hey everyone welcome to Professor Wang lectures in anatomy and physiology I'm Professor Bob long you've already heard my coronavirus disclaimer you already know what's going on if you're watching this video you've probably seen some of my other ones this video is intended for use by students enrolled in my human anatomy and physiology course at Del Mar College anyone else out there in YouTube land finds of helpful good I hope they hope you understand some stuff if not well there's some other videos out there anyway this video is intended for students in my human of his human anatomy and physiology part to course biology 2402 at Del Mar if you're watching this video hopefully you've already watched the other two videos on the eyeball this is the third in a series of videos on the eyeball and the anatomy and physiology of the eye if you're using my note set we're following along on page 12 in the last video we were covering some of the vascular tunic we talked about the choroid plexus we talked about the pupillary die later and constrictor muscles and control of the size of the pupil now what we're going to do is we're going to talk a little bit more about the fluids inside the eye when we look at the anatomy of the eye the eye actually has two giant empty spaces in it one of them in front of the ciliary body all of this area which includes the space between the iris and the ciliary body and the space between the iris of the cornea they're connected through the pupil this entire area here is called the anterior cavity everything behind the ciliary body is called the posterior cavity so we have an anterior and a posterior cavity the dividing line is the ciliary body now if we look at the anterior cavity it has somewhat of a divider here called the iris and even though these two areas are connected they had to give each one of these areas its own name the area between the cornea and the iris appear in front is called the anterior chamber of the anterior cavity if I go through the pupil on the other side between the iris and the ciliary body is the posterior chamber of the anterior cavity now these two cavities fill with fluids the fluids are actually derived from our bloodstream the fluid that fills the posterior cavity is rather thick engine it's almost like a boss snot or mucus if you ever dissect an eyeball you'll see it fall out it's a little gelatinous mass like some clear mucus and that fluid is called the vitreous body it fills this entire space back here so one of the things we know about the posterior cavity is the posterior cavity and some people and some books also called it the vitreous cavity because the fluid that fills this is called the vitreous humor now vitreous comes from the same word as viscous or similar to the word viscous which means a thick fluid it's not quite water it's not quite solid or semi-solid it's just kind of gelatinous and mucousy so the vitreous body or the vitreous cavity is filled with the fluid called the vitreous humor and it does provide some shape and support for the eyeball but by itself the eye would continue to Sag if this area were empty the fluid that fills the anterior cavity is more watery or aqueous and so that fluid that fills the anterior cavity is called the aqueous humor or the aqueous fluid the aqueous humor is constantly recycled in our eyeball the vitreous humor is somewhat recycled it is secreted and fills the eye but it's much more slowly recycled so we're going to focus on the aqueous humor now the blood vessels that are coming in under blood pressure all right going to push blood all the way up into the ciliary muscle and some of these blood vessels make a little plexus or network here and that's where the true choroid plexus is that's why this is called the colloid coat that corner right plexus is going to secrete a fluid that's going to enter into mostly the posterior chamber of the anterior cavity that fluid will start to circulate through this area through the pupil and into the anterior chamber of the end to your cabinet so the aqueous humor fills the anterior cavity the vitreous humor fills the posterior cavity so if I took a water balloon the water balloon kinda represents the vitreous cavity of the vitreous humor and if I squeeze the water balloon it would still change shape under pressure what provides what we call the intraocular pressure the pressure inside the eyeball is the constant secretion and reabsorption of the aqueous humor it plays a major role in providing what we call the intraocular pressure or the pressure inside the eyeball as this fluid pressure builds up here it'll push back on the vitreous body which pushes the vitreous body up against where the retina would be the retina would be sandwiched between the vitreous body and the colloid layer now the retina is almost like a screen so if I had a screen hanging down here and a camera focused on it or a video projector focused on it if I actually move part of the screen and curved it then part of the stuff that's being projected at part of the image would be in focus but the stuff that is out of the plane of focus of the lens would be out of focus and that's kind of what happens here our lens this kind of trying to focus all the light onto our retina if the retina is not perfectly in place then we will have blurry vision in some areas the breadline is actually not even attached to the inside of the eyeball it can completely come off and detach we get the text retinas because of this the only place where the retina really is anchored to the body physically as the axons coming from one group of cells as they run out the optic nerve so in order to hold the retina in place here's an example that I like to show my students your retina is secured in place about as well as this booklet is if I let go it doesn't stay in place and detaches so the vitreous body is pushing the retina up against the back of the eye but what's pushing on the vitreous body is the aqueous humor and the reason we have a certain amount of pressure here is that the choroid plexus here in the ciliary muscle will secrete the we assume err now if I continue the secretin without reabsorbing it then the pressure would build up and up and up in the eye and it would crush those delicate neurons in the retina and blindness would result as a matter of fact an increased intraocular pressure results on a disease that we call glaucoma glaucoma simply an increase in traction or pressure there's a number of causes of it if someone has chronically high blood pressure for long periods of time then you'll secrete more fluid faster than we can reabsorb it and by the way we reabsorb the fluid by some little blood vessels that run in the corneal limbus you may have learned this term in lab the canal of schlemm the canal of schlemm is also called nice Claro venous sinus and so as the flip and that's a blood vessel that rings around the limbus inside the eye and the fluid the aqueous humor is being secreted on one side circulates through here and then gets reabsorbed into the canal of schlemm and then we remove all that waste and debris the aqueous humor can circulate a little bit back here and it can feed all the neurons with nutrients and ions that they need they dump their waste and we reabsorb it so now what maintains our intraocular pressure is the constant secretion and reabsorption of the aqueous humor if we reabsorb it at the same rate that it's being secreted then the pressure inside the eyeball should remain relatively constant if for some reason that pressure starts to go up glaucoma can develop it can press on the the retina and cause blindness sometimes the canal of schlemm gets clogged and we secrete the fluid but you can't drain it sometimes chronic high blood pressure and other things can cause this we're not going to talk too much more about coma now what I do want you to know is this if we were to list the structures that the aqueous humor would flow through which is in my notes said the first structure is we can say that it is secreted at the ciliary muscle there's a small secretory structure inside the ciliary muscle that secretes it it will flow through the posterior chamber of the anterior cavity and will next go through the pupil it next goes through the anterior chamber of the anterior cavity and then we reabsorb it at the canal of schlemm so you should know the structures in order that aqueous humor would flow through you should know that aqueous humor place a huge role in developing intraocular pressure along with the vitreous body and they maintain the positioning of the retina on the back of the eyeball now if you're following along in the notes set one of the next things is one of the crystalline fibers who are what our lens fibers and what our crystal is well lens fibers are the special types of cells that make up the lens and they have a lot of elasticity and crystallins are proteins that fill those cells that do refraction of light so I want you to look up lens fibers and crystallins and read about them I'm going to talk about a little bit about refraction so we're still on the vascular tunic we still haven't completed it there's a lot to it and then we'll get to the retina in the next lecture so I'm going to give you the definition of refraction refraction is defined as a change in direction photons of light or sometimes we say rays of light as they pass from one medium my goodness my penmanship into another or some people call refraction the bending of light I'm gonna explain so get this definition down it's a change in the direction of photons or rays of light as they pass from one medium into another I'll explain what that means now that you got the definition down we're going to talk about it I'm also gonna erase part of the eyeball here and redraw it so that it's a little bit cleaner for us to look at okay so put our limbs back put our cornea back put our iris back in here by the way while I'm doing this I should tell you you guys should learn to take essay tests even though my exams are not going to be essays if you could do an essay test and teach this to a group of 10th graders you're gonna make an in on everything that's how you should practice so when we talk about refraction let's say you are sitting in a bathtub like this your heads up here and your eyeballs are here and your feet are here and there's water in the tub when light is coming off your toes as it hits the water and gets bent some of the light is gonna start to move towards your eyeball as that light moves towards your eyeball when it hits I'm gonna make the water a little bit straighter here if the water were pretty level here when the light is coming off your toes it's passing through one medium called water when it hits the interface those photons of light will slightly change their direction as they move towards your eyeball like this and it will make it look as if your toes we're really up here that's why your legs look longer if you're sitting in a tub or they look shorter if you're standing up in a tub because of the refraction of the bending of light so if I had a glass of water [Music] if a photon of light is moving towards the glass when it passes from air one medium into the glass and will Bend when it hits from the glass to the water it will Bend again and then it will Bend again and again and if we look it might make an object look like it's coming from a different point that it really was so light can be refracted or bent as it passes from one medium to another and by the way if we calculate what we call the angle of incidence then we'll know what the angle of refraction is there's a formula that can happen or that you do in physics for the optics of the optics and physics or the changing of light the refraction can be calculated which is what we take advantage of if someone's eyeball is not the right shape or you have blurred vision you go to an eye doctor they calculate the angles and then they put a corrective lens in here so that it bends the light a little bit more for you to help focus that's the whole purpose of contact lenses or corrective lenses if I know what these angles are and I want you to see this object here then I can actually create a lens over here that will change the light and make it appear as if it came from here again anyway that's a little side note but I do want you to understand what refraction is and the reason refraction is important is because and fix pieces of glass we can't change what's called the focal distance I'll explain all this in just a second so now when we get to page 13 sorry I got an itch on my nose and I don't want to ditch 15 or 20 minute video so sorry about that there are four refractive media of the eye so there's four things that when weight when light enters the eye it gets bent the first one is the cornea and actually the cornea has the greatest refractive power so if a photon of light is coming at the eyeball it's going to be bent at the cornea these two angles here would be slightly different then when they what when the photon of light hits the aqueous humor that's a different medium it will get bent again if it passes through the retina I'm sorry through the pupil then it will encounter the lens when it hits the lens it bends again and then when it passes from the lens to the vitreous body it bends again and it should be focused on the retina back here so those are the four refractive media of the eye the cornea the aqueous humor the lens and the vitreous umer so light is being bent and bounced around inside your eyeball like crazy so who knows what the world really looks like what's important is what we perceive it to be I guess so you should know there were four refractive media of the eye you should understand what refraction is and this is why it's really important because our lens is made up of lens fibers which have a lot of connective tissue and elasticity to them our lens can change shape changing the shape of the lens to focus at different distances is called a combination now if you're following along in my notes we're on page 13 I'm going to skip some of these definitions for a little bit and I'm going to talk to page 14 for a moment we're actually going to draw what the lens should look like so I want you to understand something before I do that if I take a straight line even though that's probably not a straight line let's pretend it is and I have a curvature here a piece of glass that is curved let's say if I measure right down the middle the arc here the perfect Bend here is exactly equal to this perfect Bend here and the same thing on the other side so that all of these angles and bends are the same if I measure in some distance on this end here then this curvature should match the curvature on the opposite side so that my lens is uniformly rounded so it's an oval structure if I have a little point of light and photons of light are going to spread out from this in an infinite number of directions as they hit the lens the angle of incidence the angle at which the light comes into the lens is going to be equal to the angle of refraction on the other side of the lens if the lens is perfectly curved equally on both sides and eventually all these photons of light will all eventually converge on the exact same point to converge means to come together to diverge means they would spread out our lens is a convergent minutes and that will focus them in a specific distance the distance at which we're going to focus that is called the focal distance now one of the problems with that is this let's say I have another point of light here as all of these photons of light come off here the angle of incidence might be so steep that they might focus at this distance now because the light of photons of light act the way that they do they would continue on and spread out and instead of seeing a nice little purple dot I would see a big purple blur for example it's like putting your pinkie up against your eye you don't really see nothing anything but a big giant blur no glass lenses can't change shape also if I made another dot that same way over here off the screen when it came in it would focus past the focal distance or the focal point here and we would still see a blur so a glass Lance can focus in a specific plane in any object in order for it to be focused at the focal distance must be in this plane for it to be in focus now that would not work for us if that were the case then everybody in the class would have to stack in the exact same distance from a board to take notes and I can't have everybody lined up right behind me in class so when you look down at your notes as you're writing you're focusing in one distance when you look up you should be able to focus at another distance the reason we can do that is because the elastic fibers or the lens fibers in our lens allow it to change shape and the ciliary muscle is going to control that so here's what's going to happen if you're following along we're on page 14 and we're going to do focusing on a near object and focusing on a distant object so if we're gonna focus on a near object I'm not going to draw the entire eyeball I'm just gonna cut out the ciliary body when we focus on the near object one of the things that you will notice is that and I'm gonna write out these details the ciliary muscle contracts because of the shape of the ciliary muscle as it contracts it squeezes down on itself like an annular muscle what happens then is the suspensory ligaments get loose and the lens has a natural desire to round out become more rounded so we can say the suspensory ligaments go slack and I didn't write out the word ligaments but and the men's is more rounded that would allow us if our retina were in a certain distance that say the red lower here then something that's up close is going to be bent and eventually focus on the retina this way now if I want to focus on a distant object then the exact opposite is going to happen with these structures and you should know this okay so I'm going to draw the ciliary muscle but now the ciliary muscles farther apart as the ciliary muscle relaxes it actually moves the ends of it or edges of it move away from each other in this direction the ciliary I'm sorry the suspensory ligaments get pulled tight or taut and the lens becomes more flattened so that's something viewed at a distance it changes this angle of incidence here and then the refraction is going to be onto the same retina and it's going to allow us to focus so we can say that the suspensory ligaments goes a pulled tight sorry I know I'm not using proper ending this year but and the lens is flattened or less rounded so a lot of times we're going to ask questions about what's happening when you're focusing on a distant distant object or near object and we call the process of focusing this way of changing the shape of the lens to focus on near objects in particular accomodation okay so I hope you have that down now before we move on to the retina which is on page 14 which is going to be another lecture I want to run quickly through all the definitions that are on page 13 of my notes set so we're gonna back up a second now I'm not going to write all these definitions out I hope you can visualize all this I hope you can see it I'm gonna erase all of this I'm gonna run through the definitions of verbally I'm not gonna write them all out but you can watch the video and back it up and rewatch it and hear the definitions and you can also look them up in the glossary of the textbook the first one that I want to know on page 13 is a cataract a cataract is defined as a loss of opacity this word opacity comes from the word opaque so when we define a cataract it is a loss of opacity to be opaque means it lets light pass through it we don't say that the lens and the cornea are truly translucent or transparent because not a hundred percent of the light makes it through without being moved around some so we say that it's opaque it's mostly translucent but translucent means light passes directly through unaffected so when the lens or the cornea becomes less opaque it becomes cloudy so that loss of opacity simply means we get a cloudy lens or a cloudy cornea you can have corneal cataracts and lens cataracts that would be like looking through a mirror that's been frying through a piece of glass that's been frosted or it's all foggy you can't see great detail so that's all that a cataract is is a loss of opacity or a cloudiness that develops in the cornea or the lens accommodation is the next definition accommodation is the change of the shape of the lens for focusing we've already defined that a stigmatism is an apple curvature of the cornea or the lens now I was just drawing this before but if I have a lens and our man's is not uniformly shaped actually one side is more rounded than the other that's why we're able to take this angle of incidence and change it so to focus on the retina now imagine if the lens were shaped like this one side is more rounded than the other this angle of incidence is not going to equal this angle of incidence and so one photon of light or one side of the structure will be focusing here the other one might focus at a much greater distance and they're not going to intersect sorry I didn't draw that very well they're not going to intersect on the retina so then you end up seeing a blur instead of a fine dot so astigmatism is an abnormal curvature of the cornea or the lens if we can calculate your astigmatism then glasses can be made that will corrective lenses can be made that will correct for that okay ametropia which is the next definition emmetropia is normal vision or what we call 20/20 vision okay they call that emmetropia myopia to be myopic means to be nearsighted in myopia or nearsightedness you can focus on near objects but you don't have the ability to focus on distant objects it becomes hard to see them that's what I am I can't I can see perfectly now but if I take my glasses off everything becomes a big blur unless it's very close to my face so I'm nearsighted I have myopia hyperopia hyperopia is farsightedness meaning you can focus on distant objects but you can't see up close and there are lenses that can correct for these things presbyopia is a hyperopia that occurs with age due to a loss of elasticity of the lens our men's has the ability to flatten or round out now because the lens has a natural desire to round out and the ciliary muscle is pulling it more flattened as we get older we can focus sometimes on near objects but because the ciliary muscle and the amount of the lens is less elastic and solidifies this way we can't flatten it out to focus on distant objects this is why people will stop and read things like this because they have pressed myopia or a hyperopia so presbyopia sometimes occurs with age as we lose elasticity of the lens and we cannot focus on near objects I'm some distant objects so presbyopia is a as hyperopia or farsightedness due to a loss of elasticity in the lens most myopia and hyperopia if it's not fresh myopia occurs because of this if this is my lens the back of the eye should be uniformly round and the lens is designed to focus all the photons on the retina if I change the shape of the eyeball over time then I change the focal distance and I can't focus you can become myopic or hyper optic due to changes the shape of the eyeball as we grow and as we age and things so 20/20 vision when we're 2020 and that means 20 feet at 20 feet so if I have the snow and I chart most of you have seen this it has the letter E and then all the little letters on here that you have to read one of those lines says 20/20 what that means is someone should be able to stand 20 feet from the board or from that chart and if this person has normal eyesight then at this distance they should be able to read that specific line if you can stand next to them and read at 20 feet but someone with normal eyesight reads at 20 feet you have normal vision err and Metropia now the next one on the list is 2015 that means you see at 20 feet when someone else would see at 15 feet they have to move five feet closer to see that detail so you actually have a greater ability to accommodate or focus at a greater distance some people refer to this as someone who has eagle eye because you can see at 20 feet what a person with normal vision can see at 15 feet they have to span five feet closer to read that same line so you have the ability to focus at a greater distance people who have that when they measure that in the military become marksmen and snipers and things if you are something like 20 40 that means you see at 20 feet what someone else can stand twice as far away and read that same line so they have twice as good a vision of you and if their normal vision that means you are not very good at your vision okay and the larger the second number the worse your vision is if someone is twenty two hundred which is the last one on the page at 2200 even with corrective lenses you're considered legally blind okay so anyway I'm gonna stop there we're finished with the vascular tunic we're gonna move on to the neuro tunic there'll be a couple of videos about that trying to finish up the eyeball so we can get on the hearing and equilibrium I hope you learned something I hope you had as much fun as I did I hope to see you in the next video thanks for watching