[MUSIC] Okay, so now we're going to talk about Near vision and to accomplish near vision we have a trio of adjustments. They're called the near triad. So, let's look at, at that, first just to remind you when we're looking at far distance at optical infinity. This is optical infinity out here. Light rays are coming in, they're coming in in parallel, they're being bent by the cornea. And then very slightly bent by, by the lens. But what happens when we look at far objects, I'm sorry, near objects, text, we're reading a book, say. Well first of all, instead of looking out there, we're, we're looking right here and now, the light rays are coming in at an angle, okay? So they're coming in at an angle onto the eye. The, the refractive index of the cornea is unchanged, it's the same. But now, this lens has changed shape a little bit, and it is still going to enable this light to be focused at the same spot on the retina. So how is that accomplished? The, as I said the, the cornea's not involved, the cornea's going to stay the at, stay doing the same thing. What's really involved are three things. First of all, adjusting your eyes, converging your eyes, so convergence, and that's something that requires actual voluntary movement, you're, you're not particularly aware of it, it's an automatic adjustment, but it is using fast muscles. And then there are two things that accompany that. First of all, we round up our lens, we're changing that fine focus so that we can get light from near objects perfectly focused onto the retina. And the second thing is pupillary constriction. Now, both of these are really slow processes because they, they go through the sympathetic and, well in this case the parasympathetic system. So they go through the autonomic nervous system and they depend on those metabotropic receptors that we, we briefly talked about that are very slow. So all of that is very slow. And I, I just want to, I, so there's, there's a few things to say here. One is that as you watch somebody change their change where they're look at from something far away to something close. You'll see their eyes converge but if you look very closely you'll also see that their pupil constricts. So to understand why pupilary constriction is so helpful. We're going to go over to the board, and think about if you have a pupil that is narrow versus if you have a pupil that is wide. The amount of light that can get through this narrow channel, the cone of light is very narrow. That's it, whereas the cone of light that can go thru here is very wide. So you could have blur that is that big, In the case of a wide diameter pupil, and blur that is only that wide in the case of a narrow pupil. And in fact this concept is what's used in in pin hole photography. And another way to think about this is if you ever go to the optometrist or optomologist and you get your eyes dilated. One of things that happens is that everything becomes blurry. It's very difficult in particular, to see near objects. And the, the final piece of narrow triad is this rounding up at the lens. It's an infinitesimal round up, but it's just enough to to contribute to the fine focus that allows us to, to read for instance text at a close distance. Now when you get into your fifth decade, when you hit 40 or so there's a law. It's, it's up there with gravity. Your, your lens is going to stiffen, it's ability to change shape is going to gradually be lost. So that by the time you're, say 60, 20 years later, you have very little ability to change it takes a long time and very, it's very ineffective to change the shape of your lens. Which means that the near triad in older people is, is now a near duo. And that is why many of us, need, reading glasses when we get older. Now the entire the entire triad, triad, in a younger person depends on this cranial nerve. This is cranial nerve three. It's ca, also called ocular motor nerve. And you can see that it's right here. This is on a stylized brain. Here's the optic nerves. Here's the optic chiasm. Here's the optic tract. The eyes would be right here. And here's the ocular motor nerve. And the ocular motor nerve is moving the eyes, but it's also rounding up the lens and constricting the pupil. And this nerve sits in a particularly vulnerable place in the situation where there's a buildup of pressure in, in the, in the brain. So if there's a large stroke in the brain, one of the things that happens is that this piece of the temporal lobe. This piece of the temporal lobe, this bump here, is called the uncus, and the uncus can slip down and actually herniate and press on that ocular motor nerve, and so under those circumstances, the near triad will will be abolishing factor. In fact, the pupil area control will be abolish in that situation. And that is, that's a medical emergency. What we're going to do next is talk about how the eye becomes the correct length, so that we can see far and then when we adopt the near triad, we can see close as well. [MUSIC]