Understanding Ocular Structures
An examination of the structures of the eye and how each plays an important role in vision
Jenean Carlton BA, ABOC, NCLC
The human eye is an amazing organ measuring about one inch in length and with approximately 65 diopters of refractive power. Each structure within the eye plays an important role in capturing light and bringing it to a sharp focus on the retina. This article explores the structures of the eye and how each plays an important role in our sense of vision.
The eye can be divided into two principal anatomical parts: the anterior and posterior segments. The dividing line between these two sections is just behind the crystalline lens.
ANTERIOR SEGMENT
The anterior segment contains the crystalline lens and all structures in front of it, including the cornea, pupil, iris, conjunctiva, anterior chamber, zonular fibers, posterior chamber, aqueous humor, and ciliary body.
CORNEA. The cornea is a convex shaped window of the eye that is completely transparent. It is avascular and nourished by capillaries at the limbus, the tear film, and the aqueous humor. With 45 diopters of power, the cornea provides the greatest focusing capacity of all ocular structures. It is the cornea, not the crystalline lens, that provides the majority of the refractive ability of the eye.
The cornea measures 1mm in thickness along the periphery and .5mm in thickness in the center. It consists of five layers: the epithelium, Bowman’s membrane, the stroma, Descemet’s membrane, and the endothelium.
The first line of defense for the cornea is the epithelium; it’s the layer of the cornea typically injured by abrasions and foreign bodies and is five to seven cells thick. It has the remarkable ability to regenerate itself without leaving a scar and accounts for 10 percent of the entire corneal thickness. Even if completely removed, the epithelium will usually regenerate and cover the cornea without scarring.
Bowman’s membrane is a cellophane-like layer made of collagen fibrils that is resistant to injury and infection, but doesn’t regenerate itself and will scar if traumatized.
The stroma layer accounts for 90 percent of the corneal thickness and is made up of 78 percent water. It is the primary structure of the cornea.
Descemet’s membrane lies between the stroma and the endothelium. It is chiefly made up of collagen.
The endothelium is the innermost corneal layer and consists of a single layer of cells. It regulates corneal hydration and doesn’t regenerate itself if damaged. The endothelium functions as a pump to maintain the necessary level of tears on the surface of the cornea.
Two Eyes, Two Colors
Heterochromia iridis is the ocular condition in which one iris is a different color from the other; the cause can be congenital or a result ocular disease. While not common in humans, it is relatively common in animals, particularly cats.
Famous folk with heterochromia iridis include:
David Bowie, Mila Kunis, Dan Aykroyd, Jane Seymour, Kate Bosworth, Alice Eve, Josh Henderson, Simon Pegg, Kiefer Sutherland, Christopher Walken, and Alexander the Great.
IRIS. The iris is a circular structure that controls the amount of light entering the eye by adjusting the size of the pupil (the aperture in the center of the iris). In humans, eye color is determined by the color of an individual’s iris, which is dependent on melanin, the pigment that gives skin, hair, and eyes color.
Human eye color can be blue, brown, green, and in rarer cases, hazel (a combination of light brown, green, and gold), violet, gray, and even pink. It is rare, but an individual can have irises of two different colors (see sidebar, previous page).
CRYSTALLINE LENS. The crystalline lens is a clear, biconvex, natural lens inside the eye that measures 9mm to 10mm in diameter and has an approximate refractive power of 16 diopters. It consists of three parts: capsule, cortex, and nucleus. It is attached to the ciliary body by zonular fibers that have the ability to change the shape and therefore, the power of the lens.
| Eyelids and Tears |
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Eyelids cover the eyes and act as a protective curtain to guard them from injury as well as excessive light. Blinking effectively removes irritants from the surface of the cornea and conjunctiva and coats the eye with tear film to provide a smooth refractive surface on the epithelium layer of the cornea. The main muscles that control the opening and closing of the upper eyelid are the orbicularis oculi and levator palpebrae superioris. The orbicularis oculi closes the eyelid, and the contraction of the levator palpebrae muscle opens the lid. Tears have antibacterial properties that protect the eye and bring oxygen to the cornea. The tear film is essential to the health of the eye and has three layers: mucin, aqueous, and lipid. The mucin layer is innermost and is produced by goblet cells in the conjunctiva. This thin mucin layer is crucial to the stability of the tear film. The middle layer of the tear film is aqueous and is produced by the lacrimal gland located above and lateral to the eyeball. The top layer of the tear film is the lipid layer produced by meibomian glands in the eyelids and, as it’s an oily layer, aids in keeping the tear film from evaporating. It is imperative for all three layers of the tear film to be in balance for adequate hydration and nourishment of the eye. Tear film produced by the lacrimal and accessory glands is drained through the punctum, lacrimal sac, and nasolacrimal duct into the nose. |
Accommodation is the adjustment of the focal power of the eye and is achieved by the contraction of the ciliary muscle, which causes the crystalline lens to change its shape and power. Presbyopia is a naturally occurring loss of accommodation that begins around the age of 40 in most people.
ANTERIOR CHAMBER
The anterior chamber is the space between the iris and the cornea’s innermost layer, the endothelium. The anterior chamber is filled with aqueous humor, a transparent fluid that maintains the intraocular pressure of the eye and inflates the globe.
AQUEOUS HUMOR. Aqueous humor is a plasma-like substance secreted from the ciliary processes which are finger-like extensions of the ciliary body. The ciliary processes provide an attachment for the zonular fibers that connect to the lens and aid in accommodation.
Aqueous is in the anterior and posterior chambers of the eye and maintains intraocular pressure while providing nutrition for avascular ocular tissues. It provides inflation and volume for expansion of the cornea and drains through the trabecular meshwork, exiting the eye via the canal of Schlemm, a channel deep in the limbus.
POSTERIOR CHAMBER
The posterior chamber is the space between the back of the iris and the front of the crystalline lens. It is filled with aqueous humor produced by the ciliary processes in this chamber.
ZONULAR FIBERS. Zonular fibers are fine suspensory ligaments attached at the edge of the crystalline lens capsule to the ciliary processes. The zonular fibers, also known as zonules, together with the ciliary body, assist the eye in accommodation in order to clearly view objects both near and at a distance.
CILIARY BODY. The ciliary body is a ring-shaped structure that connects the iris with the choroid. It contains the ciliary muscle that controls accommodation for viewing objects at varying distances and regulates the flow of aqueous humor into the Canal of Schlemm. The ciliary processes, also part of the ciliary body, are finger-like shaped extensions that produce aqueous humor and provide an attachment for the zonules that support the crystalline lens.
CONJUNCTIVA. The conjunctiva is a highly vascular mucous membrane that is loosely attached to the sclera (the white part of the eye). Its main role is to protect and repair the cornea in the event of wounds, abrasions, or infections. Tiny blood vessels in the conjunctiva leak nutrients and antibodies into the tear film to wash over the corneal surface and promote healing.
The palpebral conjunctiva lines the interior of the upper and lower eyelids; the bulbar conjunctiva lines the sclera up to the limbus, the circular border between the cornea and sclera.
Goblet cells in the conjunctiva produce mucin, an integral part of the tear film, to lubricate and nourish the eye. Mucin allows the watery layer of the tear film to spread evenly over the surface of the eye and to help keep the eye hydrated.
POSTERIOR SEGMENT
The posterior segment comprises all ocular structures that lie behind the crystalline lens including the sclera, vitreous humor, retina, macula, fovea, choroid, optic disk, and optic nerve.
RETINA. The retina is a transparent, light-sensitive vascular structure that lies between the vitreous and the choroid. It contains photoreceptor cells called rods and cones that transmit nerve impulses to the brain when stimulated by light.
Rods are employed for movement and night vision. They are located mostly outside of the fovea centralis, which is the central most area of the macula.
The majority of cone cells are found within the fovea centralis. Cone cells are used during daylight to supply detailed vision and color perception. In the event that cone cells are damaged, patients will have difficulty reading and distinguishing distant objects.
| Optic Nerve |
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The optic nerve is the bundle of nerve fibers that exit the back of each eye and transmit visual impulses from the retina to the brain. The major blood vessels of the retina radiate from the center of the optic nerve. The optic disc represents the head of the optic nerve and corresponds to the normal blind spot of the eye. The human eye is a complex organ comprised of numerous structures that all play a vital role in capturing light and focusing it onto the retina, which transmits nerve impulses to the brain that are then interpreted into vision. |
MACULA. The macula is the small, yellowish, avascular area of the retina. The fovea lies in the middle of the macula and to the temporal side of the optic nerve head. It is densely packed with cones cells that provide sharp visual acuity and color vision.
SCLERA. The sclera, often referred to as the ‘white’ of the eye, is a tough, fibrous connective tissue that covers 5/6 of the eyeball. It is the major outer layer that protects the eye from injury and prevents rupture in the case of increased ocular pressure. The six extraocular muscles that control the movements of the eye are attached to the sclera.
VITREOUS HUMOR. The vitreous humor is a clear, fibrous, gel-like structure that fills the posterior segment. It fills 80 percent of the eye and protects the internal structures of the eye during movement and prevents the globe from collapsing in the event of an injury.
CHOROID. The choroid is a continuation of the iris and ciliary body and lies between the retina and sclera. It is a highly vascular tissue that supplies the retina with oxygen and nutrients. The choroid connects with the ciliary body toward the front of the eye and is attached to the edges of the optic nerve in the back of the eye. The primary function of the choroid is to provide nourishment to the retina.
Having an understanding of the primary structures within the eye, and how each plays a role in the miracle of vision, will benefit professionals assisting with patient care and will also broaden one’s personal knowledge of the human vision system.
Understanding Ocular Structures
This puzzle will help build your knowledge about the ocular structures of the eye and how each plays a role in human vision. Look for answers in our April issue.
across
3 A highly vascular mucous membrane attached to the sclera.
4 This membrane lies between the stroma and endothelium.
5 A plasma-like substance secreted from the ciliary processes. (Two words)
7 The ___________ chamber is the space between the back of the iris and the front of the crystalline lens.
8 The _____________ conjunctiva lines the inside of the upper and lower eyelids.
9 With 45 diopters of power the _______________ provides the greatest focusing capacity of all ocular structures.
11 Accommodation is the adjustment of the focal power of the eye and is achieved by the action of the ___________ muscle.
12 This layer of the cornea is the innermost layer and consists of a single layer of cells.
14 The ___________ oculi closes the eyelid.
19 The ______________ chamber is the space between the iris and the endothelium.
20 Human eye color is dependent upon this pigment.
23 These fibers are fine suspensory ligaments attached at the edge of the crystalline lens capsule to the ciliary processes.
24 This structure is a continuation of the iris and ciliary body and lies between the retina and sclera.
25 This membrane is a cellophane-like layer of the cornea made of collagen fibrils.
down
1 A ring-shaped structure that connects the iris with the choroid. (Two words)
2 The ocular condition in which one iris is a different color from the other.
3 The anterior segment contains the ____________ lens and all structures in front of it.
6 The ____________ functions as a pump to maintain the proper amount of tears on the surface of the cornea.
10 The ______________ is the first line of defense and offers protection for the cornea.
13 A circular structure that regulates the amount of light entering the eye by adjusting the size of the pupil.
15 The human eye can be divided into two principal parts, the _________ and posterior segments.
16 These cells in the conjunctiva produce mucin, an integral part of the tear film.
17 This layer of the cornea accounts for 90 percent of the corneal thickness.
18 This gland produces the middle layer of the tear film.
21 The crystalline lens consists of three parts: capsule, cortex, and __________.
22 The____________ conjunctiva lines the sclera up to the limbus.
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