Eye- Eye Captain!

Conjunctiva The conjunctiva is the thin, transparent tissue that covers the outer surface of the eye.  It begins at the outer edge of the cornea, covering the visible part of the sclera, and lining the inside of the eyelids.  It is nourished by tiny blood vessels that are nearly invisible to the naked eye.   The conjunctiva also secretes oils and mucous that moisten and lubricate the eye.

When you look at an object, light rays are reflected from the object through the pupil to the cornea, which is where the miracle begins. 

The cornea The cornea is the transparent, dome-shaped window covering the front of the eye.  It is a powerful refracting surface, providing 2/3 of the eye's focusing power.  Like the crystal on a watch, it gives us a clear window to look through.

Corny! Because there are no blood vessels in the cornea, it is normally clear and has a shiny surface.  The cornea is extremely sensitive - there are more nerve endings in the cornea than anywhere else in the body.    The adult cornea is only about 1/2 millimeter thick and is comprised of 5 layers.  

The Iris The colored part of the eye is called the iris.  It controls light levels inside the eye similar to the aperture on a camera. 

The iris is flat and divides the front of the eye (anterior chamber) from the back of the eye (posterior chamber).  Its colour comes from microscopic pigment cells called melanin.  The colour, texture, and patterns of each person's iris are as unique as a fingerprint.

The Pupil The round opening in the centre of the iris is called the pupil.  The iris is embedded with tiny muscles that dilate (widen) and constrict (narrow) the pupil size.

The sphincter muscle lies around the very edge of the pupil The sphincter muscle lies around the very edge of the pupil.  In bright light, the sphincter contracts, causing the pupil to constrict.  The dilator muscle runs radially through the iris, like spokes on a wheel.  This muscle dilates the eye in dim lighting.  

The Lens The light rays are bent, refracted and focused by the cornea, lens, and vitreous. The zonules are tiny fibre "guy wires" that support the lens.

The lens The lens' job is to make sure the rays come to a sharp focus on the retina. This is the focussed image on the retina This is light coming into the eye. This is the lens of the eye

Get into the ZONE The zonules help control accommodation by changing the shape of the lens.  When the zonules relax, the lens thickens, increasing the eye's ability to focus up close.  When looking at a distant object, the zonules contract.  The lens becomes thinner, adjusting the eye's focus for distance vision.  

Vitreous Humour The vitreous is a thick, transparent substance that fills the centre of the eye. It is composed mainly of water and comprises about 2/3 of the eye's volume, giving it form and shape. The viscous properties of the vitreous allow the eye to return to its normal shape if compressed. 

In children, the vitreous has a consistency similar to an egg white In children, the vitreous has a consistency similar to an egg white. With age it gradually thins and becomes more liquid.

The Retina The lens' job is to make sure the rays come to a sharp focus on the retina. The retina is a multi-layered sensory tissue that lines the back of the eye.  It contains millions of photoreceptors that capture light rays and convert them into electrical impulses.

The Macula The macula is located roughly in the centre of the retina, beside the optic nerve. It is a small and highly sensitive part of the retina responsible for detailed central vision.  The macula allows us to appreciate detail and perform tasks that require central vision such reading.

There are two types of photoreceptors in the retina: rods and cones There are two types of photoreceptors in the retina: rods and cones. These are the special cells that collect the light. Rods Cones This image is a scanned electron micrograph

Fovea The fovea (arrow) is the centre most part of the macula. This tiny area is responsible for our central, sharpest vision. A healthy fovea is key for reading, watching television, driving, and other activities that require the ability to see detail. The fovea has a very high concentration of cones (photoreceptors responsible for colour vision), allowing us to appreciate colour.  

The Cone Cells The cones are the photoreceptors concerned with colour and enable high quality vision. The cones are contained in the macula, the portion of the retina responsible for central vision. The retina contains approximately 6 million cones.

The Rod Cells There are approximately 125 million rods. They are spread throughout the peripheral retina and function best in dim lighting. The rods are responsible for peripheral and night vision.

So once light has travelled to the eye the light rays are bent, refracted and focused by the cornea, lens, and vitreous. The resulting image on the retina is upside-down.  

At the retina, the light rays are converted to electrical impulses which are then transmitted through the optic nerve, to the brain, where the image is translated and perceived in an upright position!

The Optic Nerve The optic nerve is the cable of nerve fibres with carries the electrical signals from the retina to the brain for processing. The point of departure of that optic nerve through the retina does not have any rods or cones, and thus produces a "blind spot".

The Blind Spot To discover your blind spot visit the site below and do the fun activities also check out the illusions gallery. http://faculty.washington.edu/chudler/chvision.html

Eye Quiz To find out who belongs to what eye http://ebiomedia.com/gall/eyes/eye2.html

Bibliography http://www.stlukeseye.com/Anatomy.asp