The eyeball can be divided into an anterior segment and a significantly larger posterior segment.
The key structures of the globe, starting at the front of the eye and moving backwards, include:
A transparent structure, forming the anterior 1/6th of the globe, responsible for the refraction of light entering the eye. Despite being only 0.5mm thick in the centre, the cornea is composed of 5 distinct layers: epithelium, Bowmans layer, stroma, Descemet’s membrane and endothelium. Corneal transparency results from the uniform spacing of collagen fibrils in the stroma (which forms 90% of the corneal thickness). Any disruption in the orientation of collagen eg oedema, trauma, infection can result in reduced vision due to loss of corneal clarity.
The nerve supply to the cornea is via the ophthalmic division of the trigeminal (5th) nerve.
The sclera forms the posterior 5/6th of the eyeball and is opaque. Anteriorly the sclera forms the “white” of the eye and is covered by the relatively transparent conjunctiva. There are two gaps in the relatively tough sclera, one anteriorly for the cornea and the other posterior for the optic nerve. The transition zone from the sclera to the peripheral cornea is known as the limbus (corneoscleral junction).
Anterior chamber / Drainage angle
The anterior chamber is a small cavity, filled with aqueous humor, situated behind the cornea and in front of the iris. Aqueous is formed by the ciliary body and flows through the pupil into the anterior chamber where it supplies the metabolic needs of the lens and cornea (neither possess a blood supply). The peripheral margin of the anterior chamber, where the cornea, sclera, iris and ciliary body lie in close proximity, is known as the drainage angle. The trabecular meshwork is located in the angle and has multiple channels for the drainage of aqueous out of the eye.
The balance between the formation and drainage of aqueous humor gives rise to the intraocular pressure (IOP). This internal pressure, which allows the globe to maintain its optical shape, usually lies between 10-21mm of Hg in 95% of the population. An elevated IOP can give rise to glaucoma which may lead to visual loss via secondary damage to the optic nerve (glaucomatous optic neuropathy).
Glaucoma can be broadly divided into open-angle and closed-angle. In the former the drainage angle is open but there is increased resistance in the trabecular channels. In the less common latter, the iris root (peripheral margin) is displaced forward which blocks access to the trabecular meshwork and obstructs aqueous outflow.
The iris is a thin, contractile, pigmented diaphragm with a central aperture called the pupil. The pupil plays an important role in governing how much light enters the eye in different illuminations and the size of the pupil can also have an optical effect eg a smaller pupil enhances near vision focus. The iris contains two muscles which are responsible for pupil size:
- The sphincter pupillae muscle is a 1mm wide ring of smooth muscle fibres centrally located around the pupil. The nerve supply is from the postganglionic fibres in the short ciliary nerves which are derived from the oculomotor (3rd) nerve. Contraction of the sphincter papillae causes pupil constriction (miosis)
- The dilator papillae muscle fans out from the sphincter papillae to the iris periphery. The nerve supply is from the sympathetic postganglionic fibres via the long ciliary nerves. When the dilator papillae contracts the pupil enlarges (mydriasis)
The lens is a transparent, biconvex structure situated behind the iris and supported by suspensory ligaments (zonules). The lens contributes to the refractive power of the eye and can change shape to allow near objects to be focussed on the retina (accommodation). With advancing age, however, the lens becomes less elastic and the ability to accommodate is reduced (presbyopia). Consequently older people often require additional reading glasses for near tasks.
The lens continues to grow throughout life and does not discard any lens fibres. The lens thickens and undergoes metabolic changes which can cause it to become opaque which is known as a cataract. These lens opacities can result in blurred distance and/or near vision and glare.
The ciliary body is continuous posteriorly with the choroid and anteriorly with the peripheral margin of the iris. It has extensive surface folds and is involved in the production of aqueous humor. The bulk of the ciliary body is formed by the smooth muscle fibres of the ciliary muscle. Contraction of the muscle pulls the ciliary body forward which relieves zonular tension allowing the lens to become more convex (accommodation).
The vitreous body is a transparent gel which occupies the posterior segment of the globe. The function of the vitreous is to transmit light and support the posterior surface of the lens. The vitreous is attached to the optic disc, retinal blood vessels and peripheral retina (ora serrata). With age the vitreous undergoes degeneration and liquefaction which often leads to a posterior vitreous detachment (PVD), whereby the gel collapses into the centre of the posterior chamber. Rarely traction on the retina during a PVD results in retinal tears and/or retinal detachment.
The retina is a thin membrane consisting of an outer pigmented layer and an inner neurosensory layer. It is firmly attached posteriorly at the margins of the optic disc and at its anterior termination called the ora serrata (after which it merges with the ciliary body). At the centre of the posterior part of the retina is an oval, yellowish area, the macula lutea. It has a central depression, the fovea centralis, which is responsible for detailed vision.
a) Outer pigmented layer
The retinal pigment epithelium (RPE) has numerous functions including absorption of light, photoreceptor (rod and cone) turnover, vitamin A metabolism (a precursor of photosensitive pigments eg rhodopsin) and maintenance of the blood retina barrier. Tight junctions between the pigment epithelial cells form a barrier to molecules entering the retina from the deeper choroid.
b) Inner neurosensory layer
The neural retina consists of 3 main groups of neurons:
There are 2 types of photoreceptors which interdigitate with the RPE. Rods are mainly responsible for vision in dim light and cones which are adapted to bright light, colour vision and fine detail. Rods are absent at the fovea and are located in the retinal periphery. Cones are most densely found at the fovea with numbers decreasing in the periphery.
2. Bipolar cells
These are equivalent to 1st order neurons connecting the photoreceptor cells to the ganglion cells.
3. Ganglion cells
These 2nd order neurons have axons which merge to form the nerve fibre layer of the retina. Having entered the optic nerve head the axons become myelinated (once through the sclera at the lamina cribrosa) and travel in the optic nerve to the lateral geniculate body (LGB). Here they synapse and the LGB nerve cells form the 3rd order neurons which terminate in the visual cortex.
The blood supply of the retina is from two sources: the RPE and photoreceptors are supplied by the underlying choroid, whilst the inner retina is supplied by the central retinal artery and vein.
Separation of the RPE from the neural retina is known as a retinal detachment. This most commonly occurs as a consequence of vitreous traction on the retina resulting in a retinal tear. Liquified vitreous enters the defect resulting in “hydrodissection” of the neural retina away from the underlying outer pigmented layer.
The optic nerve leaves the retina about 3mm nasal to the macula. The optic disc is referred to as the “blind spot” as it is completely devoid of rod and cone receptors. The nerve is approximately 4cms long and runs backwards and medially to exit the posterior orbit via the optic canal. The optic nerve may be regarded as an anterior extension of the white matter of the brain and as it does not possess Schwann cells it is unable to regenerate if damaged. As the nerve is surrounded by an extension of the subarachnoid space it is prone to swell (papilloedema) secondary to a rise in intracranial pressure.