• The tear film is an aqueous layer that covers the surface of the eye. The main functions of tears are: to serve as a source of oxygen for the corneal and conjunctival epithelia, remove harmful substances from the surface of the eye, act as the eye’s anterior refractory surface and fill any irregularities in the corneal epithelium, and provide lubrication between the eyelids and the eye surface. Tears also contain antibodies and antibacterial substances to protect the cornea from infection, and express pain and emotion when stimulated by the nervous system.

• The cornea is a transparent, elastic lens that forms the hard, sensitive, external tunic together with the white of the eye (sclerotic) that surrounds it. The cornea’s capacity to converge light is greater than any other structure or media of the eye (approximately 42 dioptres).

• The aqueous humour is a liquid that fills the eye between the cornea and the crystalline lens. It is a colourless, clear liquid that contributes to maintaining intraocular pressure and also provides nutrients to avascular ocular tissues such as the cornea, the crystalline lens and the vitreous humour.

• The iris is a diaphragm that regulates the entry of light rays before they reach the crystalline lens. The iris is on the anterior face of the eye. It is a pigmented membrane (responsible for eye colour) that is circular in shape with an orifice in the centre (the pupil). The basic function of the iris is to regulate the size of the pupil in keeping with the intensity of the light entering the eye.

• The crystalline lens is biconvex and flexible so its power consequently varies. Its function is to maintain light focused on the back of the eye by varying its curvature. Its refraction index is very high and its power is approximately 20 dioptres. The crystalline lens is elastic and transparent and contains no nerves or blood vessels.

• After passing through the crystalline lens, light rays reach the vitreous humour, which is a transparent, colourless, jelly-like liquid that fills the area between the lens and the retina.

• Finally, the light travels to the retina at the back of the eyeball. The retina is connected to the optic nerve at the rear of the eye (the so-called "blind spot") and can be considered to be a prolongation of the brain. The optic nerve carries nervous impulses to the visual cortex, where the visual stimuli of each eye are interpreted, analysed and merged. At the periphery of the retina are photoreceptor cells called cone and rod cells:

Cone cells: Cone cells are sensitive to colour and have a high resolution, even though they are very small. However, they need bright light to be able to function properly.

Rod cells: These cylindrical-shaped cells need less light to function than cone cells, but are not sensitive to colour and provide more blurred vision. They are responsible for night vision.

The fovea also forms part of the retina and is a small "pit" where cone-cell density is highest. This also means that vision is optimal.

THE MUSCLE SYSTEM
The muscles responsible for the movements of the eye are called rectus and oblique muscles and are located in the sclera. The stimulation of the muscles of both eyes at the same time makes it possible to move the eyes and look at any given spot within the field of vision.

An anomaly in any of the muscles will lead to strabismus. Surgery to correct such deviations consists of cutting and suturing the affected muscle, thereby making it possible to align the eyes and remedy the aesthetic difference between them.

THE NERVOUS SYSTEM
With the exception of the lateral rectus and the superior oblique, all the orbital muscles are stimulated by the oculomotor nerve. The lateral rectus is innervated by the abducens nerve and the superior oblique muscle by the trochlear or pathetic nerve. These nerves belong to the Peripheral Nervous System and a dysfunction can give rise to ocular-movement disorders.