The human eye is an observation organ (a sense) which uses light to pass on an image to the brain. The eye consists of the actual eye, the eyeball, and the surrounding structures or adnexa. The eye has an average cross-section of about 2.5cm and weighs 7.5 grammes on average. The sclera of an eye has an average surface area of 17 square centimetres.
The cornea is the first structure which we see if we study the eye from front to back. This transparent structure is essential to the refraction of the eye due to its arched shape. This transparency is created by the pump function of the endothelial cells. These cells are situated on the inside of the cornea.
In recent years the cornea has been the subject of a lot of discussion in connection with the different types of refractory surgery.
Behind the cornea we enter the front chamber of the eye. This chamber is marked off at the back by the iris and the pupil opening is in the middle. The front chamber corner is situated on the periphery where the discharge of the aqueous humour is found. Naturally, this zone is important for glaucoma issues (see below).
The iris is the screen between the front chamber and the rear area of the eye. Small muscles in the iris are decisive for diameter of the central opening (the pupil).
The term pupil stands for ‘little man’, since people see themselves in the cornea reflex of someone else’s eye. The size of the pupil opening depends on the light intensity. The pupil opening also allows the circulation of the aqueous humour, which is created behind the iris screen near the corpus ciliare (ciliary body).
The lens is situated behind the iris and hangs on very thin fibres. This encapsulated and normally transparent organ owes its transparency to the maintenance of a metabolic balance. The lens refraction helps to determine the projection of an image onto the retina.
As the lens changes shape (i.e. becomes rounder), the focal distance changes and one speaks of accommodation.
Behind the lens we find the big cavity of the rearmost part of the eye chamber completely filled with a transparent viscous gel, namely the corpus vitreum or vitreous body.
Closely connected to the vitreous body is the retina. The retina consists of two layers:
- The exterior pigment layer and the interior nerve cells layer.
- In the centre of the retina we find the macula. This zone, due to its high density of cones, is the place which is responsible for the central visual acuteness.
The papilla is responsible for the ‘blind spot’. This is the start of the optic nerve which clusters together the retina nerve fibres.