VOL: 103, ISSUE: 29, PAGE NO: 26
Marion Richardson, BD, DipN, CertEd, RNT, RN, is principal lecturer, University of Hertfordshire
This article is the first in a three-part series on the eyes, which examines the structures of the eye, the visual pathway and disorders. It forms part of a larger series on the special senses.
STRUCTURES OF THE EYE
The eyes are situated in the bony orbit of the skull. Each is approximately 1.5-2cm in diameter and is attached to three pairs of extraocular muscles, which allow it to move in all directions. Its main purpose is to convert light energy into nervous impulses that can be interpreted by the brain as visual images (Allan, 2005).
Each eye is formed of three layers of tissue: the sclera, the uvea and the retina (Fig 1).
- The sclera is the tough outer layer of fibrous tissue, which keeps the shape of the eye and provides a strong attachment for the extraocular muscles. The ‘white’ of the eye is the front part of the sclera (although it ranges in colour from almost blue in childhood to yellow in old age) and the cornea, the protective ‘window’ through which light energy enters the eye. The cornea has a rich nerve supply. It is extremely sensitive to pain and is very exposed and easily damaged. As the cornea has no blood supply it is the most readily transplanted tissue in the human body as there is no immune reaction and no fear of rejection (Marieb, 2006).
- The uvea, the middle layer, contains the pupil, the iris, the ciliary muscle and the choroid (Fig 1). The choroid forms the posterior five-sixths of this layer and contains the pigment melanin which absorbs light. The iris is an involuntary muscle in the front part of the choroid, as is the ciliary muscle. Some of the fibres of the iris are arranged like spokes in a wheel - when they contract, the pupil (which is a hole in the doughnut-shaped iris muscle) opens and lets in more light; others are circular and contraction causes constriction of the pupil. In this way, the size of the pupil changes to reduce the entry of light rays in bright light and to allow more light to enter the eye when the light is dim.
- The retina is the innermost layer covering the posterior two-thirds of the wall of the eye. Its surface is covered with a ‘mat’ of nerve fibres that convert light into nerve signals. These nerve fibres collect together to form the optic nerve which leaves the eye at the optic disc, the ‘blind spot’ of each eye. The retinal nerves take information to the brain from photoreceptor cells, the rods and cones (these will be discussed in next week’s article).
The lens is biconvex and avascular (blood vessels interfere with vision) and sits directly behind the pupil, held in place by a ligament attached to the ciliary muscle (Fig 1). The lens is normally clear but can turn milky and hard and lose its transparency - a condition termed a cataract. Cataracts can be removed and replaced with an artificial lens.
The hollow spaces inside the eye are filled with fluids. Aqueous humour in the anterior cavity is similar in composition to blood plasma, while jelly-like vitreous humour fills the posterior cavity. They help to maintain the shape of the eye and bend light rays to focus them on the retina. Aqueous humour is constantly formed and drained from the eye and if this drainage is blocked, the internal pressure of the eye increases (glaucoma) and can cause blindness.
THE VISUAL PATHWAY
Light is composed of small packets of energy called photons or quanta and travels in waves at extremely high speed (186,000 miles per second).
Light normally travels in straight lines but when it passes from one transparent medium to another, its speed changes. The light waves also bend or ‘refract’ if light meets the surface of a different medium at an angle rather than ‘straight on’.
Light enters the eye through the pupil and is refracted as it passes through the cornea and through both surfaces of the lens (Fig 2). There is also minimal refraction as it passes through the humours. The result is that the light is focused on the retina. The convex surfaces also ‘turn’ the image upside down and this is how images reach the retina (Fig 2). In addition, light from the right side of the visual field is projected onto the left side of each eye (Fig 3) and vice versa.
The images are ‘gathered’ in the retina and turned into nerve impulses which leave the retina in the optic nerve. From here the nerve enters the brain and travels past the optic chiasma at the base of the brain. Fibres from the nasal side only of each nerve cross the midline to the opposite side of the brain (Fig 3). In this way, fibres leaving the right side of each eye are carried to the right side of the brain and fibres from the left sides to the left side of the brain. Since these fibres contain images of the opposite side of the visual field, the result is that the left side of the brain ‘sees’ the right side of the world and the right side of the brain ‘sees’ the left side of the world. Nerves then transmit information to the primary visual cortex in the occipital lobe where it is interpreted and we ‘see’.
DISORDERS OF REFRACTION
In a normal eye, light is focused into a clear upside-down image on the retina and the brain can interpret it the right way up. However, the brain cannot correct a poorly focused image. If our eyeballs are elongated, the image focuses in front of the retina rather than on it, resulting in a blurred image on the retina - myopia or near-sightedness. If eyeballs are shorter, the image focuses behind the retina so is again fuzzy - hyperopia or far-sightedness. Both of these defects can be corrected by eye surgery, spectacles or contact lenses. Astigmatism occurs when the curvature of the cornea in the horizontal and vertical planes is not the same, resulting in blurring of parts of the visual image.
Refraction problems also occur when the two eyes do not see the same image. Normally both eyes aim at the same object (binocular vision) and the small differences in the image caused by the space between the eyes is compensated for in the visual cortex. Squint or strabismus means that the eyes do not point in the same direction. In minor cases the brain can compensate for this but in severe cases the brain cannot construct a single image. If not corrected in early childhood, the brain will learn to ignore the information from one eye, resulting in decreased visual acuity and blindness in the affected eye.
DISORDERS OF THE VISUAL PATHWAY
Damage in the optic nerve or the brain can impair vision. Glaucoma can damage the optic nerve and diabetes can cause its degeneration. Loss of vision is not always total, depending where the problem is (Thibodeau and Patton, 2005). Cerebrovascular accident, for example, can produce visual impairment if damage occurs in one of the regions of the brain that processes visual information.
- Next week’s article will examine how the eyes see colour, detail and depth.
This article has been double-blind peer-reviewed.