Vision 

The eye

Sclera

The sclera is the white part of the eye. The cornea is a clear layer over the iris, and as it continues over the surface of the eye it becomes the white sclera. The sclera is covered by a transparent mucus layer called the conjunctiva which keeps the eye moist.

Cornea

The cornea is a clear layer on the front of the eye, in front of the iris, made of cells and protein fibres. It refracts light as it enters the eye so the light is directed through the pupil to the lens. The light is refracted in such a way that it appears upside down in the eye. There are no blood vessels in the cornea so nutrients are absorbed from the vitreous humor and from tears which contain dissolved oxygen from the air. The cornea is made up of five layers, the outermost layer (the epithelium) absorbs nutrients from tears and protects the surface of the eye from foreign substances. The second layer is the bowman's membrane, then the stroma which gives the eye it's strength and shape, then the descemet's membrane which is responsible for protection from infection and injury. The innermost layer is the endothelium which pumps excess fluid out of the stroma which has leaked into if from inside the eye. This prevents the eye from clouding up and becoming opaque, meaning the cornea would no longer be clear and light could not pass through the cornea to the rest of the eye. If either of the two outermost layers of the cornea were scratched they could repair themselves, but a large scratch may leave a scar which would cause vision loss because it would interfere with the light passing through.

Choroid

The choroid is a layer behind the retina containing blood vessels. The choroid provides some parts of the eye with blood which provides most of the nutrients for functions in the eye.

Aqueous Humor

The aqueous humor is a fluid located in two chambers called the posterior chamber and anterior chamber. The anterior chamber is the space between the cornea and the iris, while the posterior chamber is located between the iris and the lens. The aqueous humor is mostly water and supplies nutrients to the parts of the eye which don't have blood vessels to do it, including the cornea. The fluid also supports the eye and helps it keep it's shape.

Lens

Once the light has passed through the pupil it passes through the lens. The lens is able to change its shape so it becomes thicker or thinner depending on how far away the object it's seeing is, in order to focus the light onto the retina. The lens is an ellipsoid shape, meaning it's like a three-dimensional oval. The lens has three layers; the outside layer is the lens capsule which is smooth and goes around the whole lens causing it to naturally be an almost spherical shape when not being stretched by the ciliary muscles. The second layer is the epithelium which keeps the volume and state of the lens constant by pumping out materials which have entered the lens from the aqueous humor. Lens fibres are the innermost layer and are long, thin, transparent cells that make up most of the lens.

Vitreous Humor

After passing through the lens light travels through the vitreous humor, a transparent fluid located in a large cavity called the vitreous which is located behind the lens and surrounded by the retina. The fluid is comprised mostly of water and some salt, collagen, and sugar.

Optic Nerve

The optic nerve is made of ganglionic cells and nerve fibres. It uses electrical impulses to carry the information about the image detected by the photoreceptors to a part of the brain called the occipital lobe.

eye defects and diseases

Presbyopia

Presbyopia often occurs as a person ages. The proteins in the lens change so the lens become harder and stiffer, sometimes thicker, and less flexible so it is more difficult to focus on things very close-up. This is why some people need reading glasses. The glasses magnify the image at close range which the person is looking at by refracting the light since the lens is too stiff to adjust itself to the correct shape to be able to refract the light onto the retina. Bifocals can also be worn to accommodate the lens; on the top half of the lens in the glasses is a person's prescription so they can see further away, and on the bottom half is a prescription to help them see very close. Bifocal and multifocal contact lenses can also be worn to aid the lens. 

Glaucoma

Glaucoma is caused by pressure build-up inside the eye against the optic nerve which can affect it's ability to send clear images to the brain. If the pressure is hard and lasts long enough it can damage the optic nerve which can result in permanent vision loss. The pressure is caused when fluid from the aqueous humor builds up against the nerve. The aqueous humor normally flows out of the cavity it's in through a canal; if the canal becomes blocked by something the aqueous humor has nowhere to go and builds up putting pressure on the optic nerve. There are some other but much less common causes than aqueous humor build-up such as a blocked blood vessel in the eye, infection, or physical injuries to the eye. If the optic nerve has not been damaged beyond repair yet, glaucoma can be prevented with surgery or eye drops. The eye drops can decrease the amount of fluid being produced, or help the fluid to flow away from the optic nerve, releasing the pressure.

Amblyopia

Also known as a lazy eye, amblyopia is when one eye has not developed properly so the brain ignores the image detected by that eye since the image is poor or blurry. Amblyopia can be caused when one eye is much weaker than the other because of something like severe farsightedness or astigmatism. This causes the brain to just ignore the blurry image from the weak eye altogether and process the clearer image from the better eye. Amblyopia can also occur when both eyes can't focus together on one object, and one eye rolls away and looks at something else resulting in the person seeing double. The weak eye will get worse and worse so the brain will just ignore it rather than try to process the image it generates. Amblyopia can be treated with glasses or contacts with a strong prescription for the weak eye, and often also with an eye patch. The patch would cover the good eye so your brain is forced to rely on the weak eye, and the weak eye is forced to work harder than usual to generate a clear image, so the eye becomes stronger.

Cataracts

Cataracts is when the lens in your eye is blurred, blocking light from reaching the retina so you have vision loss. A cataract is formed when the proteins making up a large part of the lens grow so they are clumped together. This would mean there is a high density of proteins in a small area, creating an opaque spot which blocks light from passing through and reaching the retina. Overtime the cataract can grow, preventing more and more light from passing through the lens, and causing vision loss. Glasses or contact lenses can be worn to improve vision, although it will not get rid of the cataract or prevent it from getting worse. If the cataract has progressed a surgery may be necessary, where the opaque lens in your eye is removed and replaced by a transparent, plastic, man-made lens called an interocular lens. 

Digital Artifact: Bailey Vanderee & Esther Langendoen