Hene_Ch09.qxd 8/30/04 6:51 AM Page 348 348 4Basic anatomy and physiology The eye is a highly specialized organ with an average axial length of 24 mm and a volume of 6.5 ml. Except for its anterior aspect, it is protected by the bony orbit. The orbit is otherwise filled with orbital fat, which together with a suspensory ligament, suspends the eye in the cavity. Six muscles (four recti and two oblique) are attached to the eye and move it to the nine positions of gaze. The eyeball (globe) The eyeball itself consists of three coats that surround the contents of the eye (aqueous humor, lens, and the vitreous body). The three coats are: Tough fibrous outer layer. This layer is transparent anteriorly (cornea) and opaque in the larger posterior portion (sclera). They meet to form a small depression around the cornea (limbus). Pigmented and vascular middle layer. The uveal tissue is continuous and forms the choroid posteriorly and the ciliary body and iris anteriorly. Neural inner layer. Retina is the inner most layer and gives rise to the nerve fibres that leave through the optic disc to form the optic nerve and convey visual stimuli to the brain. The ocular contents need to be transparent for light to get through to the retina. The lens is a specialized structure situated behind the iris and can be stimulated by the ciliary body to change its shape and focus objects on the retina. It divides the eye into a large posterior and a small anterior segment. The anterior segment is further divided into a small posterior and a bigger anterior chamber by the iris and is filled with aqueous. The vitreous gel fills the posterior segment. Eyelids These protect the eyes from the environment, injury, and light, and also maintain the cornea by spreading the tears evenly over its surface. They are composed of an outer skin layer, a middle layer of muscle and connective tissue, and an inner layer of conjunctiva. The skin is thin and loosely attached to the underlying tissues. This allows significant amount of fluid to gather underneath it, and hence the characteristic swollen and bruised eye. The meibomian glands line the edge of the lids. These produce oil that helps lubricates the eye. The lashes protect the eyes from the elements and debris. The orbicularis oculi muscle closes the lids and goes into spasm for protection when the eye is painful. The conjunctiva is the mucous membrane of the eye covering the inside of the eyelids. It is continuous at the fornix with the conjunctiva covering the eyeball up to the cornea. The conjunctival sac is, therefore, only open anteriorly and it is impossible to lose a contact lens around the back of the eye (a common worry among patients). Lacrimal system The lacrimal gland is situated supero-temporally in the anterior orbit. The tears are spread on the surface of the eye by blinking and then pumped into the lacrimal drainage system. This forces the tears into the puncta at
Hene_Ch09.qxd 8/30/04 6:51 AM Page 349 CHAPTER 9 The eye 349 the inner corners of the upper and lower eyelids. The puncti are the openings for the tubes (canaliculi) leading to the lacrimal sac in the medial canthal area. The tears then go down the naso-lacrimal duct into the nose. Any disruption in this system gives rise to watery eyes, a condition many patients find very frustrating. Tear film The tear film serves several purposes: it keeps the eye moist, nourishes the front of the eye, creates a smooth surface for light to pass through the eye and provides protection from injury and infection. It is comprised of three layers: oil, water, and mucous. The mucous layer helps the film adhere to the eye. The outer oil layer, produced by the meibomian glands, reduces evaporation. The watery component is produced by the lacrimal glands. Conjunctiva This is the thin, transparent mucos membrane that covers the outer surface of the eyeball from the corneal limbus to the inside of the eyelids. It contains accessory tear glands and mucous secreting goblet cells that moisten and lubricate the eye. Sclera (the white of the eye ) This is a tough, opaque tissue that forms the eye s protective outer coat. In children, the sclera is thinner and more translucent, allowing the underlying tissues to show giving it a bluish cast. As we age, the sclera tends to become more yellow. Cornea The cornea is a dome-shaped, transparent window through which light passes. It is responsible for approximately two-thirds of the eye s focusing power. It is normally clear and has a shiny surface. It is avascular and is maintained by the aqueous and tear film. The cornea is extremely sensitive there are more nerve endings in the cornea than anywhere else in the body. There are five layers: epithelium, Bowman s membrane, stroma, Descemet s membrane, and the endothelium. The epithelium is a layer of cells that covers the cornea and quickly regenerates when the cornea is injured. However, deep injuries may lead to scarring and growth of blood vessels, and result in an opaque area. Iris and pupil The iris is a muscular diaphragm with a central aperture called the pupil. Fine muscles enable it to dilate and constrict the pupil, and thus control the amount of light that enters the eye. The iris is flat and separates the anterior chamber from the posterior chamber. The pupil constricts in response to bright light and also when looking near. If a bright light is shone in one eye, the afferent pathway leads to the stimulation of bilateral parasympathetic nuclei in the midbrain. Both pupils then constrict via the efferent fibres in the third nerve. Therefore, there is a direct and consensual response of the pupils to light. However, the near gaze response follows a different path within the midbrain and can be spared when the light reflex is affected and vice versa. Any disruption of the afferent and efferent pathways or damage to the iris muscles will affect pupillary function.
Hene_Ch09.qxd 8/30/04 6:51 AM Page 350 350 Ciliary body Part of the uveal tract, it lies between the iris and choroids, and is important in supporting the lens and changing its shape (accommodation). It also produces the aqueous humour that fills the front of the eye. Choroid A posterior portion of the uveal tract, the choroid lies between the sclera and the retina. It is composed of layers of blood vessels that nourish the back of the eye. Anteriorly it connects with the ciliary body and posteriorly it attaches to edges of the optic nerve. Aqueous humor This watery fluid is constantly produced by the ciliary body to nourish the lens and cornea, as well as maintaining the intra-ocular pressure. The aqueous circulates in front of the lens, through the pupil, and out of the eye via the drainage angle. Drainage angle The eye s drainage system is made up of several structures: the priphery of the iris, the ciliary body, the trabecular meshwork, and the canal of Schlemm. Aqueous is made in the ciliary body behind the iris and passes through the pupil into the anterior chamber. From there, fluid passes into the angle structures, where it drains from the eye. The aqueous eventually flows into the adjacent blood vessels. If the drainage angle slowly blocks, the pressure in the eye rises without causing symptoms or pain, eventually leading to glaucoma. In acute-angle closure glaucoma, the peripheral iris quickly blocks the angle and the rapid rise in pressure causes the clinical picture and pain characteristic of the condition. Lens and accommodation The crystalline lens is just behind the iris and is composed of an inner nucleus surrounded by a softer cortex. The lens is encased in a capsule, suspended from the ciliary body by tiny wires called zonules. Its purpose is to focus light onto the retina. One-third of the eye s focusing power is supplied by the lens, which can increase its refractive power to allow objects nearing the eye to stay in focus. This process is called accommodation and is possible because the young lens can change its shape. With age, the nucleus gradually hardens, diminishing its ability to accommodate (presbyopia) and reading glasses are required. Refraction For the eye to see clearly, (Fig. 9.1), the light needs to be brought into focus on the retina (refraction). In emmetropia (normal sightedness), the light rays are brought into focus on the retina. In short- and longsightedness the light is focused in front or behind the retina, respectively. In all refractive errors, the unfocused light that falls on the retina creates a circle of confusion and therefore vision is blurred. If a lens is used to move the focus of the eye onto the retina, vision improves. Astigmatism is when a point focus of light cannot be created on the retina. This occurs because the optical system of individual focuses the light differently on its
Hene_Ch09.qxd 8/30/04 6:51 AM Page 351 CHAPTER 9 The eye 351 Fig. 9.1 Ray diagrams showing the path of light in normal, myopic, and hypermetropic eyes and the pinhole principle. horizontal or vertical axis. Special cylindrical lenses are needed to correct this error. Vitreous humor This is a clear gel that occupies the posterior chamber of the eye (between the lens and the retina). It comprises about 80% of the volume of the globe. Light is transmitted through the vitreous to the retina. Retina This is a multi-layered photoreceptor tissue that lines the back of the eye, converting light into electrical impulses that are recognised by the brain as visual stimuli. There are two types of photoreceptors in the retina: rods and cones. Cones are contained in the macula and are responsible for
Hene_Ch09.qxd 8/30/04 6:51 AM Page 352 352 detailed central vision and colour perception. They are most densely packed within the fovea and function best in bright light. The rods are spread throughout the peripheral retina and function best in dim light. They are responsible for peripheral and night vision. Macula This is a small but highly sensitive part of the retina responsible for detailed vision (e.g. reading). It is located roughly in the centre of the retina, just lateral to the optic nerve, and between the vascular arcades. The fovea is the centre of the macula. Optic disc and nerve The optic nerve transmits impulses from the retina to the brain and forms the optic disc as it leaves the eye. The optic disc rim is pink and represents the retinal nerve fibres viewed head on as they dip into the disc. The cup is the area not containing any fibres and is pale. The margin of the disc should be distinct and blood vessels occupy a central position. There are no sensory receptor cells on the disc itself and hence everyone has a normal blind spot. This is not normally noticeable because the vision of both eyes overlaps and the brain completes the picture with information from the other eye. Extra-ocular muscles There are six muscles that surround the eye and control its movements. The four recti muscles control movements from left to right and up and down. The two oblique muscles rotate the eyes inward and outward, as well as assisting the recti in their function. All muscles need to work together to move the eye. As one contracts, the opposing muscle relaxes. The muscles of both eyes must also work in unison so that the eyes are always aligned.