The fovea has the highest visual acuity because its high density of cones allows for:
a) greater sensitivity to color.
b) greater sensitivity to detail.
c) greater sensitivity to color and, therefore, fine detail.
d) greater sensitivity to light and, therefore, fine detail.
B is correct. A high density of cones allows for greater sensitivity to detail. The fovea is a region located in the macula of the retina that only contains cones. Cones function to detect color and fine detail. C is incorrect because it is not the increased detection of color that results in increased detection of fine detail, as the two functions are distinct. A is incorrect because although color sensation is a function of cones, this is distinct from the function of detecting fine detail. D is incorrect because rods, not cones, detect light.
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This lesson covers the structure and function of the eye. We’ll discuss all the important structures of the eye by following the path of light through the eye.
First, notice the sclera. The sclera is the white, protective outer layer of the eye. You can see that the sclera covers the entire eye, except the part in the front. The front region is covered by the cornea. The cornea is actually continuous with the sclera. The most noticeable difference between the cornea and the sclera is that the cornea is a transparent structure. It covers the iris, pupil and anterior chamber. Because the cornea is transparent, it allows light to pass through it, towards the anterior chamber. The anterior chamber is a compartment between the cornea and the iris. This compartment is filled with a fluid called aqueous humor. From the anterior chamber, the light continues through the pupil.
The pupil is an opening whose radius is controlled by the iris. The iris is a pigmented structure, and its ability to control the pupil size is very important. Consider when you go outside on a very bright day. Your pupils are going to constrict to reduce the amount of light that enters your eyes. And that’s because if too much light enters your eye, you’ll be blinded by all the light. The opposite is when you’re in a dark environment. In darker settings, your pupils actually become dilated to let in as much light as possible so you can better see in the dark.
Once light passes through the pupil and, from there, it enters the posterior chamber. The posterior chamber is the compartment between the iris and the lens and, like the anterior chamber, it’s filled with aqueous humor. The light then reaches the lens.
The lens is a transparent structure, and it has an important role in focusing light onto the retina. When you think about the term “lens”, you might recall some of the concepts you’ve learned in physics. Our eyes actually have two physical lenses. As it turns out, both the cornea and the lens are converging lenses. And, as converging lenses, they are able to help focus light onto our retina. What’s more interesting is that, between the cornea and the lens, the cornea actually has more optical power than the lens. So why does the lens have its name when it’s actually weaker than the cornea? A major reason is because the lens is attached to a structure called the ciliary body.
The ciliary body is a muscular structure. As a muscle, it actually controls the shape of the lens in a process called accommodation. When the ciliary muscles contract, they actually change the shape of the lens in such a way to allow you to better focus on close objects.
You can test this process using your fingers. Put two fingers out in front of you, one finger close to your face and one finger far away. Now start focusing on the finger that is farther away. You’ll notice that the finger close to you has become blurry. If you then focus on the finger close to your face, you’ll notice that the finger that is far away has become blurry.
This is evidence that your ciliary body is functioning properly to allow you to focus on objects that are at differing distances by changing the shape of your lens. The unfortunate news is that this accommodation ability decreases as you age. You may have noticed that older individuals tend to read small text by holding the text far from their face as opposed to bringing it closer. This is because they’ve lost the ability to change the shape of their lens to focus on objects that are close to them.
Moving on, once the light is focused by the lens, it enters the vitreous chamber. The vitreous chamber is the compartment between the lens and the retina, and is filled with a fluid called vitreous humor. Light then passes through the vitreous humor and reaches the retina.
The retina is the back part of the eye. This region is made up of light sensitive tissue that is responsible for converting photons of light into an electrical neural signal that is sent to the brain. This process is made possible by the photoreceptors (e.g. rods and cones) that make up the retinal tissue. The fovea is the part of the retina with the high density of cones and, therefore, the highest acuity vision. So right at the center of your focus is where you have the highest visual acuity. And that’s due to the fovea.
Now, to get the signals the photoreceptors have obtained to the brain, the axons of these receptors have to leave the eye to get to the brain. The bundle of axons that leaves the retina and goes to the brain is called the optic nerve and the portion of the eye where the exit is called the optic disc. The optic disc is where that optic nerve exits the eye. It should then make sense that that part of the eye does not have any photoreceptors. Therefore, due to the lack of photoreceptors, the optic disc also corresponds to the blind spot of each eye. What that means is that there is a spot in each of our eyes that is not able to see anything. Usually we don’t notice this at all because our brain fills in this gap, making our vision continuous. But the blind spot is real and does exist.
To summarize, light hitting the eye first travels through the cornea and enters the anterior chamber. From there, it passes through the pupil and continues through the posterior chamber. It is then focused by the lens and passes through the vitreous chamber to the retina. The retina then converts the light into a neural signal that is sent to the brain via the optic nerve
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