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Eye CareAdults

Anatomy of the eyeEdited by Penelope Stanford, Senior Lecturer, University of Manchester

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Do not undertake or attempt any procedure unless you are, or have supervision from, a properly trained, experienced and competent person.Always first explain the procedure to the patient and obtain their consent, in line with the policies of your employer or educational institution.

Structures of the anterior segment of the eye

A basic knowledge of anatomy is necessary to be able to examine the eyes effectively.

Palpebralconjunctiva

Superiorfornix

Bulbarconjunctiva

Zonules

Hair follicles

Inferior fornix

Ciliary body

ConjunctivaThe conjunctiva comprises:

• The palpebral conjunctiva, which lines the inner eyelids;• The bulbar conjunctiva, which covers the entire sclera (the white of the

eyeball) from the cornea back to the optic nerve; and• The superior and inferior fornices, where the palpebral conjunctiva gently

folds back to form the bulbar conjunctiva.

To allow for easy movement of the eyeball, the conjunctiva is loosely attached, especially within the fornices where it forms folds.

CorneaThe cornea is clear, to allow unobstructed passage of light, and convex, to bend and focus light rays onto the retina; it has a refractive power of 40–44 dioptres. The cornea is 0.6 mm thick centrally, and 1.0 mm thick peripherally, with a diameter of 10–12 mm. It is highly sensitive, to protect the eye. The cornea has five layers; it is kept transparent by the sodium pump within the endothelial (inner) layer; when this pump breaks down, the stromal (middle) layer becomes waterlogged and loses transparency. The corneal epithelium is the only layer of the cornea to regenerate following damage.

Crystalline lensThe lens sits in the posterior chamber, behind the iris and in front of the vitreous (posterior cavity). The lens is a transparent structure, which is biconvex and avascular. It is supported by suspensory ligaments called zonules that arise from the ciliary processes. The lens changes shape during “accommodation”: the action of viewing either near or distant objects. The ciliary muscles contract and relax, making the lens either spherical for near vision or elongated during distance vision. The refractive power of the lens is 20 dioptres. The ageing process and some systemic conditions can cause the lens to thicken and lose transparency, and affect the ability of the lens to change shape and focus. This condition, called presbyopia, explains why some people require spectacles for reading.

Anterior chamberThe anterior chamber is behind the cornea and in front of the iris. Aqueous humour, a clear fluid produced by the ciliary body to nourish the cornea and maintain the intraocular pressure, fills the anterior chamber.

PupilThe iris controls the amount of light entering the eye through the pupil using two sets of smooth muscle fibres. In bright light, the circular muscles in the iris contract around the pupil and the radial muscles relax; conversely, the pupil dilates in dim light as the radial muscles contract and the circular muscles relax.

IrisThe iris is the coloured part of the eye. The iris is made up of muscle fibres that control the central aperture, which is known as the pupil.

Sagittal section of the eye

Eye Care

Adults

Anatomy of the eye Page 2

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Do not undertake or attempt any procedure unless you are, or have supervision from, a properly trained, experienced and competent person.Always first explain the procedure to the patient and obtain their consent, in line with the policies of your employer or educational institution.

Macula

Fovea

Optic disc

Hyaloid canal

Superior fornixThe conjunctiva folds in the fornices, forming pockets that allow for the movement of the eyeball.

Vitreous humourThis is a gel-like substance that fills the posterior cavity of the eye. It is loosely attached to the retina at the ora serrata and the optic disc.

ScleraAn opaque, tough outer coating of the eye that provides protection and an anchor for the external ocular muscles.

ChoroidThis vascular layer is responsible for most of the blood supply to the eye. The choroid also helps to drain aqueous humour via the uveoscleral route.

Ciliary processesThe ciliary processes are folds of vascular tissue in the inner surface of the ciliary body. They secrete aqueous humour which maintains intraocular pressure.

Inferior fornixThe conjunctiva folds in the fornices, forming pockets that allow for the movement of the eyeball.

Suspensory ligamentsThese ligaments, also called zonules, contract and relax to change the shape of the lens, allowing the eye to focus large or near objects onto the retina.

RetinaThis is the neural layer of the eye. It contains very sensitive photoreceptor cells that transmit light via the optic nerve to the visual cortex in the brain.

Ora serrata

Limbus

Posterior chamber

Iris

Pupil

Lens

Anterior chamber(aqueous humour)

Canal of Schlemm

By facilitating eye movement, the extraocular muscles contribute to visual function. There are six extraocular muscles: the lateral rectus, medial rectus, superior and inferior rectus and the superior and inferior oblique. Both eyes should work in a co-ordinated manner. In order to look right, for example, the right lateral rectus muscle contracts and the right medial rectus relaxes, while the left medial rectus contracts and the left lateral rectus relaxes. These muscles allow movement that defines the limit of a person’s visual field.

Superior rectus

Lateral rectus

Inferior rectus

Inferior oblique

Superior oblique

Trochlear tendon

Medial rectus

Extraocular muscles controlling eye movement

Frontal view of the eye and surrounding area

Eye Care

Adults

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Do not undertake or attempt any procedure unless you are, or have supervision from, a properly trained, experienced and competent person.Always first explain the procedure to the patient and obtain their consent, in line with the policies of your employer or educational institution.

Lacrimal gland

Upper eyelid

Iris

Limbus

Cornea

Lateral palpebral artery

Bulbarconjunctiva(covering sclera)

Transverse facial artery

Pupil

Lower eyelid

Supraorbital artery

Supratrochlear artery

Superior tarsal plate

Upper punctum

Lacrimal caruncle

Medial palpebral artery

Lacrimal sac

Angular artery

Medial canthus

Plicasemilunaris

Nasolacrimal duct

Inferior tarsal plate

Lower lacrimal punctumThe punctum at the medial aspect of the lower eyelid is the easiest to observe, having the appearance of a small hole, but there is also a punctum at the medial aspect of the upper lid. Through these openings, tears drain into the lacrimal duct and into the back of the throat.