The Anatomy Of The Eye

The eyeball is an organ approximately 2.5cm in diameter. The eye functions in the body in the same way as a camera. The clear convex structure on the outside is called the cornea. Light entering the eye through the pupil and lens on the cornea is collected on the retina at the back of the eye. Just as a picture is formed in a camera, so the retina creates an imag and this is carried to the brain by the optic nerve and what we perceive as vision is enabled by an area situated at the back of the brain.

Figure 1.  Anatomical demonstration of the eye.

The cornea is the clear layer forming the anterior surface of the eye which is in contact with the outside environment. The cornea is highly resistant to breakage. Contact lenses are worn on the cornea.

The sclera is a hard layer which appears white from the outside and protects the inner layers. On the outer surface of the sclera are 6 attachment points of the eye external muscles. These muscles provide the necessary eye movements.

The choroid is below the sclera, that is, in the inner part of the eye. This layer, which is rich in blood vessels, plays an important role in the nutrition of the eye. The choroid is adjacent to the innermost light-sensitive retinal layer. In the forward-looking part of the eye, the choroid provides thhe continuity of the iris and the ciliary body.

The eyelids function as protection for the eye against light and trauma and in addition, create the necessary fat and mucus expression to form the tear fim layer responsible for clear vision. With the blinking of the eyelids, tears are spread over the surface to form a thin tear film layer which keeps the eye moist and thus enables comfortable movement. 

The eyelashes prevent dust and similar substances entering the eye.

The lacrimal glands, also known as tear glands, are located in the upper part of both eyes inside the bone structure below the eyebrow. After the tears expressed from here have spread over the whole surface of the eye, the excess flows into the nasal passage via the ducts at the corner of the eye.

Figure 2. Color fundus photography of the posterior segment.

The anterior chamber angle of the eye is formed at the conjunction of the cornea and the iris. The fluid inside the eye, called aqueous humour, leaves the eye from this area to the outside via small canals. If the drainage is inadequate, the pressure inside the eye reaches high levels causing damage to the optic nerve and thus leading to loss of vision.

The iris is the part of the eye which appears coloured from the outside. The dark area at the centre of the iris is called the pupil and light enters the eye from here. The pupil widens in the dark allowing more light to enter. 

The lens inside the eye is a fairly clear and flexible structure, located immediately behind the iris and pupil. It is reponsible for focussing the lightwhich enters onto the retina. The lens is suspended inside the eye with zonular fibres attached to a circular structure rich in muscle fibres and known as the ciliary body. When the lens is looking close-up, the muscles of the ciliary body relax to allow the close object to be seen more clearly. With advancing age, we start to lose this ability and after the age of 40 years it may be necessary to wear glasses for close work.

The vitreous cavity is behind the lens and in front of the retina. A gel-like fluid of approximately 4ml volume is found in this cavity. This gel maintains the shape of the eyeball by protecting the light permeability of the optic system for the realisation of vision.

The retina is located at the back of the eye and perceives optic energy with photoreceptor cells and by changing this to electric signals transfers it to the optic nerve. It is adjacent to the vitreous and choroid layer. Between the retina and the choroid are the retina pigment epithelial (RPE) cells. The function of these cells is to reabsorb dispersed light and to provide vitamin A to the photoreceptor cells for the chemical function of vision. There is a multi-layered neurosensorial layer formed of nerve-rooted different cells over the epithelial layer. The most important cells of the neurosensorial layer are the rods and cones which are retinal photoreceptor cells. The rods enable night vision. Three different pigments are found in the cones which are sensitive to differences in wave length. These different pigments of red (564 nm), green (533 nm) and blue (437 nm) are activated in the presence of light in the wavelength. Thus cones enable coloured and light vision.

The optic nerve changes the light energy in the retina to electric energy and transfers the information along the nerve fibres to the occipital cortex of the brain.