Anatomy of the eye -

Anatomy of the eye -

ANATOMY OF THE EYE Haneen Omar Abu Hani THE EYEBALL The eyeball can be divided into the fibrous, vascular and inner layers. Fibrous Layer (cornea & sclera) The fibrous layer of the eye is the outermost layer. It is a

tough outer coat which is transparent anteriorly (the cornea ) and opaque posteriorly (the sclera ). The junction between them is called the limbus . The extraocular muscles attach to the outer sclera while the optic nerve leaves the globe posteriorly. Their main functions are to provide shape to the eye and support the deeper structures.

1.The cornea The cornea is transparent and positioned centrally at the front of the eye. Light entering the eye is refracted by the cornea. The cornea is 0.5 mm thick and comprises: The epithelium, an anterior non - keratinized squamous layer, thickened peripherally at the limbus where it is continuous with the conjunctiva. The limbus houses the germinative stem cells of the corneal epithelium.

An underlying stroma of collagen fibrils, ground substance and fibroblasts. The regular packing, small diameter and narrow separation of the collagen fibrils account for corneal transparency. This orderly architecture is maintained by regulating stromal hydration. The endothelium, a monolayer of non - regenerating cells

which actively pump ions and water from the stroma, controlling corneal hydration and hence transparency. The difference between the regenerative capacity of the epithelium and endothelium is important. Damage to the epithelial layer, by an abrasion for example, is rapidly repaired by cell spreading and proliferation. Endothelial damage, by disease or surgery, is repaired by

cell spreading alone, with a loss of cell density. A point is reached when loss of its barrier and pumping functions leads to over - hydration (oedema), disruption of the regular packing of its stromal collagen and corneal clouding. Functions of the cornea It protects the internal ocular structures. Together with the lens, it refracts and focuses light onto

the retina. The junction between the ambient air and the curved surface of the cornea, covered by its optically smooth tear film, forms a powerful refractive interface. The nutrition of the cornea is supplied almost entirely by the aqueous humour, which circulates through the anterior chamber and bathes the posterior surface of the cornea.

The aqueous also supplies oxygen to the posterior stroma, while the anterior stroma receives its oxygen from the ambient air. The oxygen supply to the anterior cornea is reduced but still sufficient during lid closure, but a too tightly fitting contact lens may deprive the anterior cornea of oxygen and cause corneal, especially epithelial, oedema. 2.The tear film When the eyes are open, the exposed ocular surface (the

cornea and exposed wedges of bulbar conjunctiva) are covered by a tear fi lm, 3 m thick. This comprises three layers: 1- a mucin gel layer produced by the conjunctival goblet cells, in contact with the ocular surface; 2- an aqueous layer produced by the lacrimal gland; 3- a surface oil layer produced by the meibomian glands and delivered to the lid margins.

Functions of the tear film : It provides a smooth air/tear interface for distortion - free refraction of light at the cornea. It transmits oxygen to the avascular cornea. It removes debris and foreign particles from the ocular surface through the flow of tears It has antibacterial properties through the action of

lysozyme, lactoferrin, defensins and the immunoglobulins, particularly secretory IgA. The tear film is replenished with each blink. 3.The sclera The sclera comprises the majority of the fibrous layer (approximately 85%). It provides attachment to the extraocular muscles these muscles are responsible for

the movement of the eye. It is visible as the white part of the eye. The sclera is formed from interwoven collagen fibrils of different widths lying within a ground substance and maintained by fibroblasts. It is of variable thickness 4.Conjunctiva

Anteriorly, the bulbar conjunctiva of the globe is reflected from the sclera into the fornices and thence onto the posterior surface of the lids where it forms the tarsal (palpebral) conjunctiva . A connective tissue layer (Tenons capsule) separates the conjunctiva from the sclera and is prolonged backwards as a sheath around the rectus muscles

Vascular Layer A rich vascular coat (the uvea ) lies underneath the fibrous layer, forms the choroid posteriorly and the ciliary body and iris anteriorly. 5.The choroid The choroid is formed of arterioles, venules and a dense, fenestrated capillary network.

It is loosely attached to the sclera. It has a remarkably high blood flow. It nourishes the deep, outer layers of the retina and may have a role in its temperature homeostasis. It lines the retina, to which it is firmly attached and nourishes its outer two thirds. Its basement membrane, together with that of the retinal pigment epithelium (RPE), forms the acellular Bruch s

membrane, which acts as a diffusion barrier between the choroid and the retina. 6.The ciliary body The ciliary body is subdivided into three parts: 1- the ciliary muscle ; 2- the ciliary processes ( pars plicata ); 3- the pars plana .

The ciliary muscle This comprises smooth muscle arranged in a ring overlying the ciliary processes. It is innervated by the parasympathetic system via the third cranial nerve. It is responsible for changes in lens thickness and curvature during accommodation. The zonular fibres supporting the lens are under tension during distant viewing, giving the lens a flattened profile.

Contraction of the muscle relaxes the zonule and permits the elasticity of the lens to increase its curvature and hence its refractive power. The ciliary processes ( pars plicata) There are about 70 radial ciliary processes arranged in a ring around the posterior chamber. They are responsible for the secretion of aqueous humour. Each ciliary process is formed by an epithelium two layers

thick (the outer pigmented and the inner non - pigmented ) with a vascular stroma. The stromal capillaries are fenestrated, allowing plasma constituents ready access. The tight junctions between the non - pigmented epithelial cells provide a barrier to free diffusion into the posterior chamber. They are essential for the active secretion of aqueous by the non - pigmented cells.

The epithelial cells show marked infolding, which significantly increases their surface area for fluid and solute transport. The pars plana This comprises a relatively avascular stroma covered by an epithelial layer two cells thick. It is safe to make surgical incisions through the scleral wall here to gain access to the vitreous cavity.

7.The iris The iris is attached peripherally to the anterior part of the ciliary body. It forms the pupil at its centre, the aperture of which can be varied by the circular sphincter and radial dilator muscles to control the amount of light entering the eye. It has an anterior border layer of fibroblasts and collagen and

a cellular stroma in which the sphincter muscle is embedded at the pupil margin. The sphincter muscle is innervated by the parasympathetic system. The smooth dilator muscle extends from the iris periphery towards the sphincter. It is innervated by the sympathetic system. Posteriorly the iris is lined by a pigmented epithelium two

The iridocorneal ( drainage) angle This lies between the iris, the anterior tip of the ciliary body and the cornea. It is the site of aqueous drainage from the eye via the trabecular meshwork, Schlemms canal and is composed of a lattice of collagen beams covered by trabecular cells. The spaces between these beams become increasingly small as Schlemms canal is approached. The outermost zone of the

meshwork accounts for most of the resistance to aqueous outflow. Damage here raises the resistance and increases intraocular pressure in primary open angle glaucoma. Some of the spaces may be blocked and there is a reduction in the number of cells covering the trabecular beams. Fluid passes into Schlemms canal both through giant vacuoles in its endothelial lining and through intercellular spaces.

8.The lens The lens is the second major refractive element of the eye; the cornea, with its tear film, is the first. It grows throughout life. It is supported by zonular fibres running between the ciliary body and the lens capsule. It comprises an outer collagenous capsule under whose anterior part lies a monolayer of epithelial cells. Towards

the equator the epithelium gives rise to the lens fibres. The zonular fibres transmit changes in the ciliary muscle, allowing the lens to change its shape and refractive power. The lens fibres make up the bulk of the lens. They are elongated cells arranged in layers which arch over the lens equator. Anteriorly and posteriorly they meet to form the lens sutures.

With age the deeper fibres lose their nuclei and intracellular organelles. The oldest central fibres represent the fetal lens and form the lens nucleus; the peripheral fibres make up the lens cortex . The high refractive index of the lens arises from the high protein content of its fibres.

9.The retina It is the inner layer of the eye. The retina is the light detecting part of the eye. It is a highly complex structure derived embryologically from the primitive optic cup. The retina itself is composed of two cellular layers: Pigmented layer the outer layer of the retina. It is attached to the choroid layer and acts to support the neural layer. It continues around the whole inner surface of the eye.

Neural layer the innermost layer of the retina. It consists of photoreceptors; the light detecting cells of the retina (( rods and cones ), the bipolar nerve layer (and horizontal nerve cells) and the ganglion cell layer, whose axons give rise to the innermost, nerve fibre layer. These nerve fibres converge to the optic nerve head, where they form the optic nerve). It is located posteriorly and laterally in the eye. 10.The optic nerve

It is formed by the axons arising from the retinal ganglion cell layer, which form the nerve fibre layer of the retina. It passes out of the eye through the cribriform plate of the sclera, a sieve like structure. In the orbit the optic nerve is surrounded by a sheath formed by the dura, arachnoid and pia mater, continuous with that surrounding the brain. It is bathed in cerebrospinal fluid (CSF).

The central retinal artery and vein enter the eye in the centre of the optic nerve. The extraocular nerve fibres are myelinated; those within the eye are not. 11.The vitreous The vitreous is a clear gel occupying two - thirds of the globe. Its body lies between the lens and retina.

It is 98% water. The remainder is gel - forming hyaluronic acid traversed by a fine collagen network. There are few cells. It is firmly attached anteriorly to the peripheral retina, pars plana and around the optic disc, and less firmly to the macula and retinal vessels. It has a nutritive and supportive role. Collapse of the vitreous gel (vitreous detachment), which is common in later life, puts traction on points of attachment and

may occasionally lead to a peripheral retinal break or hole, where the vitreous pulls off a flap of the underlying retina. 12.Anterior and Posterior Chambers There are two fluid filled areas in the eye known as the anterior and posterior chambers. The anterior chamber is located between the cornea anteriorly and the iris and central lens posteriorly, whose periphery is the iridocorneal angle or

drainage angle. The posterior chamber between the iris and ciliary processes. The chambers are filled with aqueous humor a clear plasmalike 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.

Damage to the nerve or weakening of the aponeurosis in old age results in drooping of the eyelid ( ptosis ). A flat, smooth muscle, innervated by the sympathetic nervous system, arises from the deep surface of the levator and inserts into the tarsal plate. If the sympathetic supply is damaged, a slight ptosis results (Horners syndrome). The meibomian oil glands deliver their oil to the skin of the

lid margin, just anterior to the mucocutaneous junction. This oil spreads onto the anterior surface of the tear film with each blink, to form a lipid layer which retards evaporation. Far medially on the lid margins, two puncta form the initial part of the lacrimal drainage system.

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