The human eye is the organ which gives us the sense of sight,allowing us to learn more about the surrounding world than any of the other five senses. We use our eyes in almost everything we do, whether reading, working, watching television, writing a letter, driving a car, and countless other activities. Sight is the most precious of the five senses, and many people fear blindness more than any other disability.

The eye allows us to see and interpret the shapes, colors, and dimensions of objects in the world by processing the light theyreflect or give off. The eye is able to see in dim light or brightlight, but it cannot see objects when light is absent. The eyechanges light rays into electrical signals then sends them to thebrain, which interprets these electrical signals as visualimages.

The eye is set in a protective cone-shaped cavity in the skullcalled the orbit or socket and measures approximately one inchin diameter. The orbit is surrounded by layers of soft, fatty tissuewhich protect the eye and enable it to turn easily.

Six musclesregulate the motion of the eye. Among the more important partsof the human eye are the iris, cornea, lens, retina, conjunctiva,the macula, and the optic nerve.

Cornea (Histology and Molecular Biology)
The cornea is the front window of the eye. It protects the interior of the eye much like the watch crystal protects the inter workings of a watch. It is clear and lets light through as it helps focus the images on the retina or film of the eye. The cornea has 5 layers (listed out side to inside) the Epithelium (the outer skin, 7 cell layers thick), Bowmanns layer (20 microns thick), Stroma (majority of the cornea), Desemets Membrane (10 microns thick) and the Endothelium (inner skin). A microscopic drawing and light microscope view shows all 5 layers.

The vast majority of the corneal stroma consists of 200 to 500 layers of flattened collagenous lamellae extending from limbus to limbus, some crossing the apex (center) of the corneal dome and others which do not . In the anterior one third of the stroma, collagen lamellae are thin (about 0.2 to 1.2 microns thick and 0.5 to 30 microns wide), run obliquely to the corneal surface, and sometimes split into two to three sub layers that become interwoven. In the posterior stroma, collagen lamellae tend to be arranged parallel to the surface and are thicker (1.0 to 2.5 microns thick and 100 to 250 microns wide).

Collagen arrangement in the anterior 1/3 of stroma Arrangement of collagen lamellae and fibers in the corneal stroma.

The lamellae have no limiting membrane and are connected to each other by several of four methods; disulfide cross links (fiber to fiber), fibers that cross over and weave into the fibers of adjacent lamellae, lamellae that often divide into smaller components that later merge with adjoining lamellae and sometimes lamellae split into two to three sub layers that become interwoven. The arrangement of the collagen lamellae show regional differences. The lamellae in the anterior region run in random directions often running oblique to the surface and often branch and interweave in an irregular manner. Those in the posterior region are piled up parallel to the corneal surface. The lamellar bundles cross irregularly and form interlaced networks in the central section of the cornea. The lamellae in the posterior layers run across their neighbors at varying angles. These lamellae often divide into smaller components that later merge with adjoining lamellae. Each collagen lamellae is composed of thin collagen fibers running parallel. Within the flattened lamellae the normal collagen fibers of the cornea are arranged in a hexagonal oriented array in a dermatan (chondroitin) and keratan sulphate ground substance matrix.

Both are glycosaminoglycans. Triple helical collagen molecules (tropocollagen) 300nm long and 1.5nm in diameter are held together by interpeptide hydrogen bonds. Several thousand tropocollagen molecules tend to align parallel to each other thus forming the individual collagen fiber 25-35 nm. (E. De Robertis (9)). This is structural collagen Type I that makes up the majority of the cornea. Type VI collagen (Telopeptides) is non-helical and binds to the C- and N- termini of the Type I collagen helix.

The central and peripheral collagen fiber matrix

About 60% of the glycosaminoglycans in the stroma consist of keratan sulphate and the remainder dermatan sulphate. There is a correlation between the increase of collagen fibril diameter in the peripheral cornea and the decrease in keratan sulphate. The normal corneal collagen fibers and its lattice is tighter centrally and looser peripherally. The central corneal collagen fibrils extend to the periphery where the fibrils weave into the limbal collagen imparting considerable strength. The normal cornea consists of 78% water. About 12-15% and 1-3% of the net weight of the tissue is composed of collagen and glycosaminoglycans respectively.

The corneal lamella is thinner centrally (2 microns) than peripherally (3 microns). This molecular lattice arrangement also holds true for the anterior to posterior corneal stroma. At any given point on the cornea, the anterior lamella are smaller then the posterior lamella.

Tropocollagen building block of the collagen fiber.

When we go from periphery of the cornea to the central cornea, the lattice structure of the collagen tightens up and the collagen lamellae become smaller. The ground substance (glycosaminoglycans) of the central cornea is predominately keratan sulphate while the periphery is predominately dermatan sulphate. The ground substance is the packing material (filler) between the collagen lattice structure and between the collagen lamellae. Both dermatan sulphate and keratan sulphate bind to the collagen fibers at specific binding sites (cross-linking). These sites are essential to the spacing of the fibrils and to the width of the interfibrillar space and thus the size of the hexagonal array. The corneal stroma is unusual in that it contains no hyaluronic acid except at the limbus where there is a gradual increase in hyaluronic acid (ground substance) concentration towards the sclera.

Lens
The lens is the clear structure located behind the pupil. Itsprimary function is to provide fine-tuning for focusing andreading. The lens performs this function by altering its shape. Atabout the age of 40-50, the lens becomes less flexible andpresbyopia sets in. At about the age of 60 or 70, the lensbecomes cloudy and hard (cataract formation), preventing lightfrom entering the eye.

Pupil
The pupil is the ‘black circle’ that you see in people’s eyes. Theprimary function of the pupil is to control the amount of lightentering the eye. When you are in a bright environment, the pupilbecomes smaller to allow less light through. When it is dark,the pupil expands to allow more light to reach the back of theeye.

Iris
This is the colored part you see in people’s eyes (i.e.blue/green/brown/hazel). The primary function of the iris is tocontrol the size of the pupil. This is acheived through contractionor expansion of the muscles of the iris.

Vitreous Body
This is the clear ‘gel like’ substance located inside the eye’scavity. Its purpose is to provide a spherical shape to the eye.The vitreous may develop small clumps known as ‘floaters’,which are more common in nearsighted people than in the restof the population.

Optic Nerve
The optic nerve carries images from the retina to the brain.

Retina
The retina consists of fine nerve tissue which lines the insidewall of the eyes and acts like the film in a camera. Its primaryfunction is to transmit images to the brain.

Sclera
This is the ‘white part’ that we see in people’s eyes. The sclera’spurpose is to provide structure, strength and protection to theeye.