Although the eye is commonly referred to as the globe , it is not really a true sphere. It is composed of two spheres with different radii, one set into the other ( Figs. 1.1 and 1.2 ). The front, or anterior, sphere, which is the smaller and more curved of the two, is called the cornea . The cornea is the window of the eye because it is a completely transparent structure. It is the more curved of the two spheres and sets into the other as a watch glass sets into the frame of a watch. The posterior sphere is a white opaque fibrous shell called the sclera . The cornea and the sclera are relatively nondistensible structures that encase the eye and form a protective covering for all the delicate structures within.
In terms of size, the eye measures approximately 24 mm in all its main diameters in the normal adult.
The eye itself is covered externally by the eyelids , which are movable folds protecting the eye from injury and excessive light. The lids serve to swab the eye and spread a film of tears over the cornea, thereby preventing evaporation from the surface of the eye. The upper eyelid extends to the eyebrow , which separates it from the forehead, whereas the lower eyelid usually passes without any line of demarcation into the skin of the cheek. The upper eyelid is the more mobile of the two and when it is open it covers about 1 mm of the cornea. A muscle that elevates the lid, the levator palpebrae superioris , is always active, contracting to keep the eyelid open. During sleep the eyelid closes by relaxation of this muscle. The lower lid lies at the lower border of the cornea when the eye is open and rises slightly when it shuts.
Normally, when the eyes are open, a triangular space is visible on either side of the cornea. These triangular spaces, formed by the junction of the upper and lower lids, are called the canthi ( Fig. 1.3 ). These canthi are denoted by the terms medial and lateral , the former being closer to the nasal bridge. Most eyes are practically the same size; therefore when we speak of the eyes appearing large or small, we usually refer not to the actual size but to the portion of the eyeball visible on external examination, which in turn depends on the size of the palpebral fissure . The shape of the fissure also determines its appearance. In Asian persons, a fold of skin extends from the upper lid to the lower lid and covers the medial fissure, giving the eye its characteristic obliquity. In the medial fissure there are two fleshy mounds: the deeper one, called the plica semilunaris , and the superficial one, called the caruncle ( Fig. 1.4 ). The caruncle is modified skin that contains sweat and oil glands. Occasionally, it also contains fine cilia or hairs. When the eyes are open, the palpebral fissures measure about 30 mm in width and 15 mm in height.
The free margin of each lid is about 2 mm thick and has an anterior and a posterior border. From the anterior, or front, border rises the eyelashes , which are hairs arranged in two or three rows. The upper eyelid lashes are longer and more numerous than the lower ones and they tend to curl upward. The lashes are longest and most curled in childhood. The posterior border of the lid margin is sharp and tightly abuts against the front surface of the globe. By depressing the lower lid, one can see the thin gray line that separates the two borders of the lid. This gray line is used in many surgical procedures to split the upper and lower lids into two portions. Also visible on both lids are the tiny openings that are the orifices of the sweat- and oil-secreting glands.
The largest oil-secreting glands, which are embedded in the posterior connective tissue substance of the lids (called the tarsus ), are called the meibomian glands . The lacrimal gland is located above and lateral to the globe. Tears are produced by the lacrimal gland and travel through fine channels, referred to as ducts , to empty onto the conjunctival surface.
On the medial aspect of the lower lid where the lashes cease is a small papilla . At the apex of this papilla is a tiny opening called the punctum (see Fig. 1.4 ). The punctum leads, by means of a small canal, through the lower lid to the lacrimal sac ( Fig. 1.5 ), which eventually drains into the nose. Tears are carried to the punctum by the pumping action of the lids and there they are drained effectively from the eye by means of tiny channels. A similar but smaller opening is found in the upper lid almost directly above it. The punctum normally cannot be seen by looking directly at the eye. It can be seen only by depressing the lower lid or everting the upper lid. The muscle underlying the eyelid skin is the orbicularis oculi , which is roughly circular. When it contracts, it closes the eye.
The portions of the eye that are normally visible in the palpebral fissures are the cornea and sclera . Because the cornea is transparent, what is seen on looking at the cornea is the underlying iris and the black opening in the center of the iris is called the pupil . The sclera forms the white of the eye and is covered by a mucous membrane called the conjunctiva . The conjunctiva extends from the junction of the cornea and sclera and terminates at the inner portion of the lid margin ( Fig. 1.6 ). The conjunctiva that covers the eye itself is referred to as the bulbar conjunctiva , whereas the portion that lines the inner surface of the upper and lower lids is called the palpebral conjunctiva . The junctional bay created when the two portions of the conjunctiva meet is referred to as the fornix . The lower fornix easily can be viewed by depressing the lower lid.
The role of the conjunctiva is to defend and repair the cornea in the event of scratches, wounds, or infections. The almost invisible blood vessels that are present dilate and leak nutrients, antibodies, and leukocytes into the tears that then wash over the avascular corneal surface. The conjunctiva also secretes mucus and oil, both of which help to keep the cornea moist and clean and to reduce friction when the lids blink over the cornea. The conjunctival mucous film over the ocular surface catches microorganisms. This mucous net then condenses into a ball and is carried to the nasal canthus where it dries and rolls onto the skin. The conjunctiva also helps to resurface the cornea with epithelial cells if the entire corneal surface is scraped or burned.
Under the conjunctiva is a fibrous layer that overlies the sclera and rectus muscles. This is Tenon’s capsule , a common surgical landmark.
The tear film is composed of three layers ( Fig. 1.7 ). The outermost layer consists of a lipid or fatty layer, mostly cholesterol esters, and is extremely thin. This layer is secreted by the meibomian glands and acts to prevent evaporation of the underlying aqueous layer. The central layer is chiefly aqueous, with some dissolved salts, as well as glucose, urea, proteins, and lysozyme. This layer is secreted by the lacrimal glands. The third layer is a very thin mucous layer lying over the surface of the conjunctiva and cornea. This layer is secreted by specific cells of the conjunctiva referred to as goblet cells and is important in the stability of the tear film. Tear film abnormalities may arise in association with a number of clinical problems in older adults and in particular problems related to contact lenses.
The precorneal tear film layer serves several functions. It forms a smooth refractive surface on the epithelium.
It maintains a moist environment for the epithelium.
It carries oxygen to the eye.
The cornea is a clear, transparent structure with a brilliant, shiny surface. It has a convex surface that acts as a powerful lens. Most of the refraction of the eye takes place not through the crystalline lens of the eye but through the cornea.
The cornea is relatively large at birth and almost attains its adult size during the first and second years. Although the eyeball as a whole increases a little less than 3 times in volume from birth to maturity, the corneal segment plays a small role in this part, being fully developed by 2 years of age.
The cornea is thicker at its periphery (1 mm) than at the center (0.5 mm). It can be divided into five distinct portions ( Fig. 1.8 ): the epithelium, Bowman’s membrane, the stroma, Descemet’s membrane, and the endothelium.
The epithelium is the part of the cornea usually injured by superficial abrasions or small foreign bodies. It is 5 to 7 cells thick (50 μm) and is composed of nonkeratinized stratified squamous cells. The epithelium functions as a barrier and as an important refractive optical surface. It regenerates rapidly and heals without leaving a scar. Injury to the deeper structures usually results in formation of an opacity in the cornea.
Bowman’s membrane consists of randomly oriented collagen fibrils of greater periodicity than the underlying stroma. This acellular layer, which is 10 μm thick, has no regenerative capabilities. Its function is unclear.
The layer just under Bowman’s membrane is the stroma . This structure is 950 μm at the periphery and about 450 μm centrally; it accounts for 90% of the corneal thickness. The stroma consists of 200 to 250 evenly spaced type I collagen lamellae, which are oriented at right angles to their adjacent lamellae. It is composed of 78% water.
Descemet’s membrane is 3 μm thick at birth, and 10 to 12 μm thick in older adults. It is composed of type III collagen. This very elastic layer retracts if cut. It forms the basement membrane of the epithelial cells.
The endothelium is a 4 to 6 μm monolayer of 500,000 cells. There is a gradual decrease in endothelial cells with age. There is no known mechanism of attachment between the endothelium and Descemet’s membrane. The endothelium is responsible for maintaining deturgescence of the cornea. No regeneration of this layer has been shown in humans. Corneal edema (swelling) can occur when contact lens materials, overwear, improper cleaning, or improper fit does not allow sufficient oxygen to reach the cornea.
The junction of the cornea and sclera is demarcated by a gray, semitransparent area referred to as the limbus . This transitional zone is only 1 mm wide and marks the point of insertion of the conjunctiva. The cornea, which contains no blood vessels, is completely nourished by three sources: a plexus of fine capillaries at the limbus, the tear film, and the aqueous humor.
In a paper published in Ophthalmology in 2013, by Dua et al, the existence of a newly described pre-Descemet’s layer, hypothetically 15 μm thick, was suggested. Time will be needed to see if others can confirm the existence of this new layer and its potential significance.
The opaque sclera forms the posterior five-sixths of the eye’s protective coat. Its anterior portion is visible and constitutes the white of the eye. In children, the sclera is thin and therefore it appears bluish because the underlying pigmented structures are visible through it. In old age, it may become yellowish because of the deposition of fat. Attached to the sclera are all the extraocular muscles. Through the sclera pass the nerves and the blood vessels that penetrate the interior of the eye. At its most posterior portion, the site of attachment of the optic nerve , the sclera becomes a thin, sieve-like structure called the lamina cribrosa , through which the retinal fibers leave the eye to form the optic nerve. The episcleral tissue is a loose connective and elastic tissue that covers the sclera and unites it with the conjunctiva above. Unlike the sclera, the episcleral tissue is highly vascular.
The uveal tract consists of three structures: the iris, ciliary body , and choroid .
The iris is the most anterior structure of the uveal tract. It is perforated at its center by a circular aperture called the pupil . The iris has many ridges and furrows on its anterior surface. Contraction of the iris, which occurs in response to bright light, is accomplished by the activity of a flat, washer-like muscle called the sphincter pupillae , buried in its substance just surrounding the pupillary opening. Expansion or dilation of the pupil is facilitated by relaxation of the sphincter muscle and by activation of the radially oriented dilator muscle of the iris found at its peripheral circumference. Expansion and contraction of the iris, like an accordion, form circular pleat lines or furrows visible on its surface. In addition to these ridges and furrows, numerous white zigzag lines are formed by the blood vessels of the iris. Between the iris and the cornea is a clear fluid called the aqueous humor . This fluid occupies the space called the anterior chamber of the eye.
The ciliary body ( Fig. 1.9 ) is in direct continuity with the iris and is adherent to the underlying sclera. Directly posterior to the iris, the ciliary body is plump and thrown into numerous folds referred to as the ciliary processes . This portion of the ciliary body is only about 2.5 mm in length and is responsible for the major production of aqueous fluid. The equator of the lens is only 0.5 mm from the ciliary processes and is suspended by fine, ligamentous fibers known as the zonular fibers of the lens. The posterior portion of the ciliary body is flat. Most of the zonular fibers of the lens originate from the ciliary body. The ciliary body in general is triangular, with its shortest side anterior. The anterior side of the triangle in its inner part enters the formation of the angle of the anterior chamber. The iris takes root from its middle portion.