The eye is the sensory organ of sight. It is an elaborate photoreceptor detecting information, in the form of light, from the environment and transmitting this information by a series of electrochemical changes to the BRAIN. The visual cortex is the part of the brain that processes this information (i.e. the visual cortex is what ‘sees’ the environment). The eyes sit within bony cavities (orbits). Each orbit is situated on the front of the skull, one on each side of the nose. The eye consists of an outer wall of three main layers, and a central cavity divided into three.
consists of the sclera and the cornea; their junction is called the limbus.
This is white, opaque, and constitutes the posterior five-sixths of the outer coat. It is made of dense fibrous tissue. The sclera is visible between the eyelids, as the ‘white of the eye’. It is covered by the transparent conjunctiva. At the back is a hole through which nerve fibres from the retina leave the eye in the optic nerve.
The transparent, colourless anterior (front) one-sixth of the eye which allows light in. It is more steeply curved than the sclera and provides most of the focusing power of the eye (the lens acts as the ‘fine adjustment’). The cornea is supplied with very fine nerve fibres which make it exquisitely sensitive to pain.
is the junction between cornea and sclera. It contains the trabecular meshwork, a sieve-like structure through which aqueous humour leaves the eye.
consists of the choroid, ciliary body and iris.
A sheet of tissue lining the posterior (back) two-thirds of the sclera. It contains a network of blood vessels which supply the outer half of the retina.
A ring of tissue extending 6 mm back from the anterior edge of the sclera. The muscles of the ciliary body change the shape of the lens during ACCOMMODATION. The ciliary body is lined by cells that secrete aqueous humour.
A flattened muscular diaphragm that is attached at its periphery to the ciliary body, and has a round central opening – the pupil. By contraction and relaxation of the muscles of the iris, the pupil can be dilated or constricted (dilated in the dark or when aroused; constricted in bright light and for close work). The iris lies in front of the lens and forms the back wall of the anterior chamber of the eye. It is visible from in front, through the transparent cornea, as the ‘coloured part of the eye’. The pupil is merely a hole in the centre of the iris and appears black.
The retina is a multi-layered tissue (ten layers in all) which extends from the edges of the optic nerve to line the inner surface of the choroid up to the junction of ciliary body and choroid. It contains light-sensitive cells of two types: (i) cones, that operate at high and medium levels of illumination; they are for fine discrimination of vision and colour vision; (ii) rods, that function best at low light intensity and deal with black-and-white vision.
The retina contains about 6 million cones and about 100 million rods. Information from them is conveyed by the nerve fibres in the inner part of the retina, which leave the eye in the optic nerve. There are no photoreceptors at the optic disc (the point where the optic nerve leaves the eye). This is the blind spot.
The retina can be subdivided into several areas:
contains mainly rods and a few scattered cones. Visual acuity from this area is fairly coarse.
It occupies an area 4.5 mm in diameter to one side of the optic disc and can produce a high level of visual acuity. Cones are abundant here but there are few rods.
A small central depression at the centre of the macula. Here the cones are tightly packed; rods are absent. It is responsible for the highest levels of visual acuity.
There are three: the anterior and posterior chambers, and the vitreous cavity.
Limited in front by the inner surface of the cornea, and behind by the iris and pupil. It contains a transparent clear watery fluid, the aqueous humour. This is constantly being produced by cells of the ciliary body and constantly drained away through the trabecular meshwork of the limbus.
A narrow space between the iris and pupil in front and the lens behind. It too contains aqueous humour in transit from the ciliary epithelium to the anterior chamber, via the pupil.
The largest cavity of the eye. In front it is bounded by the lens and behind by the retina. It contains vitreous humour.
Transparent, elastic and biconvex in cross-section, it lies behind the iris and in front of the vitreous cavity. Viewed from the front it is roughly circular and about 10 mm in diameter. The diameter and thickness of the lens vary with its accommodative state. The lens consists of:
A thin transparent membrane surrounding the cortex and nucleus.
This comprises newly made lens fibres that are relatively soft. It separates the capsule on the outside from the nucleus at the centre of the lens.
The dense central area of old lens fibres that have become compacted by new lens fibres laid down over them.
Numerous radially arranged fibres attached between the ciliary body and the lens around its circumference. Tension in these zonular fibres can be adjusted by the muscles of the ciliary body, thus changing the shape of the lens and altering its power of accommodation.
A transparent jelly-like structure whose shape conforms to that of the vitreous cavity within which it is contained: that is, it is spherical except for a shallow concave depression on its anterior surface. The lens lies in this depression.
These are multilayered curtains of tissue whose functions include spreading a tear film over the front of the eye to prevent desiccation; protection from injury or external irritation; and the control of light entering the eye. Each eye has an upper and lower lid which form an elliptical opening (the palpebral fissure) when the eyes are open. At the free margin of each lid are the eyelashes and the openings of tear glands which lie within the lid.
The bony cavity within which the eye is held. There are various openings into the posterior part of the orbit – namely the optic canal, which allows the optic nerve to leave en route for the brain, and the superior orbital and inferior orbital fissures, which allow passage of nerves and blood vessels to and from the orbit. The most important structures holding the eye within the orbit are the extra-ocular muscles; a suspensory ligament of connective tissue that forms a hammock on which the eye rests and which is slung between the medial and lateral walls of the orbit; and the orbital septum, a sheet of connective tissue extending from the anterior margin of the orbit into the lids which helps keep the eye in place. A pad of fat fills in the orbit behind the eye and acts as a cushion for the eye.
A transparent mucous membrane that extends from the limbus over the anterior sclera or ‘white of the eye’.
There are six in all, the four rectus muscles (superior, inferior, medial and lateral rectus muscles) and two oblique muscles (superior and inferior oblique muscles). They are attached at various points between the bony orbit and the eyeball. By their combined action they move the eye in horizontal and vertical gaze. They also produce torsional movement of the eye (i.e. clockwise or anticlockwise movements when viewed from the front).
There are two components: a tear-production system, namely the lacrimal gland and accessory lacrimal glands; and a drainage system.
Tears keep the front of the eye moist; they also contain nutrients and various components to protect the eye from infection. Crying results from excess tear production. The drainage system cannot cope with the excess and therefore tears overflow on to the face.
Located below a small depression in the bony roof of the orbit. Numerous tear ducts open from it into predominantly the upper lid. Accessory lacrimal glands are found in the conjunctiva and within the eyelids: the former open directly on to the surface of the conjunctiva; the latter onto the eyelid margin.
This consists of:
An elevated opening toward the medial (inner) side of each lid. Each punctum opens into a canaliculus.
A fine tube-like structure running within the lid, parallel to the lid margin. The canaliculi from upper and lower lid join to form a common canaliculus which opens into the lacrimal sac.
A small sac on the side of the nose which opens into the nasolacrimal duct. During blinking, the sac sucks tears into itself from the canaliculus. Tears then drain by gravity down the nasolacrimal duct.
A tubular structure which runs down through the wall of the nose and opens into the nasal cavity.
Light stimulates the rods and cones of the retina. Electrochemical messages are then passed to nerve fibres in the retina and then via the optic nerve to the optic chiasm (junction). Here information from the temporal (outer) half of each retina continues to the same side of the brain. Information from the nasal (inner) half of each retina crosses to the other side within the optic chiasm. The rearranged nerve fibres then pass through the optic tract to the lateral geniculate body, then via the optic radiation to reach the visual cortex in the occipital lobe of the brain.