The Protective Structures of the Eye. The two orbits, sometimes referred to as “sockets,” that protect the human eyes are situated at the front of the skull, each with a wider opening to the front narrowing to a small opening at the rear where the optic nerve exits to connect through the visual pathways and the brain. The orbits are angled outward approximately 23° with respect to the midline of the skull. The human eye itself is approximately 24 millimeters (mm) (0.94 inches [in]) in diameter and occupies about 25% of the volume of the orbit, allowing for the extraocular muscles, blood vessels, nerves, orbital fat and connective tissue that surround and support the eye (Figure 6-1). The orbit surrounds and supports most of the human eye, while the cornea and part of the anterior globe extend somewhat beyond the orbital rims. These structures are protected by the eyelids.
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| Figure 6-1. The position f the eye in its socket (Wolff, 1933). |
The upper and lower eyelids form an aperture that is generally 30 mm (1.2 in) wide and 10 to 12 mm (0.4 to 0.5 in) high when the eye is “open.” The lids themselves have cartilage-like tarsal plates within their structure that provide shape to the lids and additional strength for protection of the eye. Each lid has a row of cilia or eyelashes that are very sensitive to touch or particles near the eye, which when stimulated bring on the blink reflex. The lids also contain the glands responsible for maintenance of the tear layer. The globe itself is predominately formed of and protected by the sclera that extends from the edges of the clear
cornea at the front of the eye (the “limbus”) to the optic nerve at the back of the eye. The sclera is a thick, opaque white tissue that covers 95% of the surface area of the eye. It is approximately 530 microns (μm) in thickness at the limbus, thinning to about 390 μm near the equator of the globe and then thickening to near 1 mm (0.04 in) at the optic nerve. At the posterior aspect of the eye, the sclera forms a netlike structure or “lamina cribrosa” through which the optic nerve passes. The sclera also serves as the anchor tissue for the extraocular muscles.
The Anterior Segment of the Eye
The portion of the eye visible to the observer without special instrumentation is considered the anterior (or
“front”) segment of the eye. Most of the structures responsible for focusing images onto the retina of the eye are here. The cornea is the primary focusing structure, providing about 75% of the focusing power of the eye. The crystalline lens provides the remaining variable focusing power and serves to further refine the focus, allowing the eye to focus objects at different distances from the eye. The iris controls the aperture or pupil of the eye for different light levels. The iris is actually an extension of the ciliary body, a structure that has multiple functions in the anterior segment, from production of the fluid that fills the anterior segment (aqueous humor) to suspension and control of the shape of the crystalline lens of the eye. Figure 6-2 shows most of the major structures of the human eye, including the components of the anterior segment, the protective sclera and the posterior segment (described in the next section).
The cornea
The cornea is a unique biological tissue that is transparent to light and contains no blood vessels. This small transparent dome at the front of the eye is approximately 11 mm (0.43 in) in diameter and 500 μm thick in the center, thickening to around 700 μm at the periphery. At the very edge of the cornea, transparency is slowly lost over a 1-mm (0.04-in) range in an area known as the “limbus”, which is where the cornea integrates into the opaque sclera. The cornea is more curved than the rest of the globe with an average radius of curvature of 7.7 mm (0.3 in), while the radius of curvature of the globe is approximately 12 mm (0.5 in).
“front”) segment of the eye. Most of the structures responsible for focusing images onto the retina of the eye are here. The cornea is the primary focusing structure, providing about 75% of the focusing power of the eye. The crystalline lens provides the remaining variable focusing power and serves to further refine the focus, allowing the eye to focus objects at different distances from the eye. The iris controls the aperture or pupil of the eye for different light levels. The iris is actually an extension of the ciliary body, a structure that has multiple functions in the anterior segment, from production of the fluid that fills the anterior segment (aqueous humor) to suspension and control of the shape of the crystalline lens of the eye. Figure 6-2 shows most of the major structures of the human eye, including the components of the anterior segment, the protective sclera and the posterior segment (described in the next section).
The cornea
The cornea is a unique biological tissue that is transparent to light and contains no blood vessels. This small transparent dome at the front of the eye is approximately 11 mm (0.43 in) in diameter and 500 μm thick in the center, thickening to around 700 μm at the periphery. At the very edge of the cornea, transparency is slowly lost over a 1-mm (0.04-in) range in an area known as the “limbus”, which is where the cornea integrates into the opaque sclera. The cornea is more curved than the rest of the globe with an average radius of curvature of 7.7 mm (0.3 in), while the radius of curvature of the globe is approximately 12 mm (0.5 in). |
| Figure 6-2. Cross-sectional view of the eye (http://www.gimbeleyecentre.com/images/ Cross_Section_Labelled.gif). |
With the primary function of transmitting and focusing light into the eye, all the structures of the cornea are very specifically arranged (Figure 6-3). About 90% of the cornea is made up of evenly spaced collagen fibrils arranged in sections that crisscross to cover the entire extent of the cornea. This layer is known as the “stroma” and it provides not only transparency, but strength. Four more layers make up the remaining 10% of the cornea, the epithelium and Bowman’s layer at the front of the cornea and Descemet’s membrane and the endothelium at the back of the cornea. The epithelium of the cornea, much like the epithelium of the skin, serves as a barrier to bacteria or other pathogens.
Additionally, the epithelium helps to maintain the stroma at a proper level of hydration by preventing fluid from entering the stroma through its tight cell junctions and the pumping of a small portion of fluid out of the stroma. Bowman’s layer is a very thin (12 μm) membrane right beneath the epithelium and, in mammals, is only found in primates. Its purpose is not entirely known, although it may aid in protection of the stroma. At the back or posterior aspect of the cornea is another very thin membrane called Descemet’s membrane that is between 10 to 15 μm thick. It also is felt to have some protective function. The endothelium is a single layer of cells at the very posterior aspect of the cornea. The endothelium is in direct contact with the aqueous humor, the fluid that fills the anterior chamber of the eye. The endothelium pumps nutrients, such as glucose, from the aqueous humor into the cornea while actively pumping fluid out of the cornea. The hydration balance maintained by the endothelium and somewhat assisted by the epithelium is important to the transparency of the cornea, since excess fluid would disturb the regularity of the corneal fibrils and result in increased light scatter. In mild cases of edema, such as may occur when contact lenses are worn too long or under hypoxic conditions, the cornea may become slightly cloudy (Jones and Jones, 2001; Liesegang, 2002; Morris et al., 2007). Under more extreme conditions, such as anoxic conditions, or in cases of endothelial dystrophies, the swelling of the stroma could result in complete opacity of the cornea.
Additionally, the epithelium helps to maintain the stroma at a proper level of hydration by preventing fluid from entering the stroma through its tight cell junctions and the pumping of a small portion of fluid out of the stroma. Bowman’s layer is a very thin (12 μm) membrane right beneath the epithelium and, in mammals, is only found in primates. Its purpose is not entirely known, although it may aid in protection of the stroma. At the back or posterior aspect of the cornea is another very thin membrane called Descemet’s membrane that is between 10 to 15 μm thick. It also is felt to have some protective function. The endothelium is a single layer of cells at the very posterior aspect of the cornea. The endothelium is in direct contact with the aqueous humor, the fluid that fills the anterior chamber of the eye. The endothelium pumps nutrients, such as glucose, from the aqueous humor into the cornea while actively pumping fluid out of the cornea. The hydration balance maintained by the endothelium and somewhat assisted by the epithelium is important to the transparency of the cornea, since excess fluid would disturb the regularity of the corneal fibrils and result in increased light scatter. In mild cases of edema, such as may occur when contact lenses are worn too long or under hypoxic conditions, the cornea may become slightly cloudy (Jones and Jones, 2001; Liesegang, 2002; Morris et al., 2007). Under more extreme conditions, such as anoxic conditions, or in cases of endothelial dystrophies, the swelling of the stroma could result in complete opacity of the cornea.
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