Structure of the eyeball

Function

The function of the Tenon’s capsule is to protect the eyeball, to position it within the orbit and to allow the actions of the extraocular muscles. Although the episcleral potential space exists between the fascia and the eyeball, there is actually very little movement between the eye and the sheath, meaning that the fascia and the eye move together within the orbital fat.

Sclera

The sclera is an opaque, white, outer layer that surrounds the posterior five-sixths of the eyeball. The sclera is thickest posteriorly, becoming progressively thinner anteriorly. The posterior pole of the sclera is perforated by the optic nerve and this site is marked as the posterior scleral foramen. Here, the outer two-thirds of the sclera are continuous with the dural sheath of the optic nerve. 

The inner third of the sclera is pierced by the numerous fibers of optic nerve forming a sievelike structure known as the lamina cribrosa. Besides the optic nerve axons, lamina cribrosa allows the passage of the central retinal artery and vein.

The sclera features three more sets of apertures; the anterior, middle and posterior. 

• The four anterior apertures are located at the scleral attachments of the rectus muscles, and they transmit the anterior ciliary arteries. 
• The 4-5 middle apertures are found posterior to the equator of the eye, and they transmit the vorticose (vortex) veins. 
• The numerous posterior apertures are found around the posterior scleral foramen and they serve for the passage of the long and short ciliary arteries, veins and nerves.

The anterior margin of the sclera is continuous with the cornea. The line of their junction is called the corneoscleral (sclerocorneal) junction or the corneal limbus. Posteriorly to the junction and within the inner surface of the sclera is a circular canal called the internal scleral sulcus that contains the scleral venous sinus (canal of Schlemm). The posterior lip of the internal scleral sulcus shows a projection directed anteriorly and inwards, called the scleral spur, which serves as an attachment point for the ciliary muscle.

Structure

Most of the authors divide the sclera into three distinctive layers; 

• The episclera is the outermost connective tissue layer. Superficially, it is connected to the Tenon’s capsule, while its deep surface overlies the scleral stroma. The anteriormost part of the episclera contains an arterial episcleral plexus formed by the branches of the anterior ciliary arteries. This plexus is normally not visible, however during inflammation it becomes congested, giving the characteristic appearance of the ‘red eyes’ in the affected person.
• The scleral stroma is composed of the dense irregular connective tissue which gives the sclera its distinctive white color. The change of scleral color can indicate a pathological process in the body; for example, a yellow sclera may indicate liver diseases such as hepatitis.
• Lamina fusca is the innermost layer of the sclera and receives its name from the large number of melanocytes. The lamina fusca overlies the choroid, the outermost layer of the uveal tract. The potential space between the lamina fusca and choroid is called the perichoroidal space, and it is traversed by the long and short posterior ciliary arteries and nerves.

Blood supply and innervation

The anterior part of the sclera is vascularised by the episcleral plexus. The posterior part of the sclera is supplied by the branches of the long and short posterior ciliary arteries.

The anterior part of the sclera is innervated by the long ciliary nerves, while the posterior part is supplied by the short ciliary nerves.

Function

The function of the sclera is to protect the inner contents of the eye from the mechanical trauma. Moreover, its rigid structure contributes to maintaining the shape of the eyeball and keeping the eye structures in place, especially during the contractions of the extraocular muscles.

Cornea

The cornea comprises the anterior one-sixth of the fibrous layer of the eyeball. It is a circular transparent layer that covers the pupil, iris and anterior chamber of the eye. The cornea is noticeably more convex to the outside than the sclera. For this reason, the corneoscleral junction features a shallow sulcus on its outer surface, called the sulcus sclerae.

The cornea is the thickest at its periphery, becoming gradually thinner towards its center. Microstructurally, the cornea is entirely composed of proteins and cells organized in five layers, which from superficial to deep are:

• The nonkeratinized stratified squamous corneal epithelium consists of the 5 layers of cells centrally, while on the periphery it contains up to 10 layers.
• The Bowman’s membrane (layer), also known as the anterior limiting lamina of the cornea. It is an acellular layer, consisting of the meshwork of irregularly arranged collagen fibrils.
• Substantia propria (corneal stroma). It is the thickest corneal layer, comprising around 90% of the thickness of the cornea. This layer consists of the parallelly arranged collagen fibers. 
• Descemet’s membrane, also known as the posterior limiting lamina of the cornea. This is in fact the basement membrane of the underlying corneal endothelium and it consists of the collagen fibers. At the periphery of the cornea, the Descemet’s membrane gives off protrusions that project into the anterior chamber of the eye, called the Hassal-Henle bodies. The Descemet’s membrane is continuous with the trabecular meshwork of the Schlemm’s canal, and the line of their junction is called the line of Schwalbe.
• The corneal endothelium consists of a single layer of squamous endothelial cells. It covers the entire deep surface of the Descemet’s membrane, and it is continuous with the endothelium of the iridocorneal angle and the anterior surface of iris. The corneal endothelium forms a barrier between the cornea and the surrounding structures, controlling the inflow of the aqueous humor into the cornea, and maintaining the proper corneal hydration and nutrition. Note that the corneal endothelial cells don’t undergo mitosis, so if they get injured, the corneal surface can become permanently opaque.

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Blood supply and innervation

The cornea is a completely avascular structure, meaning that it doesn’t receive arterial blood supply. Instead, it is nourished by the nutrients from the aqueous humor that it receives via the active transport through its endothelial layer. The anterior ciliary arteries abruptly terminate at the corneal margins, so the periphery of the cornea uptakes some nutrients from these vessels via diffusion.

The innervation to the cornea comes from the long ciliary nerves, whose branches form the annular plexus within the perichoroidal space.

Function

The main function of the cornea is to participate in the refraction of light. In fact, the cornea is the most important refractory structure of the eye as it has the highest optical power (42 diopters). The refraction of light occurs in the center of the cornea, where its refractive power is significantly higher than that of the atmospheric air. 

The preservation of corneal transparency is very important for maintaining its proper function. In case of any damage to the corneal epithelium, the fluid from the anterior chamber may enter the corneal stroma and cause the cloudiness of the cornea. Being part of the outermost (fibrous) layer of the eye the cornea also has a protective role, protecting the eye's delicate components from foreign particles.

Choroid

The choroid is a highly vascular layer accounting for almost 90% of the total blood flow in the eye. Its vascular component is formed by the branches of the short posterior ciliary arteries, as well as the tributaries of the vorticose veins. The outer surface of the choroid is firmly attached to the inner surface of the sclera, while its inner surface attaches to the retina. The choroid extends from the site of exit of the optic nerve posteriorly, to the ciliary body anteriorly. The choroid is divided into three layers:

• The vessel layer which contains numerous melanocytes as well as many blood vessels.
• The capillary layer which consists of the numerous melanocytes and the smaller branches of the vessels from the vessel layer.
• The Bruch’s membrane which consists of five distinctive components; the basement membrane of the endothelium of the capillaries of the capillary layer, collagen and elastic fibers, and the basement membrane of the pigmented layer of the retina.

The choroid is innervated by the branches of the long and short ciliary nerves, which access choroid from the perichoroidal space. The function of the choroid is to provide the blood supply to the outer layers of the retina, as well as to provide the passage of the blood vessels from the back to the front regions of the eye.

Ciliary body

The ciliary body lies deep to the scleral spur and superficial to the ora serrata of the retina. It is continuous with the choroid posteriorly and with the iris anteriorly. 

The ciliary body forms a complete ring around the iris. On the cross-section, the ciliary body is triangular in shape. Its narrow base faces the periphery of the iris, while its apex faces posterolaterally and is continuous with the choroid. The base of the ciliary body is rough and it is called the corona ciliaris or pars plicata, while its posterior surface is smooth and it is called the orbiculus cialiaris or pars plana. The corona ciliaris gives off the thin projections called the ciliary processes. The intervals between the ciliary processes serve as the attaching sites of the zonular fibers of the lens.

Structure

The ciliary body consists of three parts:

• The ciliary epithelium is a double-layered cuboidal epithelium that covers the deep surface of the ciliary body. The inner layer contains non-pigmented cells which are continuous with the nervous part of the retina posteriorly. The posterior layer consists of the pigmented cells, which are continuous with the pigmented epithelium of the retina. 
• The ciliary stroma is composed of loose connective tissue and rich in blood vessels, which are the branches of the ciliary arteries and veins. These vessels form the major arterial circle at the base of the ciliary body, adjacent to the peripheral margin of the iris.
• The ciliary muscle is a smooth muscle embedded in the ciliary stroma. When this muscle contracts, it pulls the ciliary body anteriorly. This leads to the loosening of the zonular fibers of the lens, allowing the lens to shrink and become more convex. This process enhances the refractive power of the lens and plays an important role in the process of accommodation.

The innervation to the ciliary body comes from the short ciliary nerves. These nerves carry the parasympathetic input from the oculomotor nerve (CN III), which is why the ciliary muscle is controlled by the parasympathetic nervous system.

Functions

The ciliary body has several important functions:

• The ciliary processes produce the aqueous humor into the posterior chamber eye. The humor flows through the pupil into the anterior chamber of the eye, where it is absorbed into the scleral venous sinus (the canal of Schlemm).
• The ciliary muscle enables the accommodation of the eye.
• Its posterior surface faces the vitreous body and provides it with glycosaminoglycans.

Iris

The iris is a contractile, heavily pigmented, circular diaphragm that is analogous to the diaphragm of a camera. It contains numerous melanocytes, whose number greatly varies among individuals. Thus, the color of iris, or simply the eye color, varies from light blue to dark brown. In people who lack melanin due to certain health conditions (e.g. in albinism) the iris of the eyes appears as red due to visible blood vessels of the iris.

The iris represents the border between the anterior and posterior chambers of the eye. It is located anterior to the lens and posterior to the cornea, being immersed in the aqueous humor. The periphery of the iris is marked as its root or the ciliary margin. The acute angle formed by the root of iris and the cornea is called the iridocorneal angle (filtration angle). This angle contains the trabecular meshwork that facilitates the drainage of the aqueous humor into the Schlemm's canal, and as such is an important point in the pathway of the aqueous humor. An injury of the iris can squeeze the iridocorneal angle and obstruct the aqueous humor outflow, which leads to a condition called closed-angle glaucoma.

The iris contains two smooth muscles that enable its contractile property. These are the sphincter pupillae muscle and dilator pupillae muscle. The center of the iris features a circular opening called the pupil. The inner margin of the iris that bounds the pupil is called the pupillary margin. The size of the pupil can change by the action of the two pupillary muscles and usually varies from 1-8 millimeters. The purpose of these changes in the size of the pupil is to control the amount of light that enters the eye.

Surfaces

The anterior surface of iris shows many textural elements. It is divided into two zones; central (pupillary) and peripheral (ciliary) zone. The border between the two is marked by a wavy line called the collarete, which lies around 2 millimeters from the pupillary margin and it is the thickest region of the pupil.

The anterior surface is marked by the radial streaks, which are the bands of the collagen fibers that converge towards the pupil. The intervals between the streaks are called the Fuch’s crypts. The ciliary part of the anterior surface shows several circular lines that are called the contraction furrows, caused by the dilation of the pupil.

The posterior surface of the iris is black and features numerous radial contraction folds, especially in the pupillary region. The ciliary region is marked by the contraction furrows, just like the anterior surface.

Blood supply and innervation

The root of the iris contains a circular anastomotic arterial network called the major arterial circle, formed by anterior and posterior ciliary arteries. This circle gives off small radial branches that converge towards the pupillary margin of iris. At the level of the collarette of iris, the radial arteries anastomose with each other and form the minor arterial circle of iris. 

The venous drainage mirrors the arterial supply; small veins from the pupillary margin form the minor venous circle, from which the larger veins convey the blood into the vorticose veins.

The iris is sensory innervated by the long and short ciliary nerves, branches of the ophthalmic division of trigeminal nerve (CN V1). The two pupillary muscles receive autonomic motor innervation; 

• The sphincter pupillae is innervated by the parasympathetic fibers from the oculomotor nerve (CN III), via short ciliary nerves.
• The dilator pupillae is innervated by the sympathetic fibers from the superior cervical ganglion.

Function and pupil movements

The function of the iris is to control the size of the pupil through the actions of the sphincter and dilator pupillae muscles. 

• The dilation of the pupil is called mydriasis. The mydriasis is a result of dilator pupillae muscle contraction. It occurs when the intensity of light is low, and also in the states of fear and excitement (sympathetic predominance).
• The contraction of the pupil is called miosis. It is a result of the sphincter pupillae muscle contraction. The miosis occurs in conditions of high light intensity, convergence (simultaneous inward movement of both eyes toward each other) and sleeping.

Neural retina

The neural retina contains 6 types of cells which are distributed throughout its nine layers;

• Photoreceptors. These consist of rods and cones. The rods are cylindrical cells adapted for absorbing the dim light, being responsible for producing the images in the grayscale. The cones are the conical cells that are specialized for high-intensity light, enabling the color vision. The distribution of the cones and rods varies across the retinal surface; the rods are absent within fovea centralis, increasing in density going to the periphery of the retina. The cones, however, are the most abundant at the fovea, decreasing in number towards the periphery.
• Bipolar cells. These cells possess an axon on one end and a dendritic tree on the opposite end of their body. They are radially oriented within the retina, with the dendritic tree synapsing with the rods and cones, and the axon being directed towards the deeper layers of the retina. These cells are the second-order neurons in the visual pathway, as they collect the information gathered in the photoreceptor cells and pass them further to the ganglion cells.
• Ganglion cells. These cells are the third-order neurons in the visual pathway. They are multipolar cells, which synapse with the bipolar and amacrine cells via their dendrites. They have long non-myelinated axons, which stem from their basal ends. The axons take a sharp horizontal turn and converge towards the disc of the optic nerve. They pass through the lamina cribrosa of sclera, after which they become myelinated. Therefore, the axons of the ganglion cells comprise the optic nerve.
• Horizontal cells. These cells are distributed around the apices of the rods and cones and synapse with them. Additionally, they have long processes which synapse with the distant ganglion cells. The function of these cells is to release the inhibitory neurotransmitter GABA that inhibits the distant ganglion cells. This process enables the optic nerve to transmit the signals from the photoreceptors that are the most excited, thus contributing to the formation of a clear image.
• Amacrine cells. They are dispersed close to ganglion cells and synapse with the dendrites of the ganglion cells and axons of the bipolar cells. The bipolar cells stimulate the amacrine cells, which in turn stimulate the ganglion cells with which they synapse. Therefore, the amacrine cells are the indirect connection between bipolar and ganglion cells and their function is to modulate the photoreceptive process by ensuring that all the relevant ganglion cells are stimulated.
• Supporting cells. The most abundant supporting cells are the Müller cells, which are dispersed throughout the entire neural retina. The Müller cells show many radial processes that connect with the photoreceptor cells. These connections are seen as a dense layer known as the outer limiting membrane. The processes of Müller cells reach the anterior surface of the retina, where they feature a terminal dilation covered by the basement membrane. This termination forms another dense strap, known as the inner limiting membrane. Aside from the Müller cells, the retina features retinal astrocytes, perivascular glial cells, and microglial cells.

These six types of cells are distributed in a way to form the 9 layers of the neural retina:

1. The inner limiting membrane (described above)
2. The nerve fiber layer consists of the axons of the ganglion cells that converge towards the optic disc. 
3. The ganglion cell layer consists of the nuclei of the ganglion cells. 
4. The inner plexiform layer is comprised of the synapses between the bipolar, amacrine, and ganglion cells.
5. The inner nuclear layer consists of the nuclei of the bipolar, horizontal cells, amacrine, and Müller cells. 
6. The outer plexiform layer consists of the synapses between the terminal processes of the rods and cones, bipolar and horizontal cells.
7. The outer nuclear layer is made up of the nuclei of the cones and rods. 
8. The outer limiting membrane (described above)
9. The layer of rods and cones, containing the photoreceptor cells.

Retinal pigment epithelium (RPE)

The retinal pigment epithelium is the deepest layer of the retina which sits on the Bruch’s membrane of the choroid. It consists of a layer of cuboidal cells which extends from the optic disc to the ora serrata. Anteriorly, it is continuous with the pigmented epithelium of the ciliary body.

The cells of the RPE contain a high amount of dark pigment. Their function is to absorb light which passes through the retina and prevent it from reflecting back to the neurosensory layer. This feature is of great importance for a clear vision. Additionally, the cells of the RPE contribute to nourishing of the retina and it forms the blood-retinal barrier. The barrier is composed of the tight junctions between the cells of the RPE and its function is to prevent the diffusion of large and/or toxic molecules from the choroid into the retina.

Blood supply

The layers 1-6 of retina are supplied by the branches of the central retinal artery, while the layers 7-10 are supplied by the capillaries from the choroid.

Lens

The lens is a circular biconvex structure found anterior to the vitreous body and posterior to the iris. The outer margin of the lens (equator) divides the lens into anterior and posterior surfaces. The central points of these surfaces are called the poles and they are connected by an imaginary line called the axis of the lens.

A convenient feature of the lens is that it can change its dioptric power by changing its shape, which makes its refractive power flexible, unlike any other refractory medium of the eye. Although the cornea is the most powerful refractory structure, the lens’ contribution of up to 21 diopters makes it important for the maintenance of clear vision.

The lens consists of three parts:

• The capsule, which wraps the outer surface of the lens.
• The epithelium of the lens, which is a layer of cuboidal epithelial cells that lie deep to the lens capsule. 
• The lens fibers, which are actually the transformed, elongated, epithelial cells and comprise most of the lens’ substance. 

The lens is held in place by the series of small ligamentous bands that extend from the ciliary processes to the equator of the lens. These fibers are known as the zonular fibers (zonule of Zinn). Collectively, the zonular fibers are termed as the suspensory ligament of lens.

This ligament plays an important role in changing the shape of the lens in the process of the accommodation of the eye. In the resting state, when the person is looking far away, the ciliary body maintains tension on the zonules that keep the lens in a "flatten" state. When the focus shifts to close objects, the ciliary muscles contract resulting in the relaxation of the suspensory ligament of lens. This allows the lens to increase its anterior curvature which results in the increase in refractive power. Since the miosis of the pupil happens simultaneously, the rays of light are focused to pass through the thickest, central part of the lens and be directed towards the retina.

Vitreous body

The vitreous body is the largest structure of the eyeball, occupying the four-fifths of the entire eye. It fits into the concavity of the retina, being posterior to the lens. The anterior concavity that is adapted to fit with the convexity of the lens is called the hyaloid fossa. 

The vitreous body is a gelatinous structure, with a dense cortex that attaches to the surrounding structures. Its core is looser and features a narrow and somewhat oblique channel that extends from the optic disc to the posterior pole of the lens. This channel is called the hyaloid canal and it serves to transmit the hyaloid artery in fetal life, which supplies the lens in this period. The function of the vitreous body is to contribute to the refraction of light, although its dioptric index is significantly smaller than that of cornea and lens.

Aqueous humor

The aqueous humor is a nutrient-rich fluid that fills the anterior and posterior chambers of the eye. The amount of aqueous humor in a healthy human eye is 200 microliters. The aqueous humor is produced by the ciliary processes and delivered into the posterior chamber of the eye. 

The humor then passes between the zonular fibers and then through the iris, to reach the anterior chamber of the eye. Further, the aqueous humor flows through the trabecular meshwork of Schlemm’s canal and drains into it. The function of the aqueous humor is to nourish the lens and cornea, which are devoid of arterial blood supply (avascular).