in today's video we're going to take a look at the structure of the eye and see how it responds to changes in light intensity with the iris reflex if you look at this image here this is actually a cross section of the eye which means that it's as if we had an entire eyeball looking to the left but then we sliced it in half and removed the half closest to us so that we could see inside it as light hits the eye the first structure that it meets is the cornea and to allow all the
light to pass through the cornea is completely transparent and so has no blood vessels this means that all the oxygen it needs has to diffuse into it from the outside air rather than being supplied by the blood like everywhere else in the body the reason we need the cornea is because it causes all the light that passes through it to refract or bend by a certain amount the next important structure is the iris which is the coloured bit of your eye and controls how big or small the pupil is the pupil itself is just here
and it's not actually a structure it's just this gap in the middle of the iris that allows the light to pass through to the lens just like the cornea the job of the lens is to refract or bend light but importantly the lens can change its shape which allows it to control how strongly it refracts the light and this helps it to always focus the light perfectly onto the retina at the back of the eye the retina itself is made up of two different types of receptor cells cone cells are sensitive to the color of
light and so allow us to see in color however they don't work very well in low light conditions so we also have rod cells which are more sensitive to light but only allow us to see in black and white and this is actually why you can't see colors when it's dark because it's only your rod cells that are working the next structure to know is the fovea which is a special spot on the retina which is full of only cone cells and it's this region that we try to focus light on so that we can
see things most clearly and lastly we have the optic nerve which takes all of the impulses generated by the receptor cells and transmits them to the brain the next thing we need to consider is the iris reflex and to understand it let's take a look at the eye from the front in very bright light conditions like when you are outside in the sun or you look towards a light bulb the intensity of the light can actually cause damage to your retina so to prevent this you have a reflex that controls the size of your pupil
making it smaller in bright light conditions so that less light gets in and damages your retina but larger when in low light conditions which allows more lighting as it helps you to see in the dark both of these cases have their own names so in this image on the left when the pupils are small we call it constricted but when it's larger like on the right we say it's dilated to control this process the iris is made up of two different types of muscles on the inside are these circular muscles that stretch around the pupil
like circles and then around the outside are the radial muscles and you can remember that they're called radio muscles because they stretch from the inside to the outside like the radius of a circle in order to make the pupil smaller in bright light these circular muscles contract squeezing the pupil smaller and the radial muscles have to be relaxed so that they can be stretched longer if it's dark though and the pupil needs to let in more light these circular muscles relax and the radio muscles now contract making them shorter and pulling the pupil open anyway
that's everything for today's video so i hope you enjoyed it if you have any thoughts about any of it then do let us know down in the comments and we'll see you again soon
