The eyeball is a bilateral and spherical organ, housed within a bony cavity of the facial skeleton known as the bony orbit, which serves as a structure for vision. It is composed of three layers: the fibrous, vascular, and inner. In this article, we shall consider the anatomy of the eyeball in greater detail, as well as its various clinical correlations.
The fibrous layer of the eye is the outermost layer and it comprises of two components: the sclera and the cornea. The sclera makes up approximately 85% of the fibrous layer and serves as the anchoring point for the extraocular muscles, which are responsible for eye movement. Visible externally, the sclera appears as the white part of the eye. The cornea is transparent, located centrally at the front of the eye, and light entering the eye is refracted by its lens.
The vascular layer of the eye appears beneath the fibrous layer and is comprised of three components: the choroid, ciliary body, and iris. The choroid is made up of connective tissues and blood vessels, and provides nourishment to the outer layers of the retina. The ciliary body is composed of the ciliary muscle and ciliary processes, which are responsible for controlling the shape of the lens and aiding in the formation of aqueous humor. The iris is a circular structure with an aperture in the center (the pupil) and its diameter is determined by the smooth muscle fibers within the iris, which are innervated by the autonomic nervous system. It is located between the lens and the cornea.
The inner layer of the eye is the retina, its light detecting component. It is further divided into two layers: pigmented (outer) and neural (inner). The pigmented layer is made up of a single layer of cells and is attached to the choroid, helping to absorb light and preventing its scattering within the eyeball. This layer also continues around the entire inner surface of the eye. The neural layer consists of photoreceptors, the cells that detect light and is located posteriorly and laterally within the eye. Anteriorly, the pigmented layer continues but the neural layer does not, this region is known as the non-visual retina.
The vitreous body is a transparent gel which is located in the posterior segment of the eyeball, anterior to the lens. It is marked by a narrow canal, the hyaloid canal, running from the optic disc to the lens, which is a remnant from the fetal stage. The vitreous body has three main functions: to contribute to the magnifying power of the eye, supporting the lens, and holding the layers of the retina in place. In addition, the lens of the eye is found anteriorly, between the vitreous humor and the pupil.
The anatomy of the eyeball is a complex and multifaceted structure, composed of several layers with specialized functions. The fibrous layer is responsible for providing shape to the eye and support, while the vascular layer consists of the choroid, ciliary body, and iris, each with its own specialized purpose. The inner layer is the retina, which contains the photoreceptor cells that detect and interpret light. Finally, the vitreous body is a transparent gel that provides three main functions. Together, these components form the eyeball and ensure vision is possible in both eyes.
The anatomy of the human eye is composed of three main parts – the choroid, ciliary body and iris. The choroid is a layer of connective tissue located between the sclera and retina, which contains a network of blood vessels that supply nourishment to the outer layers of the retina. The ciliary body is located posteriorly to the iris and is comprised of two parts, the ciliary muscle and ciliary processes. The ciliary muscle is a collection of smooth muscle fibres that are attached to the lens of the eye by the ciliary processes. The shape of the lens is altered by the ciliary body which changes its refractive power. In some cases, the lens may become opaque, a condition known as cataract.
There are two fluid-filled areas in the eye - the anterior and posterior chambers. The anterior chamber is located between the cornea and the iris, and the posterior chamber between the iris and ciliary processes. The chambers are filled with aqueous humor, a clear plasma-like fluid that nourishes and protects the eye. The aqueous humor is produced constantly and drains via the trabecular meshwork, an area of tissue at the base of the cornea near the anterior chamber. If the drainage of aqueous humor is obstructed, a condition known as glaucoma can result.
The eyeball receives arterial blood primarily via the ophthalmic artery, a branch of the internal carotid artery that arises immediately distal to the cavernous sinus. The ophthalmic artery gives rise to many branches, which supply different components of the eye. The most important branch is the central artery of the retina, which supplies the internal surface of the retina. Occlusion of this artery will result in blindness.
Venous drainage of the eyeball is carried out by the superior and inferior ophthalmic veins, which drain into the cavernous sinus, a dural venous sinus in close proximity to the eye.
Glaucoma refers to a group of eye diseases that result in damage to the optic nerve. There are two main clinical classifications of glaucoma – open angle glaucoma and closed angle glaucoma. Open angle glaucoma is where the outflow of aqueous humor through the trabecular meshwork is reduced. It causes a gradual reduction of the peripheral vision until the end stages of the disease. Closed angle glaucoma occurs when the iris is forced against the trabecular meshwork, preventing any drainage of aqueous humor. It is an ophthalmic emergency, which can rapidly lead to blindness.
Papilloedema refers to swelling of the optic disc that occurs secondary to raised intracranial pressure. The optic disc is the area of the retina where the optic nerve enters and can be visualised using an ophthalmoscope. Common causes of papilloedema include intracerebral mass lesions, cerebral haemorrhage, meningitis and hydrocephalus. In papilloedema, the high pressure within the cranium resists venous return from the eye. This causes fluid to extravasate from blood vessels and collect in the retina, producing a swollen optic disc.
The iris is a circular structure with an aperture in the centre (the pupil). The diameter of the pupil is altered by smooth muscle fibres within the iris, which are innervated by the autonomic nervous system. It is located between the lens and the cornea.
The inner layer of the eye is formed by the retina; its light detecting component. The retina is composed of two layers – the pigmented (outer) layer and the neural (inner) layer. The pigmented layer is formed by a single layer of cells and is located anteriorly, continuing around the whole inner surface of the eye. The neural layer consists of photoreceptors, the light detecting cells of the retina. It is located posteriorly and laterally in the eye, and is the optic part of the retina. The optic part of the retina can be viewed during ophthalmoscopy. The centre of the retina is marked by an area known as the macula. It is yellowish in colour, and highly pigmented. The macula contains a depression called the fovea centralis, which has a high concentration of light detecting cells. It is the area responsible for high acuity vision. The area that the optic nerve enters the retina is known as the optic disc, which does not contain light detecting cells.
The vitreous body is a transparent gel located in the posterior section of the eye, directly behind the lens. This gel is largely composed of a fetal remnant known as the hyaloid canal, which runs from the optic disc to the lens. The vitreous body serves three main functions: it contributes to the magnifying power of the eye, it supports the lens, and it holds the layers of the retina in place.
The lens, which rests between the vitreous humour and the pupil, is held in place by the ciliary body. This body is responsible for altering the shape and refractive power of the lens. As people age, their lenses can become opaque - a condition known as a cataract.
Within the eye there are two fluid-filled areas, known as the anterior and posterior chambers. The anterior chamber is situated between the cornea and the iris, and the posterior chamber between the iris and the ciliary process. The chambers are filled with aqueous humour: a clear, plasma-like fluid that nourishes and protects the eye. The aqueous humour is constantly produced and drains through the trabecular meshwork, a tissue area located at the base of the cornea, near the anterior chamber.
If this drainage of aqueous humour is at any point blocked, a condition known as glaucoma can occur.
Papilloedema is a condition in which the optic disc - the area of the retina where the optic nerve enters -swells as a result of raised intracranial pressure. This swelling of the disc is caused by the high pressure in the cranium, which prevents venous return from the eye. As a result, fluid extravasates from the blood vessels and collects in the retina, creating a swollen optic disc.
Common causes of papilloedema include intracerebral mass lesions, cerebral haemorrhage, meningitis, and hydrocephalus.
