The Eye

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This section provides a basic overview of the structures that make up the anatomy of the eye.

Diagram of human eye anatomy

Cornea

The cornea is a clear, dome-shaped structure forming the most anterior (front) part of the eye. It consists of five layers of tissue laminated together with each layer having a specific role in maintaining the integrity, health, and function of the cornea.

The primary role of the cornea is to bend light entering the eye and to focus it clearly on the back of the eye (retina). The outer layer, the epithelium, also has a protecting function. It consists of highly regenerative cells allowing the whole epithelial layer to be replaced every ten days. This is necessary to facilitate the fast healing of superficial injuries and is of particular interest to the Optometrists when fitting contact lenses and Orthokeratology.

Pupil

The pupil is the “black circle” within the coloured part (Iris) of the eye. The pupil is a hole formed by the Iris tissue. The pupil diameter is controlled by the Iris muscle tissue opening and closing the pupil size based on the amount of light that needs to enter the eye.

The iris size aperture function is similar to the F-stop aperture function found with SLR photography. The intensity of the light (brightness) reaching the retina must be within a constant range. If the light is too bright, the pupil size will become smaller to allow less light through and in dark light the pupil size will become bigger to allow more light to reach the retina.

Iris

The iris is the coloured part of the eye surrounding the pupil. The word Iris comes from the Greek word iris (gen.iridos) meaning “rainbow.” This is due to the many colours the iris can be, ranging from different hues of brown to green and grey to blue

The primary function of the pigmented iris is to regulate the aperture size of the pupil through contraction or dilation of the iris muscles.

Lens

The crystalline lens is a clear biconvex lens structure behind the iris. Its primary function, together with the cornea, is to focus the light entering the eye onto the macula of the retina.

Diagram explaining the eye accommodation system

The crystalline lens is flexible and can change shape and thus the focus of the eye, due to forces exerted on it by the ciliary muscle. For any object that is further than 6 meters from the eye, the focal system is at rest or in a neutral position to allow the light to focus on the retina. For any object closer than 6 meters the focal system activates and the lens shape is changed to maintain clear near focus, a process known as accommodation. The closer the eye needs to focus the more energy is spent by the ciliary muscle and the more the lens changes shape.

As we age, the lens also ages, becoming less flexible. When we reach our 4th decade of life the lens has lost so much of its flexibility that it becomes difficult to focus on near objects. This condition is called Presbyopia. It is important to note that the main cause of Presbyopia is not the ciliary muscles that have become weak or lazy but it is the crystalline lens material that has thickened and lost elasticity.

As the lens ages further and loses more elasticity, it also starts to lose its clarity. Typically from the 6th to 8th decade of life the lens gradually begins to turn yellow and later will form opacities, obscuring vision. When clouding or opacities are noticed in the eye, it is called a cataract.

Ciliary Body

The ciliary body is located just behind the iris and is a round tissue made up of the ciliary muscles and the ciliary processes.  The ciliary muscles are located above and below the crystalline lens and are attached to the lens via tiny fibrous strands called the zonule of Zinn.

During the accommodation process, the ciliary muscles will contract, relaxing the zonule fibres and forcing the lens more convex, allowing the eye to focus on near objects. When the eyes focus on distance objects, the ciliary muscles will relax which tightens the zonule fibres and flattens the lens shape, resulting in improved distant focus.

Ora Serrata

The ora serrata is the junction between the retina and the ciliary body. It is the area where the retina transitions from a non-photosensitive area to a multi-layered photosensitive region. One of the primary functions of the ora serrata is to secrete the aqueous humour to provide nutrients to the internal structures of the eye.

Retina

The retina consists of a complex network of light-sensitive nerves lining the inside wall of the eye. The innermost retinal layer contains millions of photoreceptors, called rods and cones that convert light rays into electrical impulses.

Most of the cones are found in the macula, the area responsible for central vision. The fovea which is the center part of the macula is responsible for our high definition colour sight and contains the most densely packed cones.

The rods are more prominent in the peripheral regions of the retina and are responsible for our night vision (light and dark contrast) and movement perception.

The electrical impulses generated by the photosensitive retina are transferred via the optic nerve to the visual processing centers in the brain.

Optic Nerve

The optic nerve or intracranial nerve II is part of the central nervous system. The primary function of the nerve is to carry electrical impulses through a complex tract from the retina to the occipital lobes of the brain for visual processing.

Anatomically the optic nerve is very susceptible to damage due to increase pressure in the ball of the eye. A condition called glaucoma. It is, therefore, important to have regular eye examinations to rule out signs of glaucoma.

Sclera

The sclera is the outer white part of the eye, providing structure, strength, and protection to the eye. High amounts of myopia (near-sightedness) are mainly due to the sclera thinning in the posterior (back) section during our early years of life (approximately years 6 to 28). The stretched sclera elongates the ball of the eye causing the light to focus in front of the macula (see retina).

It is important to note that the sclera contains no muscle tissue and exercising the muscles of the eye has no effect on the thickness or shape of the sclera and therefore has no effect on the refractive error of the eye.

There is, however, six extra-ocular muscles attached to the sclera which controls the eyes movements. By exercising these muscles, the tracking capabilities of the eye can be improved resulting in better hand-eye coordination and increased reading speed.

Vitreous Humour

The vitreous or vitreous humour is a transparent, gel-like substance filling the space between the crystalline lens and the retinal lining in the back of the eye. It is composed mainly of water and helps to give the eye is spherical form and shape. The outer part of the vitreous, the cortex, is in contact with the retina but is not adhered to it except in three places, namely the ora serrata, the macula and the optic nerve disc.

With aging in the eye, the vitreous tends to liquefy in the center which is a condition known as syneresis. Syneresis allows cells to float freely in the vitreous and is seen as black spots or cobwebs in the visual field. This occurrence is commonly called floaters or vitreous floaters and is harmless.

Another result of syneresis of the vitreous is a condition called posterior vitreous detachment or PVD. With PVD, the vitreous cortex physically pulls away from the retinal layer causing a temporarily short circuit in the retina. This is typically observed as a flash of white light normally followed by numerous floaters. Flashes and floaters should be seen as serious as it can indicate a retinal tear and should be examined immediately.

Aqueous Humour

The aqueous humour is a watery substance filling the space between the cornea and the crystalline lens. It is secreted by the ciliary body and then flows between the lens and the iris to the anterior (front) chamber of the eye. The aqueous drains out of the eye through the trabecular meshwork which is an area of spongy tissue situated in the angle formed by the base of the cornea and the iris-ciliary body.

Some of the functions of the aqueous humour include the provision of nutrients and immunoglobulins (antibodies) and the removal of waste products from the anterior (front) chamber of the eye. Most importantly it is part of the optical system that helps to focus the light onto the macula and maintains a constant intraocular pressure to inflate and maintain the shape of the eye.

If the intraocular pressure exceeds a threshold of approximately 22 mmHg of pressure, damage can occur to the optic nerve head causing a condition called glaucoma. Uncontrolled glaucoma typically will lead to peripheral visual field loss and ultimately to full visual loss.

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