A basic knowledge of structure and function is necessary to understand how symptoms and signs relate to disease manifestations. From the front to the back of the eyeball, the structures are:
- the cornea, which is clear
- the anterior chamber, a clear fluid-filled region
- the iris, which is coloured and has a small aperture called the pupil, through which light passes
- the lens, a crystalline structure
- the vitreous cavity, a jelly-like cavity posterior to the lens
- the retina, a delicate neurovascular lattice at the back of the interior of the globe.
Beyond the clear cornea the exterior of the eyeball is composed of a robust collagenous structure known as the sclera. The globe sits within a bony orbit (Figure 1.1) with a periocular soft-tissue support. The eye rotates within the orbit by the action of the six extraocular muscles (Figure 1.2). Anteriorly the globe is protected by the eyelids. Posteriorly, the optic nerve, the main neural output of the eye, travels from the globe towards the brain, exiting the orbit via a small hole in the skull known as the optic canal, before decussating at the optic chiasm above the pituitary gland (Figure 1.3).
The eye forms from the optic vesicle, an outpouching from the primitive forebrain which invaginates to form a cup-like structure. It also folds along the optic fissure, the edges of which fuse inferonasally (Figure 1.4).
Figure 1.2: Actions of the extraocular muscles. Int., internal rotation; Ext., external rotation; Add., adduction; Abd., abduction.
The inner eyelid and ocular surface are covered in a stratified columnar epithelium called the conjunctiva. The tarsal conjunctiva on the inner aspect of the eyelids (Figure 1.6) is continuous with the bulbar conjunctiva, which covers the globe.
The fornices, where the bulbar and tarsal conjunctiva join, are visible by retracting the lid and asking the patient to look in the opposite direction.
Cornea and sclera
The cornea (Figure 1.7) is a transparent, convex ‘window’ at the front of the eye that consists of five layers. It is composed of collagen fibrils that are aligned in such a way as to allow light to pass easily.
The cornea is one of the most highly innervated parts of the body with a subepithelial plexus of nerves. Any break in the contiguity of the corneal epithelium results in pain/irritation through activation of the trigeminal nerve and reflex lacrimation through activation of the facial nerve (Figure 1.8). This system has evolved in order to keep the cornea clear and free of foreign bodies.
The circumference of the round cornea is known as the limbus. Corneal stem cells are found at the limbus; these help to replace lost or damaged epithelial cells (Figure 1.9).
The sclera is continuous with the cornea at the limbus. It is visible through the transparent conjunctival mucosa as the white of the eye. It consists of a dense non-transparent arrangement of collagen. Attached to it are the extraocular muscles and periocular soft tissue.
The pupil is the central aperture of the iris. Dilation of the pupil occurs via sympathetic signals to the radially arranged dilator pupillae muscles; constriction of the pupil occurs via parasympathetic signals to the circumferentially arranged constrictor pupillae muscle (Figure 1.10).
The crystalline lens (Figure 1.11) is composed of a multilayered and interdigitating arrangement of lens fibrils that consist of crystalline proteins. These are laid down in chronological order of development such that the foetal nucleus is at the core of the lens and all the fibres are added to the outer layers, much like the rings of tissue in a tree trunk. There is a dense nuclear region surrounded by a softer cortex region, both of which are encapsulated by an elastic outer capsule to which lens zonules, fibres from the ciliary body controlling lens shape, are attached.
Behind the lens and filling most of the eyeball volume is the vitreous humour (Figure 1.12). This is a hydrated gel with fine collagenous fibres holding fluid to form a jelly-like structure. The jelly has several attachments to the retina. The strongest of these are around the optic nerve, on retinal blood vessels and at the anterior border of the retina in a region known as the vitreous base.
The uvea is formed by the iris anteriorly and is continuous with the posterior aspect of the choroid, which underlies the retina. The choroid's function is to provide the highly metabolic retina with oxygenated blood and nutrition; consequently, the uvea is highly vascular and has one of the highest blood flows of any tissue.
The retina is a thin neurovascular layer composed of a multitude of specialised cells and is similar in thickness to tissue paper (Figure 1.13a).
There are two types of photoreceptors (light-sensitive cells): rods and cones (Figure 1.13b). They are located in the outermost part of the neural retina. Rods are responsible for dim light vision and cones are responsible for colour vision in bright illumination.
Processing cells, found further in, include the horizontal and bipolar cells. These eventually synapse with ganglion cells. The unmyelinated axons of the retinal ganglion cells form the innermost layer of the retina – the nerve fibre layer. These axons course towards the optic disc, where they leave the eye through a perforation in the sclera to form the optic nerve.
The most sensitive portion of the retina with the greatest resolving capability is known as the macula. The most sensitive portion of the macula is the fovea, which is located between the upper and lower temporal vascular arcades.
The main refracting elements of the eye are:
- the cornea –fixed focus
- the crystalline lens –variable focus.
Accommodation is the process by which the eye is able to change its optical power in order to maintain focus at different distances. This is achieved through contraction of the ciliary muscle, which allows relaxation of the zonular fibres attached to the equatorial zone of the lens capsule (Figure 1.14).
Light transmission to the retina relies on transparent ocular media, which include the cornea, aqueous humour, lens and vitreous. These structures are devoid of blood vessels to maintain optical clarity. Since photoreceptors are found on the outer aspect of the neural retina, light must pass through the other retinal layers before being detected. Accommodation maintains clear focus of light rays in order to improve the definition of vision.
The tear film contains an inner proteinaceous layer, the main function of which is to adhere the tear film to the cornea.
The middle layer of the tear film is aqueous in nature, whereas the outer layer has a high lipid content in order to reduce tear evaporation (Figure 1.15). The spent tears are channelled towards two tiny orifices at the medial aspects of the eyelids, pass through their connecting tubes into the bridge of the nose and are flushed down the nasolacrimal duct into the nasopharynx.
Increased hydration of the corneal stroma leads to disruption of the collagen fibril matrix causing loss of transparency of this tissue, which in health requires a tightly controlled hydration level.
This dehydration is maintained by pumping fluid into the aqueous by corneal endothelial cells found on the innermost aspect of the cornea (Figure 1.16).
The aqueous humour, a watery but nutrient-rich liquid produced by the ciliary body, flows through the anterior part of the eye to the outflow at the angle between the cornea and the iris, known as the trabecular meshwork (Figure 1.17). The aqueous provides important nutrients to the avascular cornea and lens. Intraocular pressure is also manipulated by relative inflow or outflow of aqueous, a concept important to remember in understanding the pathogenesis and treatment of glaucoma.
Figure 1.16: The endothelial pump generates a net ion movement from the stromal to anterior chamber side of the corneal endothelium. ① Passive diffusion into cell of H2O. ② Passive diffusion into cell of HCO3. ③ Carbonic anhydrase converts H2O and CO2 to HCO3− and H+. ④ Anion-dependent ATPase. ⑤ H2O passively follows bicarbonate to the aqueous-facing surface.
The outer capsule of the lens is attached to collagen fibrils known as zonules (Figure 1.18). These fibrils attach the lens to the ciliary body, which contains a circular and circumferentially arranged muscle. This apparatus permits some variability in focusing capability of the lens.
Light falling upon the macula (Figure 1.19) serves a central visual function, such as reading. The highest resolving power is at the fovea. It is here that the finest blood vessels terminate in a perifoveal vascular plexus to support this highly metabolic neuroretinal tissue. The foveal avascular zone is the site at which light falls directly on the photoreceptor layer, with the overlying neuroretinal layers splayed to reveal a yellow dot visible with the ophthalmoscope. This luteal pigment is yellow to protect the central macula against photic damage.
Retinal pigment epithelium and Bruch's membrane (basal lamina of choroid)
Critical to the function of the photoreceptors is the retinal pigment epithelium (RPE) that separates the neuroretina from the underlying choroid (Figure 1.20). The basement of the RPE is known as the Bruch's membrane and is prone to age-related changes affecting function.
The RPE acts in a similar way to the blood– brain barrier and selectively allows chemicals in and out of the retina.
Below the RPE is the highly vascular choroid, which provides oxygen and nutrients to the highly metabolic retina as well as removing waste products. It is important that Bruch's membrane remains highly permeable for retinal homeostasis.