Diagram of the eye showing its different structures: cornea, lens, etc. With legend (sclera not shown in this diagram).
The cornea is a transparent structure, which delimits the anterior part of the eye. It is largely responsible for total ocular refraction which forms an accurate image on the retina.
It consists of six layers positioned to obtain a fully transparent structure free from blood vessels. It contains many nerve endings and is therefore highly sensitive.
It is nourished from the front due to tears and from the back due to the aqueous humour (the liquid found between the cornea and the iris).
The lens is a transparent, biconvex structure free from blood vessels. It is situated directly behind the iris. It is formed of 65% water and 35% proteins.
The lens has the capacity to change its shape depending on the focal distance, to be able to see clearly both up close and at a distance. When it needs to focus on a distant object, the lens relaxes; by contrast, when it needs to focus on a near object, it contracts giving more power to the eyeball to be able to see in detail.
This ability diminishes gradually with age and presbyopia appears. For this reason, optical aids are needed to read or perform up-close tasks. Additionally, when the tissues of the lens lose their properties and transparency, a cataract appears, causing deterioration in vision.
The sclera is a thick, resistant, opaque membrane which forms the white part of the eye. Its consistency is the result of its formation from highly resistant connective tissue fibres. It is the outermost layer of the eyeball and its role is to offer it protection and structure. Thanks to the help of intraocular pressure (IOP) produced by the aqueous humour, the eye retains its spherical shape. The sclera also acts as an insertion point for the extraocular muscles, responsible for making the eye move.
The iris is a structure located between the cornea and the lens, which is pigmented and defines the colour of the eye. The more pigment, the darker the iris. It is circular in shape and has a round opening in the centre called the pupil, which is black in colour and varies in size according to the amount of light, due to the muscular fibres distributed throughout the iris. It is mainly composed of connective tissue.
The pupil is the black hole situated in the centre of the iris. Due to the muscular fibres of the iris, it can change size, and contract (miosis) or relax (mydriasis) to let in more or less light. This provides the retina with the right amount of illumination at all times to obtain the best possible visual quality.
The aqueous humour is a transparent liquid situated in the anterior chamber (between the cornea and the iris) and the posterior chamber of the eye (between the iris and the lens). These two small spaces are connected together by the pupil. This substance is responsible for providing nutrition to the cornea and the lens and also contributes to maintaining the correct shape of the anterior pole of the eye.
The amount of aqueous humour inside the eye is directly related to intraocular pressure (IOP), which is a very important value as it determines potential conditions such as Glaucoma.
The innermost layer of the eye is called the retina and is responsible for transforming light into electrical impulses, which reach the brain via the optic nerve to provide an accurate image. This transformation of light takes place in specific layers, which contain the two types of photoreceptors: cones and rods. Cones are responsible for colour vision whilst rods detect black and grey shades.
The retina plays an important role in the visual process. It is vital to perform examinations to ensure it remains healthy. When certain areas of this tissue become weaker or deteriorate, a condition known as “retinal detachment” can occur. This causes a severe loss in visual function and it is important to intervene as quickly as possible through surgery to try to return the retina to its original position.
The optic nerve is composed of many nerve fibres running from the retina to the brain. It is the structure responsible for transporting the nerve impulses produced by the cones and rods (photoreceptors situated on the retina) to the brain. This is how image recognition takes place.
For accurate vision, the optic nerve must be healthy. When there are lesions in this area and the nerve fibres are destroyed, a visual field defect can occur resulting in a loss of vision. This is the case with Glaucoma, a condition caused by an increase in intraocular pressure (IOP) when an imbalance occurs between the segregation and drainage of the aqueous humour. In this case, it is very important to monitor the state of the optic nerve because once it has been affected, the damage is irreversible.
The macula is the area of the retina with the highest concentration of cones (responsible for colour vision) and which provides the most detailed and clear vision possible. In a healthy eye, the rays must focus on this area to produce an optimum image in terms of quality, precision and detail.
When there is a lesion in this area, all high-precision visual activity is compromised, as the central visual field is affected. The patient encounters problems when reading, driving or even seeing people’s faces. This also happens with AMD (Age-related Macular Degeneration), a condition which often affects people over the age of 50 and is caused by degenerative changes in the tissues.
The vitreous humour is a transparent gelatinous substance which fills the largest cavity in the eyeball: the vitreous cavity. The anterior cavity wall is formed by the lens whilst the rear is delimited by the retina. It is composed of hyaluronic acid and collagen fibres which give it its structure. Any residue which becomes detached from the wall of the retina ends up floating in the vitreous humour which is why “floaters” often appear, which manifest themselves as moving black dots. The pressure exercised on the walls helps the eye to retain its shape.
The sclera is the “white” of the eye. It is a relatively thick and opaque layer, consisting of two finer layers of tissue – conjunctiva and episclera. In these two layers there are visible blood vessels. The thickness of the sclera varies and has its strongest structure in the posterior part of the eye.
The conjunctiva is a transparent mucous membrane which coats the anterior part of the eyeball and the posterior surface of the eyelids, forming what is known as the conjunctival sac. Despite being a continuous layer, it can be divided into three parts:
- Palpebral conjunctiva: the part which is attached to the eyelid at the anterior part, from the lid margin.
- Fornix conjunctiva: situated between the palpebral and bulbar conjunctiva, at the fold of the eyelid
- Bulbar conjunctiva: coats the anterior sclera, from the conjunctival sac to the limbus.
Its cells produce a mucus which helps to lubricate and disinfect the eye. This layer of mucus constitutes the innermost layer of the tear film, which is of vital importance for correct functioning and hydration of the eye.
On occasion, the conjunctiva may become inflamed. This phenomenon is called “conjunctivitis” and can be caused by many factors, such as bacteria or viruses. An allergy may also be the cause of this inflammation of the blood vessels. It does not usually affect vision but does cause redness, stinging, foreign body sensation, excessive tearing, palpebral swelling, etc.
The upper and lower eyelids are folds of skin which are responsible for protecting the eyeball, closing the orbits and helping to lubricate the eye through the tear film. Each eyelid has two surfaces, the anterior (formed of skin) and the posterior (coated by the palpebral conjunctiva). They have two edges, one free and the other attached. On their free edges, are the eyelashes and next to them the sebaceous glands.
The eyelids comprise four layers: the skin, the muscular layer, the tarsus (fibrous layer) and the conjunctiva. The Meibomian glands are located inside the tarsus, and produce a lipid secretion which forms part of the tear film. The tarsus also contains the glands of Zeiss and Moll, situated on the lid margin near the eyelashes, which offer additional protection to the eye.
Poor lipid secretion by the palpebral glands can cause a condition known as blepharitis, an inflammation in the eyelids which causes stinging and irritation.
The tear system starts with the production of the tear film. The principal constituents of tears are produced in the main tear gland, situated on the superior lateral side of the eyeball. Tear formation is also contributed to by the Meibomian glands found on the eyelids. Due to the mechanism of the eyelid, tears are distributed throughout the ocular surface before being directed to its inner edge, where they are drained through the tear duct to the nose. This tear duct starts at the superior and inferior lacrimal puncta, situated on the edges of the eyelid, and continues with the canaliculi, which move the tears to the lacrimal sac. Through this process, the tear reaches the nose through the nasolacrimal duct.
The tear film is composed of three layers: the innermost mucous layer, which is in direct contact with the cornea; the middle aqueous layer which comes from the main tear gland; and finally, the outermost lipid layer, which is produced by the Meibomian glands situated on the eyelid.
It is extremely important for all of the structures of the tear system to function correctly and for the tears to be of a good quality. Low tear production can cause discomfort and visual problems in the cornea and conjunctiva, as the surface of the eye is not being properly lubricated. This phenomenon is known as “dry eye”.
The ciliary muscles are situated on the anterior part of the choroids (middle layer of the eyeball) and form a ring of striated muscles. Secured to this muscle are the ligaments which suspend the lens. This area is known as the “zonule of Zinn”. The contraction and relaxation of the ciliary muscles changes the shape of the ligaments of the zonule, to enable the lens to focus on near or distant objects according to visual needs and to provide the correct accommodation.