Visual Anatomy: Study with Video Lessons, Practice Problems & Examples

Anatomy of the Eye

Alright, so the sense organ for vision is, of course, the eye. We're just going to go over some basic eye anatomy. We're going to begin with the outermost structure, which is called the cornea. The cornea is essentially the outer protective layer of the eye. It helps prevent dust or debris from entering your eye and also helps to bend light as it enters your eye; we'll talk about the path of light in just one moment.

Next up, we have a few structures that I'm sure you have heard of before. We have the pupil, which is the black dot right in the center of our eye, which is actually an opening that allows light to get in. Surrounding the pupil, we, of course, have our iris. The iris is the colored circle around your pupil, and the iris has muscles that can relax and contract, which can make your pupil larger or smaller, which then will determine the amount of light that is able to get into your eye. Right behind the pupil, we have this structure here called the lens, and the lens can actually change shape to help you focus on images.

The process of that is something called accommodation. Accommodation is the ability of our lens to change shape to help us focus on objects at different distances. Basically, there are muscles around the lens, and those muscles will contract or relax to change the shape of the lens. If you're trying to focus on an object that's far away, your lens will basically flatten a little bit, and if you're focusing on an object very close to you, your lens is going to bend. Fun fact: if you are nearsighted or farsighted, that is often caused by some kind of problem with the accommodation of your lens. The lens is just not accommodating correctly.

Now finally, we have the retina. The retina is everything here depicted in yellow. This is actually neural tissue that lines the inner back of the eyeball, and this tissue contains the receptors for vision. These receptors are sometimes called photosensitive cells. "Photo," of course, meaning light in this context, so they are light-sensitive cells. The retina contains the cells that are actually going to be sending neural impulses to the brain to tell the brain that we’re seeing some light. To trace the path of light as it's entering the eye, light would be coming in, the cornea would help to bend it to the pupil, which is the opening that allows the light to get in. It would then hit the lens depending on how far away the object is that you're focusing on.

The lens would be accommodating. The purpose of that accommodation is to help the light hit the retina at just the right spot. You may have noticed how our retina has a little bump right here, and that is actually a very special and important part of the retina, and we will talk about that in our next video. I will see you there.

Visual Anatomy Example 1

Alright, so for this one on our image here we're going to be drawing the pathway of light as it enters the eye, then using these words we're going to put these structures in the order that they would come in contact with light, which I think after drawing it out will be very easy to do. Right? So when we're thinking about how light enters the eye, it is of course an external stimulus. So it's going to be entering from over here. It would hit the cornea.

It would then get bent toward the pupil, the opening. It would then hit the lens. The lens would of course be accommodating depending on how far away the object that you're looking at is and the point of that accommodation is to basically help that light hit the retina right in the center in just the right spot there. So that is kind of the general path of light as it enters the eye. Now based on this, we know that the light is first going to hit the cornea, we'll put C there, it is then going to hit the lens, and then finally it's going to hit the retina.

We'll put A there. And the retina of course contains all of the actual receptors for vision that are going to be sending a neural impulse to the brain to let the brain know, hey, we're seeing some light basically. Alright. So that is kind of the pathway of light as it goes through our eye, and I will see you guys in the next one. Bye bye.

As we age, our eyes lose the ability to accommodate, causing many people to need reading classes. Which structure of the eye is likely affected?

• A. Cornea.
• B. Lens.
• C. Retina.
• D. Pupil.

Photosensitive Cells

Alright. So, like we talked about, the retina contains those photosensitive or light-sensitive cells. We have two types of photosensitive cells. The first type is called rods. If you're looking at our zoom-in of the retina here, the rods are all of these gray, literally rod-shaped cells, and rods are going to be responding to dim light.

So if you were in a dark or dimly-lit room your rods are going to be working really hard and they basically allow us to have monochromatic vision or our ability to see in kinds of blacks and whites and grays. This is why if you wake up in the middle of the night and your room is dark, everything almost looks like it's in grayscale. It's because in that very dimly-lit environment your rods are responding, and rods allow us to see in a monochromatic palette. Now, rods are located mainly along the periphery of our retina. If we're looking at our image here, the rods are going to be mainly located in these regions that I'm highlighting in purple.

We have relatively fewer of them located in the center of the retina and we have a lot of rods. Each of your eyes is going to have between 80 to 120 million of these cells, so they are very, very numerous. Now our other type of photosensitive cells are called cones. I'm going to pop off the screen just for a second so that you can see this a little bit better. But if we're looking at our image, our cones are these little rainbow colored cells there.

Again, they're literally going to be cone-shaped just like how our rods are rod-shaped, and cones respond to bright light. Anytime you are in a well-lit environment, your cones are going to be working really hard and cones basically give us color vision. So cones allow us to see in color. Now cones are going to be located mainly in the center of our retina so they are very densely packed right into the center of the retina and then we have relatively fewer of them along the periphery.

So basically the opposite of what we see with rods. And compared to rods, we have relatively few cones. Each eye has between 5 to 7 million cones. And again, they're going to be very densely packed into the center of the retina. Now, speaking of the center of the retina, as I mentioned previously, that is a very important part of the eye.

This region is actually called the fovea. The fovea, which you can see here, we kind of illustrated it with like a little bump in the retina. The fovea is the center of our retina and because of that, it's going to contain a lot of cones of course because cones are located mainly in the center. And the fovea has very high visual acuity. It's basically the region of our retina where vision is the sharpest.

So that's why every time I was kind of tracing the path of light as it entered the eye, I was always having it hit, like, right there in the center of the retina because we want light to be hitting there for optimal visual acuity. Alright? So that is the fovea. Now we know we're talking about these photosensitive cells responding to light, but that signal has to actually get to our brain somehow. Right?

So, it's going to be getting to our brain via the optic nerve, but that is actually going to create a blind spot in our eye. So our blind spot is basically going to be the location where the optic nerve leaves the eye, and in this location because we have the optic nerve literally exiting the eyeball, we have no receptors in that region. So we have no rods and no cones in that area, and so because of that, we literally can't see. It's a little tiny blind spot in our eye.

Now, you basically never notice this blind spot and it rarely causes problems. Your eye does certain things to kind of make up for it and your brain does as well. So, we basically never notice it, but it is there, which is kind of cool to think about. So just to kind of sum everything up, just as a kind of memory tool, It might be helpful to think about how cones see in color. Cones are near the center, so all those kind of c words going together, and they are scarce.

I know scarce is a bit of a stretch, but think of that kind of c sound that we have going on there. So cones that see in color, they are in the center and they are scarce. And if you know what cones do, you know that rods are basically the opposite of all of those things. Right? Alright.

So those are our photosensitive cells, and I will see you guys in our next video. Bye bye.

Visual Anatomy Example 2

Alright. So for each of these situations, we're going to be determining what type of photosensitive cells we would be relying on. So if we are looking at Van Gogh's beautiful sunflowers, we're obviously going to be trying to appreciate the gorgeous colors in that painting. Right? So we're going to be using our cones.

Cones see color. Alright, now if we are walking through a forest at night, if it's nighttime, odds are there is very little light or we have very dim light and we know that rods are what respond to dim light. Right? So we're going to be using our rods mostly. Cones need nice bright light and that is what helps us see color, which is why when it's nighttime and we are responding to very dim light everything looks kind of monochromatic because our rods are the ones responding then.

Alright, so if we are deciding what color shirt to wear we're obviously going to be using our cones. And then if we see movement in our peripheral vision, that one is a little bit trickier, but remember when we're thinking about our retina, along the periphery of our retina that is where our rods are located. Right? We tend to have cones clustered in the center, rods around the periphery. So if we're using our peripheral vision, that is mainly going to be using our rods.

Alright. And there you have it, and I will see you guys in the next one. Bye bye.

Why do our eyes have a blind spot?

• A. Having a place with no rods or cones saves biological resources.
• B. A blind spot helps create a sense of depth perception.
• C. Our blind spot is rich in rods, so it isn’t fully blind but lacks visual acuity.
• D. Our blind spot is where the optic nerve leaves the eye to go to the brain.