Vision stands out among human sensoperceptive systems because of its high complexity. The structure of the eye, the main organ of sight, is a good example of this, to the extent that it has come to be used as a supposedly irrefutable argument by those who defend that life was created and designed by a god.

The analysis of the parts of the eye can be extended to a great extent as the organs of vision are composed of many structures. In this article we will focus on the main ones and on the general description of the process of transduction that makes light energy to be perceived as images.

What is the eye?

The eyes are the basis of the visual system. These organs transform light energy into electrical impulses which, when transmitted to the visual cortex of the occipital lobe, allow the three-dimensional perception of shape, movement, colour and depth.

Eyeballs are spherical in shape and have a diameter of approximately 2.5 cm. They are divided into two sections: the anterior and posterior chambers, filled respectively with aqueous and vitreous humour, liquids that regulate intraocular pressure. The anterior chamber is smaller and is located between the cornea and the iris, while the posterior chamber consists of the rest of the eye.

Unlike other sensory organs, the eye is partially derived from the central nervous system . Specifically, the retina, which receives light information, develops from the diencephalon, the embryonic structure that also gives rise to the cerebral hemispheres, the thalamus and hypothalamus.

In the retina we find two types of photoreceptors, rods and cones . While the cones allow for daytime vision and the perception of colour and detail, the rods are adapted for night vision and produce low resolution black and white images.

Parts of the eye and their functions

The eyes work in a similar way to cameras. The lens is adjusted according to the distance of the stimulus, serving as a sort of lens that allows the refraction of light; the pupil is the diaphragm through which the image enters the eye and is projected onto the retina, from where it will be sent to the brain through the optic nerve.

1. Cornea

The cornea is the front part of the eye and is in contact with the outside. It is a transparent structure that covers the iris and the crystalline lens and allows light refraction . Tears and aqueous humor allow the correct functioning of the cornea, since they perform functions equivalent to those of blood.

2. Iris

This structure separates the anterior and posterior chambers of the eye. The iris dilator muscle increases the pupil size (mydriasis) and the sphincter muscle reduces it (myosis). The iris tissue is pigmented due to the presence of melanin ; this gives rise to the colour of the eye, by which we can easily identify this structure.

3. Pupil

There is a circular hole in the centre of the iris which allows to regulate the amount of light entering the eye as it changes size as a result of mydriasis and myosis; this opening is the pupil, the dark part which is situated in the centre of the iris.

4. Crystalline

The crystalline lens is the “lens” behind the iris that allows visual focus. Accommodation is the process by which the curvature and thickness of the lens is modified to focus on objects as a function of their distance . When light rays pass through the lens, the image is formed on the retina.

5. Aqueous humor

The aqueous humor is found in the anterior chamber of the eyeball, between the cornea and the lens. It nourishes these two structures and allows the eye pressure to remain constant . This liquid is composed of water, glucose, vitamin C, proteins and lactic acid.

6. Sclera

The sclera covers the eyeball, giving it its characteristic white colour and protecting the internal structures. The anterior part of the sclera is attached to the cornea, while the posterior part has an opening that allows the connection between the optic nerve and the retina.

7. Conjunctive

This membrane covers the sclera. It contributes to the lubrication and disinfection of the eyeball as it produces tears and mucus, although the lacrimal glands are more relevant in this respect.

8. Choroids

We call “choroids” the layer of blood vessels and connective tissue that separates the retina and the sclera. The choroid provides the retina with the nutrients and oxygen it needs to function properly, as well as maintaining a constant temperature in the eye.

9. Vitreous humor

The posterior chamber of the eye, which is situated between the lens and the retina, is filled with vitreous humour, a gelatinous liquid of greater density than the aqueous humour of the anterior chamber. It constitutes the majority of the eyeball and its functions are to provide rigidity, cushion impacts, maintain intraocular pressure and fix the retina.

10. Retina

The retina is the true receptor organ of the visual system since it is in this structure that the rods and cones, the photoreceptor cells, are located. This membrane covers the back of the eye and has a function similar to that of a screen: the lens projects the perceived images onto the retina, from where they will be transmitted to the brain via the optic nerve.

Specifically, the rays of light are received by the area of the retina known as the phovea , which, being very rich in cones, has great visual acuity and is therefore the main responsible for the vision of detail.

11. Optic nerve

The optic nerve is the second of the twelve cranial pairs. It is a set of fibres which transmit light impulses from the retina to the cerebral optic chiasm . From this point, visual information is sent to other areas of the brain in the form of electrical signals.