer of the eye is called the retina.
Rod cells and cone cells are located at the back of the retina closest to the choroid.
Many rod cells share one cell, but cone cells don't.
Cone cells distinguish more detail than rod cells.
The rod cells and cone cells connect with the bipolar cells, which in turn connect with the ganglion cells that initiate nerve impulses.
The human retina has 150 million rod cells, 6 million cone cells, and 1 million ganglion cells.
The sensitivity of cones versus rods is related to how directly they connect to ganglion cells.
The signals from rod cells are about the size of a thumb tack hole.
The stimulation of rods results in blurred vision.
There are cone cells in the fovea centralis.
Cones give us a sharper, more detailed image of an object.
The axons of cells in the eye form the nerves.
The eyes carry nerve impulses from them to the brain.
An X shape is formed by a crossing over of the nerve fibers.
The fibers from the right half of the retina converge and continue on in the right eye, while the fibers from the left half converge and continue on in the left eye.
Both eyes can see the field.
We can see the entire visual field with the combined data.
The thalamus is a part of the visual pathway to the brain.
Most of the fibers in the thalamus connect with the neuron cell bodies in the hypothalamus.
Information about the right portion of the visual field and the left portion of the visual field have been split by the thalamic nuclei.
The right and left visual areas must communicate to see the whole field.
Because the image is inverted and reversed, it must be righted in the brain for us to see it correctly.
Even though we see a unified visual field, the brain has taken it apart.
The visual association areas are believed to give us an understanding of the field at the same time.
Problems with visual focus and diseases of the eye are some of the common disorders of vision.
The most common cause of blindness in adults is diseases of the retina.
People who smoke are 20 times more likely to get this disorder.
Drugs that are injected directly into the vitreous humor can be used to treat these diseases.
The choroid layer protects the retina from eye trauma or other diseases.
A laser can be used to repair a detached eye.
Artificial retina technology is becoming an option for some blind people who have their visual pathways intact.
Glaucoma occurs when the drainage system of the eyes fails.
In the early stages, this pressure tends to destroy the nerve fibers, but in the later stages, it can lead to total blindness.
Eye doctors always check intraocular pressure, but the disorder can come on quickly and cause permanent nerve damage.
Drugs that increase the outflow of humor are usually used in treatment.
Cataracts occur in 50% of people between the ages of 65 and 74 and 70% of people 75 or older.
In most cases, vision can be restored by removing the unhealthy lens and replacing it with a clear plastic artificial lens.
As the average age of the population increases, the number of Cataract surgery in the United States will increase.
Some patients may no longer need glasses if surgeons are able to replace cataracts with multifocal lens.
People who can read 20 letters on a chart 20 feet away are said to have good vision.
People who can't read the letters but can focus on close objects are said to be nearsighted.
These individuals have an eye that has a Page 718, and when they try to look at a distant object, the image is brought to focus in front of the retina.
The problem of the image being focused on the retina is usually corrected by cave lens.
25 million people are blind due to diseases of the retinas.
There are few effective treatments in most cases.
Artificial retina technology has been developed that can restore vision to people who were completely blind.
The rods and cones are dead but the ganglion cells and neurons are not.
Scientists have been working on ways to take the place of the diseased photoreceptors.
In March of 2011, a California company received approval in Europe to sell the Argus II, a device that can be used in blind patients with intact connections between their retinas and brain.
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