The Human Eye Structure & Function Abnormalities Dissection of the Eye Lesson 23 - The Human Eye The Human Eye Structure & Function Abnormalities Dissection of the Eye Other types of Eyes

The Human Eye Our eye is designed to gather light for vision The eye does not “see” for us… but rather is uses light to create nerve impulses to send to the brain It is our brain that turns that information into images that we “see” We will discuss this concept in more detail latter on

Structures of the Eye Choroid Cornea Sclera Iris Pupil Fovea Retina Lens Optic Nerve Ciliary Muscle Blind Spot Vitreous Humor

EYES vs. CAMERAS

The Cornea The cornea is a clear, dome-shaped surface that covers the front of the eye It is the first and most powerful lens in the eye's optical system To keep it transparent the cornea contains no blood vessels Instead, tears that flow over it and aqueous humor in the chamber behind it keep it nourished

The Iris This is the colored part of the eye: brown, green, blue… It is a ring of muscle fibers located behind the cornea and in front of the lens It contracts and expands, opening and closing the pupil, in response to the brightness of surrounding light

The Pupil The pupil is not really a structure at all The pupil is the hole in the eye ball that allows light to pass into the eye and to the retina It is the iris that controls the size of the pupil A “dilated pupil” is actually a wide open iris

Lesson 23 - The Human Eye Demonstration Time

The Lens The lens is a fairly solid structure which can be flexed by the joining muscles around the periphery of the eye The lens is stretched or relaxed to either make it thinner or fatter depending on the distance the eye is looking at If working properly, the lens will focus light to a point on the retina

The Ciliary Muscle The ciliary muscle is the controlling factor that dictates how fat or flat the lens will be When contracted, the ciliary muscle pulls on the lens causing the lens to become flatter so the light from far off images can be focused When the ciliary muscle is relaxed, the lens pulls back to become more round and will focus on up-close objects

To the right is a magnified view from an electron microscope that shows “zonule” fibers attaching the lens to the ciliary muscle

The Vitreous Humor The vitreous humor is a sack of clear gel like fluid that fills the interior of the eye ball The vitreous humor serves two purposes… It helps the eye maintain its shape by “filling” the hollow of the eye and preventing outside tissue from changing the shape of the eye Provides a clear path for light to travel to the retina

The Retina The retina is a “blanket” of photo recepting nerves that covers the back 2/3 of the eye The retina is composed of millions of RODS and CONES which are photon activated nerves The retina is the most delicate portion of the eye

The Optic Nerve The optic nerve is the eyes link to the brain and to the body It is the only other opening to the eye other than the pupil The optic nerve is a channel that funnels all of the nerve impulses created by the photoreceptors (Rods & Cones) to the brain The opening is also the chorid’s connection to the circulatory system for nourishment

The Optic Nerve The optic nerve has 4 main wires running to the brain As you can see, two of the wires cross so that info seen from the right visual field is sent to the brain together and info seen from the left visual field is seen together The optic nerves travel to the occipital lobe of the brain

The Blind Spot Where the retina and nerves (wires to the brain) funnel out of the eye is called your blind spot It is at the one area where there aren’t any photo receptors to be activated by light Therefore we “don’t see anything” at this point

The Blind Spot

The Blind Spot You can experience your blind spot by playing around with two images equidistant to your eyes and then covering one eye Move the images back and forth until you can’t see one

The Sclera The sclera is the white part of the eyes some of which you can see from the front of the eye The sclera is the “flexibly rigid” portion of the eye that provides the eye with its shape The muscles that move the eye up & down and side to side are attached to the sclera

The Choroid The choroid is a black layering of blood vessels on the interior of the eye The blood vessels provide the cells in the retina and the interior of the eye with nourishment The black pigment is meant to absorb all the light that enters the eye

The Fovea The fovea is a pinpoint space located in the center, at the back of the eye directly behind the center of the lens The fovea is a concentrated node of photoreceptors (mostly cones) where the lens tries to focus the light on

Cones in the Retina Cones are cells that are activated by bright light and allow us to see in great detail Cones are mostly concentrated at the back of the eye directly behind the lens Cones allow us to see colour

Rods in the Retina Rods are cells that are activated in dim light and allow us to see at night time in low light Rods are mostly concentrated around the periphery of the eye and provide peripheral vision Rods see in Black & White

Lets Recap the Eye Choroid Cornea Sclera Iris Pupil Fovea Retina Lens Optic Nerve Ciliary Muscle Blind Spot or Optic Disk Vitreous Humor

Lesson 23 - The Human Eye Your Task You are being given a short worksheet that deals with the structures and the functions of the eye Your task is to complete this sheet in a timely manner before we move on to the next portion of the lesson Be sure to ask for help on any concept that is confusing you so that we can get it cleared up before your oral examination on the eye