Eye

An eye is a sensory organ found in animals. Its purpose is to detect light; more highly-evolved eyes are capable of forming images. It is positioned in an orbital cavity to provide it with maximum protection whilst still allowing full range in order for the eye to function.

Overview of Anatomy
The eyeball itself is held in position for it is embedded into the fat layer within the orbital cavity. However, in order to allow for movement, the eye is covered in a thin membrane called the capsule of Tenon. This membrane has a smooth inner layer and is perforated in certain positions such as the connection to the muscles. Aside from these perforations, the membrane covers the entirity of the eye.

The eye can predominantly be divided into three different layers, each containing different aspects of the organ. The outer layer contains the sclera and the cornea. The sclera is the white part of the eye and is made up of connective tissue. This makes up the majority of the outside of the eye but at the front sits the cornea which is the structure that allows light to enter the eye. The next layer is the middle layer and this is made up of the ciliary body, the choroid and the Iris. The ciliary body contains muscles called ciliary musles which are attached to the lens of the eye by zonular fibres. The zonular fibres are made up of connective tissue. The choroid contains blood vessels which are involved in supplying the eye with nutrition and are located underneath the sclera. The iris is made up of two layers of pigmented muscle and is located at the front of the lens. The pigments are responsible for giving individuals their eye colour. In the centre of the iris lies the pupil which is a hole in the iris that allows light to enter the eye. The iris is in control of the pupil and can cause it to constrict or dialate depending on the environment. The final layer is the inner most layer which holds the retina. This is made up of neural tissue and contains the photoreceptors. The receptors are either rod or cone cells which detect different sorts of light. The main role of the retina is to convert light to electrical energy. Outside of the retina is the retinal pigmented epithelium which is attached to the choroid. It contains melanin which will absorb any light that hits the back of the eye to prevent reflection.

There are a number of other structures that play a role in the structure of the eye. The lens and ciliary body split the eye into two chambers which are filled with fluid. There is an anterior segment which sits in front of these and this can be further divided into two chambers; the anterior chamber and the posterior chamber. This segment contains aqueous humor which is a watery fluid responsible for supplying nutrients to the cornea and the lens. The fluid is essential because it is transparent and therefore light can pass through easily. If this part of the eye used blood vessels to provide nutrients, the red of the blood would hinder the light entering. There is another segment of importance called the posterior segment or also known as the vitreous chamber which contains a jelly like substance to maintain the shape of the eye. This segment is located behind the lens.

Retina
The retina boasts a complex structure and is important to the function of the eye. There are over ten main regions of the retina varying from membranes to pigment layers. Two areas especially are of high importance and these are the fovea and the optic disk. The fovea is located in the centre of the retina where the light from the visual field hits and the fovea is the area with the greatest visual acuity. The optic disk is the point at which the optic nerve and blood vessels pass through the retina. There are no photoreceptors in this part of the retina and it is therefore referred to as the blind spot so light striking this area cannot be perceived.

There are three main layers that make up the retina. The inner layer is made up of neurons called ganglion cells. The middle layer contains bipolar neurons and amacrine and horizontal cells which are responsible for the communication between neurons. The outer layer contains the rod and cone cells which are the photoreceptors. In order to make sure that light gets directly to the fovea where the cone cells reside, there is a depression in the centre of the retina called the macula lutea. Cone cells are situated within the fovea and rod cones are found in increasing proportion away from the fovea. Only rod cells are present at the periphery of the retina.

Rod and Cone cells
The photoreceptors are responsible to recieiving and transducing wavelengths of light. In order to see both colour and black and white, two types of photoreceptors need to be present. Rod cells are sensitive to dim light and they are the cells responsible for seeing black and white light. They can be activated by one photom of light. When the light is very bright, the rod cells become saturated or bleached and they cannot recognise additional brightness. Cone cells however, are not so easily acitvated. They need high intensity light in order to become activated and there are three types of cone cells which each respond to a different wavelength of light. This provides an individual with colour vision.

At low levels of luminance only the rod cells are activated and at high levels only the cone cells are activated. At intermediate levels however, both types of photoreceptors are activated which creates mesopic vision.

Colour Vision
In highly evolved eyes, the individual can percieve colour. This is carried out by the cone cells and can only occur in bright light. Different types of cone cells respond to different types of wavelength. For example, blue cones respond to light at a wavelength of 420 nm whereas green cones are more sensitive at a wavelength of 530 nm and red cones at 560 nm. These spectrums do overlap so that many different colours can be perceived. Therefore, our brain can decide what colour we are seeing based off the pattern of cone cells and the amount of cone cells activated at a given time.

Colour Blindness
Some individuals have a condition that renders them incapable of seeing certain colours. This is referred to as colour blindness and the cone cells are responsible for this trait. If an individual lacks one of the types of cone that perceives colour then they cannot register that wavelength of light. Red-green colour blindness is the most common condition whereby an individual cannot tell the difference between the two colours. This is usually caused by a genetic defect in either of the red or green cones and involves the photopigments.

Individuals are colour blind because they have inherited the genetic defect. The genes that code for the photopigments are recessive and can be found on the X chromosome. Therefore as males only have one X chromosome, they are more likely to inherit the genetic trait. Blue colour blindness is another condition but it is much rarer and does not have a link to the X chromosome.

Defects in Vision
There are a number of different conditions that can affect the workings of the eye. If the waves of light entering the eye are not focused properly then an individual will experience blurred vision and corrective lenses will be used to help improve vision.

Myopia is a common visual defect and it is referred to as near-sightedness. The individual will be able to see objects that are close to them quite clearly but will struggle to see distant objects. This problem arises due to the fact that the lens or cornea is bending the light rays too much because they are too strong for the length of the eyeball. The light from distant images appears blurred. In order to solve this problem, a concave lens is used which will diverge the waves of light before they reach the eye.

Hyperopia or far-sightedness is another common problem. The lens/cornea is too weak in this instance for the length of the eyeball. Therefore it is the opposite of myopia and distant images are clearer than close images. In this case, a convex lens is placed over the eye causing the waves of light to converge before reaching the eye.

Many other conditions can affect the eyes. Astigmatism is where the light waves bend erratically due to irregularities on the surface of the lens. Other conditions increase with age such as Presbyopia where the lens actually hardens and loses elasticity so that it is harder to see nearby objects. Cataracts are also problematic and a sign of ageing. The lens decreases in transparency due to the appearance of an opacification. Also, glaucoma is a serious eye condition usually affecting the elderly. The aqueous humour increases in volume which puts pressure on the eye cavity which can cause the eyeball itself to distort and the lens will shift in position. This in turn puts pressure on the vitreous chamber which compresses the optic nerves or blood vessels. This can dramatically decrease the blood supply to the eye and result in blindness.