The eye (latin: oculus) is a paired sensory organ, which is responsible for the sense of sight. The eyes are located in the orbital cavities, an opening in front of the skull. The frontal part of the eye is visible, while the posterior part is in the orbital cavity.
The frontal part is called the anterior segment of the eye. Even though, most people think that the eyeball is shaped like a ball, it is more like a two-piece shaped unit consisting of the anterior and posterior segments. The eyeball is less tall than it is wide. The height or vertical size is around 23.7 mm, the width or horizontal size – 24.2 mm, the anteroposterior or depth – 22-24.8 mm.
The eyeball is a spheroidal structure containing sense receptors for vision. The eyeball contains many structures, with each being responsible for different functions. The eyeball is surrounded by the adipose tissue and lies within the orbit. The eyeball is covered by a thin fascial sheath known as the Tenon’s capsule.
The anterior segment of the eyeball is small, transparent, and creates around 1/6 of the eyeball. The posterior segment of the eyeball is larger and forms 5/6 of the eyeball. The eyeball has an anterior and posterior pole. Both poles are connected via the optic axis. The midway between both poles is known as the equator. The eyeball consists of the layers of the eyeball and the refractive media of the eyeball.
The wall of the eyeball consists of three layers surrounding the vitreous body:
The fibrous layer of the eyeball consists of the sclera - the largest posterior part, white in colour, and the cornea - the anterior part of the fibrous layer, a transparent plate. The sclera and cornea are continuous with each other.
The sclera is a white outer layer surrounding the eyeball’s posterior five-sixths. The sclera is the thickest posteriorly. The outer surface of the sclera is smooth, with an exception in the place where the tendons of the orbital muscles attach to the sclera. In the frontal part, the sclera is continuous with the cornea. The connection site is called the corneoscleral junction. A scleral venous sinus (the canal of the Schlemm) lies behind the junction. It collects the aqueous humor from the eye’s anterior chamber.
The sclera has a posterior pole that is perforated by the optic nerve – the posterior scleral foramen. The sclera has also other openings: four anterior openings, four to five middle openings, numerous posterior openings. The sclera has three layers: the episclera, scleral stroma, and lamina fusca.
The episclera is the outer layer of the sclera. The scleral stroma consists of dense irregular connective tissue. The lamina fusca is the inner layer of the sclera. It covers the choroid.
The sclera is supplied by the episcleral plexus and the long and short posterior ciliary arteries. Venous drainage happens through the vorticose veins into the superior and inferior ophthalmic veins. The sclera is innervated by the long and short ciliary nerves.
The sclera acts as a barrier for the eye components and protects those from mechanical trauma. The rigid structure of the sclera is the reason why the eyeball can maintain its shape and keep all the structures in place.
The cornea makes up the frontal one-sixth of the eyeball. The cornea is a circular and transparent layer covering the pupil, iris, and anterior chamber. The cornea does not contain any blood vessels but has receptors. The cornea consists of five layers: stratified corneal epithelium, Bowman’s membrane, substantia propria, Descemet’s membrane, and corneal endothelium.
The stratified corneal epithelium is the superficial layer of the cornea. Bowman’s membrane has no cells within it, consisting only of irregularly arranged collagen fibers. Substantia propria is the thickest layer making up almost 90% of all the cornea. Descemet’s membrane is considered as the basement membrane for the following layer (the corneal endothelium). Descemet’s membrane connects to the network of the canal of Schlemm. The corneal endothelium is the deepest layer. The endothelium has only one cell layer.
The corneal endothelium continues with the endothelium of the iridocorneal angle. The cornea participates in the refraction of light. The cornea has the most considerable refractive power, and its refractive index is different from the refractive index of the air.
The cornea is avascular, so the nutrients are supplied by the aqueous humor. It has no lymphatic drainage. The central part of the cornea receives oxygen indirectly from the air, while the peripheral part - by diffusion from the anterior ciliary arteries. The cornea is innervated by the fibers from the ophthalmic division of the trigeminal nerve, mainly by the long ciliary nerves.
The middle vascular layer is rich with blood vessels and pigment, and it includes the choroid - the largest part of the vascular layer, the ciliary body, and the iris in the anterior part. One of the main functions of the ciliary body is the production of aqueous fluid. The iris contains the pigment that determines the eye color.
There is a hole in the middle of the iris called the pupil. The narrowing of the pupil is provided by the sphincter pupillae, while the widening is provided by the dilator pupillae. These are smooth muscles surrounding the pupil and innervated by the autonomic nervous system.
The choroid is a thin coat lining the inner surface of the sclera. The choroid is rich in blood vessels and accounts for almost 90% of the total blood flow in the eye. It lies between the optic nerve and the ciliary body. The choroid is the thickest at the posterior pole. Between the sclera and the choroid is a space called the perichoroidal space. The choroid has three layers: the vessel layer, the capillary layer, and the Bruch’s membrane.
The vessel layer is the outer layer and consists of arteries that are branches of the short posterior ciliary arteries. The capillary layer consists of a network of capillaries that have saclike dilatations. Bruch’s membrane is the innermost layer with five components: the endothelium’s basement membrane of the capillaries, the outer layer of collagen fibers, the network of elastic fibers, the inner layer of collagen fibers, and basement membrane of the retina’s pigmented epithelium.
The main function of the choroid is to get the blood to the outer layers of the retina.
The choroid is supplied by the posterior ciliary arteries branching off of the ophthalmic artery. Venous drainage happens through vorticose veins. Innervation is received by the long and short ciliary nerves.
The ciliary body is found in front of the choroid. The ciliary body consists of blood vessels, connective tissue, and smooth muscle cells. The frontal part of the ciliary body is called corona ciliaris, while the dorsal part – orbiculus ciliaris. Corona ciliaris gives around 70 radial extensions called the ciliary processes that produce the aqueous humor. The anterior and long posterior ciliary arteries meet in the anterior part of the ciliary body.
The ciliary body is a passage for the nerves. The ciliary body has four layers: the ciliary epithelium, ciliary stroma, ciliary muscle, and supraciliary layer. The ciliary epithelium has two layers of cubital cells. The epithelium has pigmented and unpigmented parts. The stroma consists of loose connective tissue. The ciliary muscle plays a role in the accommodation. The supraciliary layer is made of collagen fibers. The ciliary body is involved in the accommodation.
The ciliary body is supplied by the anterior ciliary and long posterior ciliary arteries. The venous blood is drained by the vortex veins. Innervation is received by the short ciliary nerves.
The iris is found in front of the ciliary body. The iris consists of blood vessels, connective tissue, and smooth muscle cells. The iris is a thin, contractile structure with pigment and a central opening – the pupil. The anterior surface of the iris has two margins or zones: the large ciliary zone and the small pupillary zone. Both zones join together in a place called the collarette.
The iris is the structure that divides the space between the cornea and the lens into the anterior and posterior chambers. From anterior to posterior, the iris is divided into three parts: the anterior border layer, the stroma, and the epithelial layers. The anterior surface of the iris has no epithelium. The anterior surface contains large depressions called the crypts of Fuchs. The stroma houses extensively vascular connective tissue containing collagen fibers, fibroblasts, melanocytes, and matrix, as well as nerve fibers, the smooth muscle cells.
The sphincter pupillae muscle is found in the pupillary zone of the iris. The dilator pupillae muscle is found as a myoepithelium layer extending from the root of the iris to the sphincter pupillae.
By controlling the dilatation and constriction of the pupil, the iris controls how much light enters the eye.
The iris is supplied by radial vessels from the stroma of the iris. The arteries originate from the major arterial circle found in the stroma of the ciliary body. The venous drainage of the iris happens through the veins that follow the arteries and also create a minor venous circle. The iris is innervated by the long and short ciliary nerves.
The nervous layer or the retina is the deepest layer. The retina can be further divided into two layers: pigmented or outer layer and neurosensory (neural retina) or inner layer. The space between both of the layers is called the subretinal space. The retina is the light-sensitive structure of the eye, where the neural visual pathway begins.
The neural retina contains three layers of neural cells: light-sensitive photoreceptor cells, bipolar cells, and ganglion cells of the retina. The retina is a thin, transparent membrane. The retina is housed between Bruch’s membrane of the choroid externally (the retina’s outer surface) and the aqueous humor internally (the retina’s inner surface). The retina has numerous types of cells and layers. The retina is made up of ten layers, from the outside inwards:
The pigmented layer of the retina or the retinal pigmented epithelium has a single cell layer extending between the optic nerve and the ora serrata. The neural retina comprises different cells: the photoreceptors, the bipolar cells, the ganglion cells, the horizontal cells, the amacrine cells, and the supporting cells.
Within the retina, it has characteristics that work as topographic landmarks for orientating around the retina. The two notable landmarks are the macula lutea and optic disc. The macula lutea is an oval-shaped, highly pigmented area in the center of the inner retinal layer. A few millimeters medially to the macula lutea is the optic disc also known as the optic nerve head.
The retina has two types of blood supply. The outer five layers of the retina are avascular and have an indirect supply from the choroidal capillaries. The inner layers are supplied directly from the capillaries connected to branches of the central retinal artery and vein. Venous drainage happens similarly to the blood supply, with all tributaries ending up in the central retinal vein. The retina receives its efferent innervation through axons within the optic nerve.
The refractive media of the eyeball is made up of structures that take part in focusing the ray of light onto the retina so the photoreceptors can detect it. The eye has four refractive media structures: the cornea (described above), lens, vitreous body, and aqueous humor (associated with the chambers of the eye).
The lens is a double convex, circular structure located anterior to the vitreous body and posterior to the iris. Together with the cornea, the lens transmits light to the retina. The lens is encapsulated structure swimming in aqueous humor. The lens is divided into anterior and posterior surfaces by the outer margin of the lens, also known as the equator.
The lens is avascular and has no nerve fibers. No other structures are preset as the lens needs to keep its transparency. The lens has three parts: the lens capsule, the lens epithelium, and the lens fibers. The elastic capsule surrounds the outer surface of the lens. The lens epithelium lies below the capsule. The lens fibers are responsible for the substance of the lens.
The majority of the lens consists of the lens fibers, which anteriorly are covered by a single layer of epithelium and are adjective by the outer lens capsule. By the ability to change its shape, the lens selectively transmits light and adjusts the focus of the eye so that the retina can form a sharp image. This process is known as accommodation. The suspensory ligament of the lens takes part in changing the shape of the lens through the accommodation of the eye.
The vitreous body is the largest structure of the eyeball located posterior to the lens. The vitreous body does not contain blood vessels, and nerve ends. It consists of gel-like liquid - the vitreous humor - surrounded by a capsule. The vitreous body has a dense cortex but a liquid center. The center of the vitreous body is looser.
Within the margin of the optic disc, the vitreous body attaches to the neural retina. In front of the vitreous body is the hyaloid fossa (the anterior concavity), where the lens sits. The hyaloid canal, a remnant of the hyaloid arteria from the embryonic period, passes through the vitreous body.
The vitreous body or the vitreous humor has no colour, and it is almost filled with water. The vitreous body contributes to the refraction of light, even though its dioptric index is much smaller than the cornea index and lens index.
The aqueous humor is a fluid filling the anterior and posterior chambers of the eye and produced by the ciliary processes. The aqueous humor is loaded with nutrients. The aqueous humor supplies the cornea and lens supports the eyeball, and maintain the intraocular pressure. The aqueous humor originates from the plasma in the ciliary processes and is constantly secreted in the posterior chamber.
The fluid flows between the zonular fibers, around the equator of the lens, then through the pupil and the iris into the anterior chamber. Within the anterior chamber, the aqueous humor circulates and then leaves the chamber through the iridocorneal angle in two possible ways. The first and main route is via the canal of Schlemm, while the other route - through the ciliary muscle, an alternative uveoscleral pathway.
Production of the aqueous humor occurs due to active transport by the unpigmented cells of the ciliary epithelium. The aqueous humor supplies the cornea and lens with nutrients. It contains glucose, amino acids and has a high concentration of ascorbic acid.
The anterior chamber is a small space found behind the cornea and anterior to the iris, a part of the ciliary body, and a small area of the anterior surface of the lens. The posterior chamber is a small space between the iris in the front, the ciliary processes surrounding it, and the lens behind it.
Accessory structures of the eye are responsible for the movement of the eyeball and its protection. The accessory structures are:
The eyebrows are situated at the junction of the forehead and the upper eyelid. The eyebrows have a medial and lateral end, with the medial end being just below the orbital margin, while the lateral end lies above the orbital margin.
The eyebrows are supplied by the branches of the ophthalmic artery. Venous drainage happens through the corresponding veins. The muscles affecting the eyebrows are innervated by the facial nerve.
The eyelids are structures working as a barrier to protect the eyes from injury and too much light. The protection is acquired by closing the eyelids. Blinking allows the eyelids to distribute the tears over the eyeball's anterior surface. Humans have upper and lower eyelids. Between the eyelids is a palpebral fissure - the space between the lateral and medial canthi.
The eyelid has two parts: an orbital and a tarsal part. The division is made by a horizontal furrow called the superior palpebral sulcus. The upper eyelid is an insertion site for a powerful muscle - the superior levator palpebrae muscle. The eyelids have six layers: the skin, subcutaneous tissue, muscle fibers of the orbicularis oculi, orbital septum and tarsal plate, smooth muscle, and conjunctiva.
The skin of the eyelids is thin and folds easily. The skin has tiny hair with sebaceous glands and sweat glands. The subcutaneous tissue is loose and contains many elastic fibers. The orbicularis oculi muscle is an elliptical muscle surrounding the orbital margin and extending onto the temporal region.
The eyelid forming base is made of a membranous sheet called the orbital septa. The orbital septa attaches to the orbital margin. The tarsal plates are made of dense fibrous tissue and are responsible for the shape of the eyelids.
The superior and inferior tarsal muscles are made by the smooth muscle. The superior tarsal muscle raises the upper eyelid and helps the superior levator palpebrae muscle. The inferior tarsal muscle lowers the lower eyelid. The conjunctiva is a mucous membrane covering the inside of the eyelids. The conjunctiva covers part of the sclera.
The eyelids receive blood from the lateral and medial palpebral arteries. Venous drainage happens via the veins that drain into the ophthalmic, angular, and superficial temporal veins. The eyelids are innervated by the branches of the trigeminal nerve.
The conjunctiva is a thin mucous membrane covering the inner surface of the eyelids and the sclera. The conjunctiva helps to lubricate the eye. The conjunctival epithelium changes into the epidermis of the skin at the eyelid's margin and into the corneal epithelium at the limbus. The conjunctiva can be divided into three parts or regions: the palpebral conjunctiva, conjunctival fornices, and bulbar conjunctiva.
The palpebral conjunctiva is the part that attaches to the posterior surfaces of the tarsal plates and lines the inner surface of the eyelids. The conjunctival fornices are the regions between the palpebral and bulbar conjunctiva. The bulbar conjunctiva is in direct contact with the eyeball. It is translucent.
The conjunctiva receives its blood supply from the two palpebral arches and anterior ciliary arteries. Venous drainage happens via veins running along the arteries and drain into the palpebral veins or directly into the superior and inferior ophthalmic veins. The different parts of the conjunctiva receive innervation from different branches of the trigeminal nerve.
The orbit is the cavity of the skull where the eyeball lies. The orbit contains not only the eyeball but also extraocular muscles, some of the cranial nerves, orbital and retrobulbar fascia, blood vessels, ligaments, and lacrimal gland. The orbit houses and protects the eyes.
The lacrimal apparatus is a set of connected anatomical structures located within the orbit that are responsible for the production and drainage of tears.
The lacrimal apparatus include the following structures:
The lacrimal gland is a paired lobular gland located at the upper lateral corner of the orbit that is responsible for the production of tears. The tears protect and help to maintain the eyeball and refractive structures viable and functional. The lacrimal gland consists of orbital (larger) and palpebral (smaller) parts that are continuous posterolaterally around the concave lateral edge of the aponeurosis of the levator palpebrae superioris muscle.
The lacrimal gland has lobulated tubulo-acinar structure. The lobules are separated by loose connective tissue. The lacrimal gland receives its blood from the lacrimal artery originating from the ophthalmic artery. Venous drainage happens through the superior ophthalmic vein. The lacrimal glands are innervated by branches of the ophthalmic, maxillary, and facial nerves.
The lacrimal canaliculi are small channels between the puncta lacrimalia and the lacrimal sac that participate in the tear drainage system of the lacrimal apparatus. Each lacrimal canaliculus opens with punctum lacrimalis, an opening on the margin of each lower eyelid in the medial corner of the eye, where the drainage of tears begins. There are two lacrimal canaliculi - superior and inferior.
The lacrimal sac is the upper dilated segment of the nasolacrimal duct that is a part of the tear drainage system of the lacrimal apparatus. The main function of the lacrimal sac is to serve as a reservoir for the overflow of tears. The lacrimal sac is supplied by branches of the ophthalmic, facial, and maxillary arteries. It is innervated by the trigeminal nerve’s branches.
The nasolacrimal duct is a channel that is directly continuous with the lacrimal sac and opens into the nasal cavity, forming the final part of the tear drainage system of the lacrimal apparatus. The nasolacrimal duct drains excess tears from the nasolacrimal sac into the nasal cavity, which explains the phenomenon of a running nose when a person is crying. The nasolacrimal duct is supplied by branches of the ophthalmic, facial, and maxillary arteries. It is innervated by the trigeminal nerve’s branches.
There are seven extraocular muscles, also known as the extrinsic muscles of the eyeball, in each eye. The extraocular muscles controlling eye movements include six striated muscles: the superior rectus, inferior rectus, medial rectus, lateral rectus, superior oblique, and inferior oblique muscles. Additionally, there is a muscle that provides elevation of the upper eyelid, the levator palpebrae superiors. The muscle actions are:
Even though the eye is a small structure, it still has many conditions affecting it.
Strabismus, also known as the crossed eye, is a disorder when the eyes do not line up with one another, meaning that the eyes are turned in different directions. In normal conditions, the eye's muscles control eye movement, so the eyes work together. But in the case of strabismus, the eye movement control is affected, and the eye cannot keep ocular alignment.
Strabismus is caused by abnormal neuromuscular control. Less commonly, the reason is a problem of an actual muscle. Strabismus is often inherited. Strabismus can be associated with different medical conditions:
Based on the direction of the turned or misaligned eye, strabismus is categorized:
Strabismus has several forms, while three forms are more commonly seen:
Strabismus is generally treated with glasses or contact lenses, eye exercise, medications, eye patching, eye muscle surgery.
Astigmatism is a common disorder with imperfect curvature of the eye resulting in the blurred distance and near vision. Astigmatism can be present already at birth.
The cornea and lens have round curvature. In astigmatism, one or both structures have an egg-shaped shape with two mismatched curves. It causes the light rays to be bent differently, creating two different images.
Astigmatism's symptoms are blurred vision, eyestrain or discomfort, headaches, squinting, and poor night vision.
Treatment is corrective lenses or refractive surgery.
Myopia or nearsightedness is a very common vision disorder when people can clearly see near objects, but objects in the distance are blurry. The eye's shape makes light rays bend wrongly and results in focusing images in front of the retina, not on the retina. Myopia occurs when the eyeball is longer, or the cornea is curved too steeply. Myopia can progress slowly or rapidly. The worsening is usually seen during childhood.
Symptoms of myopia are blurred distant objects, eyestrain, headaches, squinting. Myopia can be first noticed when children want to sit closer to the TV, in front of the classroom, having persistent squinting.
Most commonly, myopia is treated with prescription lenses. Refractive surgery is also possible.
Cataract is a disease when a cloudy area forms in the lens. Cataract is more common with age. Cataract usually is age-related, but sometimes an eye injury or eye surgery may be a reason for it. In these cases, cataract can also affect younger people. In older people, len's proteins start to break down and stick together, causing cloudiness. People can get cataract in one or both eyes, but cataract cannot spread from one eye to another. People may not notice having cataract at the beginning.
Risk factors: smoking, excess alcohol usage, family history, diabetes, eye surgery, eye injury, upper body radiation therapy, steroid usage.
Symptoms of glaucoma are blurred vision, seeing pale colours, bad vision in poor lightning, bright lights seem too bright, seeing a halo around lights, seeing double.
At the beginning of cataract a home treatment is used. Start by using brighter lights at home, wearing anti-glare sunglasses, or using magnifying lenses. A new prescription for glasses may be needed. If none of the previous is helping or the cataract worsens, surgery may be done. The surgeon will remove the lens and replace it with an artificial one.
Glaucoma is a disorder affecting the optic nerve due to abnormally high intraocular pressure. Glaucoma can be considered as one of the leading causes of blindness in people over the age of 60.
The damage of the optic nerve leads to blind spots in the visual field. Increased intraocular pressure is caused by a buildup of aqueous humor. If the fluid is produced too much, the drainage does not work properly, and the fluid builds up. Glaucoma has a genetic background.
Glaucoma has many types:
Risk factors: age over 60, high intraocular pressure, ethnicity (Asian or Hispanic), family history, other diseases like diabetes, high blood pressure, extreme nearsightedness or farsightedness, eye injury, corticosteroid medication.
Symptoms of glaucoma:
Treatment is using eye drops, like prostaglandins, beta-blockers, cholinergic agents, etc. Oral medication can be used together with eye drops. Also, surgery can be done by opening the blockage with laser therapy, filtering therapy to create an opening in the sclera, drainage tubes.
Horner syndrome is characterized by signs and symptoms caused by the disruption of the nerve pathway going traveling from the brain to the eye. In most cases, Horner syndrome results from other medical conditions – stroke, tumor, spinal cord injury, neck injury, demyelinating diseases. In rare cases, no reason can be found.
Symptoms are persistently small pupil, lowering of the upper eyelid, delayed opening of the affected pupil, the difference in pupil size, sunken appearance to the eye. Horner syndrome does not have a specific treatment. Usually, the symptoms go away when the underlying reason is treated.
Presbyopia is a condition of progressing loss of the eye’s ability to focus on nearby objects. Presbyopia is a natural age-related condition when the lens gets harder. The lens becomes less flexible, disabled to change its size, thus no longer being able to focus. Risk factors are age, drugs, diabetes, multiple sclerosis. Symptoms: blurred vision when reading at a normal distance, headaches, eyestrain, holding reading material farther away. Treatment options are eyeglasses, contact lenses, refractive surgery, and lens implants.
Amblyopia is also known as the lazy eye. It occurs due to abnormal visual development in early life. The affected eye receives fewer visual signals. Amblyopia develops during the birth and the age of 7 years.
The top reason for amblyopia in children is an imbalance of the muscles that position the eyes. Risk factors: premature birth, family history, developmental disabilities, small birth size.
Symptoms: one eye wanders inward or outward, head tilting, eyes that do not work together, poor depth perception.
Treatment is glasses, contact lenses, eye patches, eye drops, surgery.
Corneal abrasion is the same as a scratch on the eye. The abrasion can occur by poking the eye with a pointy end, foreign materials, dirt, sand getting into the eye, chemicals, eye infection, dirty contact lenses.
Symptoms are feeling grit in the eye, pain in the eye, tearing, redness, sensitivity to light, blurred vision.
Treatment is antibiotic eye drops or ointment. The doctor may also prescribe eye drops for the pain.
Retinal detachment occurs when a thin layer of the retina pulls away from its normal position. Retinal detachment blocks the blood supply to the cells in the layer pulled away.
Causes are based on three types of retinal detachment:
Risk factors: aging, previous detachment, family history, myopia, eye surgery, eye injury.
Symptoms: blurred vision, reduced side vision, flashes of light, a curtain-like shadow over the visual field, appearance of floating particles.
Retinal tears are treated with laser surgery or freezing. Retinal detachment is treated by injecting air or gas into the eye, indenting the surface of the eye, draining and replacing the fluid in the eye.
Age-related macular degeneration (AMD) is a disease that blurs central vision. AMD develops with age caused by damage to the macula. AMD has two types: dry and wet. Dry-type is more commonly seen. Dry-type is caused by macula getting thinner. On the other hand, the wet type is caused by abnormal blood vessels growing in the back of the eye, thus damaging the macula.
Most people above 60 years of age are affected. The macula is the structure responsible for the sharp and straight-ahead vision. AMD is one of the leading causes of vision loss in older people. People do not lose vision completely but lose central vision.
Risk factors for AMD: smoking, Caucasian ethnicity, family history of AMD. Symptoms of AMD depend on the stage of AMD. In the case of early AMD, people do not have any symptoms. In the case of intermediate dry AMD – symptoms may not present, but some people may have mild blurriness in their central vision or affected vision in poor lighting.
In the late stage, patients see wavy lines instead of straight lines, have blurred vision. Eventually, the blurred vision can expand, and blank spots are seen.
Early-stage AMD does not have any treatment. If a person has intermediate AMD, dietary supplements may stop the progression. In the case of wet AMD, injections of anti-VEGF need to be done. Also, photodynamic therapy can be used. There is no treatment for late dry AMD.