This paper will take a close look at the anterior portion of the human eye, investigating such properties as structural and chemical makeup, optics, and function. The anterior portion of the human eye can be separated into four major areas: the sclera, cornea, aqueous humor, and iris.

The sclera, (Greek, skleros, hard) is the hard white outer wall of the eye, i.e. the "white" of the eye (1). Closely interwoven fibres, the main component of the sclera, create this white color (2).

Directly in the "front and center" of the eye, the surface of the eye bulges out, and the fibres of the sclera are arranged in a regular fashion. This creates an area in the sclera that is transparent, known as the cornea (2). The cornea is primarily responsible for letting light into the eye. Approximately 70% of the focusing of light is accomplished by the cornea (mostly due to the large change in refractive index from air to cornea) (2) (the fine-tuning is, of course, completed with the aid of the eye lens). The cornea consists of six distinct layers: epithelium, epithelial basement membrane, Bowman’s layer (also known as Bowman’s membrane), substantia propria (stroma), Descemet’s membrane, and the endothelium (3).

The epithelium consists of five or six layers of cells. The first layer or so tends to have flat, overlapping squamous cells, similar in design to those found on the surface of the skin. The major difference between these two sets of cells results from the need of the epithelial surface cells to be transparent, thus the epithelium is not keratinzed. Despite this, epithelial cells are often referred to as keratinocytes. The middle layers are made up of cells that get progressively more columnar the deeper the layer. These are known as the wing cells. The innermost, or basal layer, is made up of columnar cells packed closely together. All of the cells are held together by a cement substance, and some of the cells form protrusions that are connected into the indentations of adjacent cells by means of attachment bodies, known as desmosomes. Between the basal cells of the epithelium, and Bowman’s membrane, is an extremely thin basement membrane. This epithelial basement membrane is only about 60 to 65 nm thick. It is connected to the basal cells by means of attachment bodies known as hemidesmosomes (3). The hemidesmosomes maintain anchoring filaments and adhesive glycoproteins such as laminin and fibronectin. Laminin and fibronectin are thought to provide the key attachment between the basal cells and the major integral components of the basement layer, type IV collagen and heparan sulfate. Bullous pemphigoid antigen, fibrin, and type VII collagen have also been identified in the human corneal basement membrane (4).

Through electron microscopy it has been discovered that tiny microvilli extend fingerlike from the outer cell membranes of the surface cells. It is suspected that these microvilli, which project into the tear film, trap tear fluid to prevent the drying of the epithelial cells (3).

The epithelium represents 10% of the total wet weight of the cornea. Of this wet weight, 70% consists of water. The 30% solid portion is comprised of nucleic acids (DNA, RNA), lipids (phospholipids and cholesterol) in the cell membranes, and proteins. The epithelium also contains: ATP, 2000 m mol/kg wet weight; glycogen, 10 mg/g; glutathione, 75 to 180 mg/g; ascorbic acid, 47 to 94 mg/100 g; traces of acetylcholine (ACh) and cholinesterase’s (3).

All major classes of cytoskeletal proteins are apparent in the cornea: keratins (as mentioned), actin microfilaments, and microtubules (present during cell mitosis). Keratin filaments are about 10nm in diameter, and are divided into two major categories: acidic and basic. In humans, about 20 different types of keratins have been discovered, several of these found in the cornea (both acidic and basic types). Acting filaments run around 5-7nm in diameter, most often found in the surface cytoplasm of the two outermost (surface) cells (4).

Although it is still a major source of research, it is evident that vitamin A is an essential nutrient for the cornea, its differentiation, development, and maintenance. The Cornea primarily makes use of the metabolized retinol, which is suspected to be supplied the cornea via the tear film. The two major functions of retinol appear to be control of keratin expression and synthesis of glycoconugates (4).

The cornea is a major center of oxidative metabolism in the cornea. Oxygen reaches the epithelium from the tear film at a rate of 3.5 – 4.0 m l/cm² per hour. The primary substrate metabolized is Glucose, supplied mainly by the stroma. The Glucose first goes through phosphorylation to glucose 6-phosphate, which is in turn metabolized three different ways. The first, which accounts for 85% of the glucose metabolized, is glycolosis, which aids in the production of ATP (adenosine triphosphate). The second is the conversion into glycogen, which ends up being localized in the basal cell layer. Glycogen can serve as an energy source during periods of oxygen deprivation, trauma, or as a consequence of poorly fitting contact lenses, however the glycogen stores are rapidly depleted under such conditions. The third way is through the pentose phosphate pathway, or hexose monophosphate shunt. The pentose phosphate pathway both metabolizes pentoses and produces NADPH, which aids in the protection of the cornea by keeping free radicals in a reducing state, free radicals that could otherwise supply potential oxidative damage to the cornea (4).

Bowman’s membrane is a sheet of transparent tissue, approximately 12 m m thick. Bowman’s membrane contains no cells, but rather appears to be made up of uniform fibrils, most likely of collagenous material, that run parallel to the surface of the cornea. This modified superficial stromal layer is interestingly enough found only in primates (3).

The substantia propria, or stroma, comprises 90% of the cornea. It is composed of layers of lamellae, each running the full length of the cornea. Each lamellae is made up bundels of collagen fibrils, each fibril being separated by a ground substance 64 to 66 nm thick. The bundles do interlace with one another, although they are roughly parallel to the surface. As well, the lamellae are very loosely adherent to each other. The cell bodies, known as corneal corpuscles, or keratocytes, are flattened and also lie parallel to the surface. Their flatness also leads to interlacing of cells and cell processes. The arrangement of the stroma, having all bodies parallel to the surface, allows for optic uniformity (3).

The corneal stroma is differentiated connective tissue containing 75%-85% water (on a wet weight basis). The remaining solids, 20%-25%, constitute mainly collagen, other proteins, and gluycosaminoglycans or mucopolysaccharides. The skeleton of the corneal stroma is primarily made up of corneal fibrils, neatly organized and with a periodicity of 64 to 66nm (as mentioned)(3). They are typically 25-30nm in diameter. Four collagen types have been identified: types I (50%-55%), III (£ 1%), V (8%-10%, and VI (25-30%). These collagen fibrils are fairly similar to tendon and skin collagen, containing high nitrogen, glycine, proline, and hydroxyproline content (3). The design of the fibrils overall creates a natural diffraction grating, a key suspect for the cornea’s ability to scatter 98% of incoming light. It is not known to what degree of regularity is needed to ensure transparency. However, according to the "lattice theory" of D.M. Maurice (1957), scattered light waves interact in a perfectly ordered fashion, resulting in the elimination of destructive interference (4). 4% to 4.5% of the dry weight of the cornea consists of glycosaminoglycans (GAG or mucopolysaccharides). GAGS are suspected to attach to collagen fibrils or soluble proteins, and act as anions to bind cations and water. Three major fractions of GAG are found in the cornea: keratan sulfate (50%), chondroitin (25%), and chondroitin sulfate A (25%). There are few if any GAG to be found in the sclera, leading to the suspicion that GAG function to maintain hydration level and transparency (3). It is imperative that the degree of hydration remains constant, and that the arrangement of the collagen fibers remains orderly, as these are key issues in determining the transparency of the cornea (4).

Corneal collagen is formed during embryonic and early postnatal development, resulting in very little synthesis of collagen, if any, in a normal adult stroma. Although little research has been accomplished in this area, it is suspected type III collagen is synthesized by the endothelial cells, which deposit it in the growing Descemet’s membrane of a neonatal cornea. The other three types, I, V, and VI, appear to be synthesized by stromal-derived fibroblasts. It is further speculated that type VI collagen, due to its unusual structure and role as a space-filling element, may be involved in the development of the transparency of the neonatal cornea (4).

Descemet’s membrane is a 10 m m (although this varies with age) thick sheet which separates the deep stroma from the endothelium. It is suspected that Descemet’s membrane is the product of the secretion of endothelial cells (3). It seems to consist of three sections: a granular basement membrane-like material at the stromal surface, wide-spaced collagen in the center, and again a basement membrane-like layer at the endothelial surface (4).

Descemet’s membrane consists of type IV collagen, and a high content of glycine, hydroxyglycine, and hydroxyproline (3). It is hypothesized that type VIII collagen also exists in Descemet’s membrane. Amounts of laminin and fibronectin have also been discovered (4).

Lining Descemet’s membrane is a single cell layer, known as the endothelium. It is the bottom layer of the cornea, and therefore its inner surface contacts with the aqueous humor. The endothelium strives to keep its single cell layer between the aqueous humor and Descemet’s membrane. In humans, the endothelium has extremely limited, if any, reproductive capability. Therefore, as cells are lost due to age or trauma, the remaining cells increase in size and spread to maintain the barrier. This means that as a person gets older, the density of their endothelium decreases (3).

The hexagonally shaped cells of the endothelium seem to serve two major functions. The first involves continuously pumping fluid and ions out of the stroma and into the aqueous humor to maintain corneal dehydration and consequently transparency. The second is the act of the endothelium as a barrier to control the entry of fluid and dissolved solutes into the stroma via the aqueous humor. In between the cells of the endothelium are maculae occludentes, or focal tight junctions, however these junctions do not appear to prevent the penetration of ions and certain small molecules. The endothelium is therefore thought of as more of a leaky barrier, where the pump and the leaks act as balanced processes to maintain corneal dehydration (3).

Because of this leak and pump process, the endothelium, while sharing similar charbohydrate metabolism processes as the epithelium, is considerably more active. Calculations of respiration have shown that the oxidative activity in the endothelium is in fact five to six times greater than that of the epithelium. The endothelium also has an abundance of mitochondria, to handle the extra energy needs. As far as the metabolic processes go: 93% of the glucose 6-phosphate is converted to pyruvate through anaerobic glycolysis; 70% of this pyruvate is converted to lactic acid dehydrogenase; the remainder is oxidized through trycarboxylic acid cycle to produce ATP. NADH is also metabolized in the endothelium (through a similar pentose phosphate pathway), again with the main purpose being to keep potentially damaging free radicals in a reducing state (4).

The nerves supplying the cornea derive from the ciliary nerves, the end-branches of the ophthalmic division of the fifth cranial nerve. The nerves enter the cornea in the middle and anterior stromal layers, and move in a radial fashion toward the center of the cornea. About a millimeter or so into the cornea, the nerves lose the myelin sheaths, meaning that the corneal nerves are fairly easy to observe within this first millimeter. After that, they become extremely hard to observe without high magnification, yet another modification necessary to keep the cornea as a highly transparent section. As they run into the center of the cornea, the nerves divide dichotomously. In Bowman’s membrane, deep within the cornea, nerve fibrils form a plexus just beneath the epithelium. The free nerve endings in the epithelium run between the epithelial cells, making the cornea one of the most sensitive tissues in the body. This sensitivity is a form of protection, guaranteeing that if anything should happen maliciously to the cornea, it will be noticed, even if it is relatively small (3).

The refractive index of the cornea is generally placed at 1.377 for humans. Interestingly enough, the cornea exhibits birefringent properties. This arises from the anisotropic (meaning the material has a different structure in different directions) nature of the lamellae in the stroma. The fibrils that make up these lamellae are arranged in such a way that they behave similar to a positive uniaxial crystal, with the optic axis placed parallel to the axis of the fibril rods. The from birefringence, b_t, can than be described as:

Here, n_e = refractive index along extraordinary axis; n_o = refractive index along ordinary axis; n₂ = refractive index of ground substance; n_s = mean refractive index of the stroma; f_I = fraction of the volume occupied by the fibres. The total birefringence of an individual lamella is equal the sum of the form birefringence (b_t) and the intrinsic birefringence of the fibres themselves. The optic axis for this lamellar birefringence lies along the direction of the fibres within the lamellae. The corneal stroma has about 100 lamella, and the fibrils within each lamella being more or less randomly oriented with respected to those in nearby lamella. For calculating the total birefringence of the stroma, the optical axis will have to be placed perpendicular to the stroma. Because of the curvature of the cornea, incident light entering the cornea that is not perpendicular to optic axis will make an angle with the normal to the corneal surface, and therefore result in retardation. This retardation can be represented in two formulas:

Here: d = phase difference in radians (retardation); r = optical path length through cornea; q = angle between optic axis and direction of light in the cornea; l = wavelength of light. This formula can also be represented as:

Here: b = birefringence of the cornea; a = angle between incoming light ray and the normal to corneal surface; n_c = refractive index of the cornea; d_c = thickness of the cornea. To calculate the birefringence of a single lamella, simply take 2b . It should be noted that these formulae only hold in a first approximation (5).

 The contour of the cornea has been an important area of research, especially with regards to the fitting of contact lenses. The general picture places the radius of curvature at the center of the cornea at around 7.8mm, with the surface progressively flattening towards the outer edges. Although corneas tend to be slightly asymmetrical, an approximation can be made, assuming the surface to be conicoid with rotational symmetry about the z-axis. This can be represented as:

 Here: x and y are Cartesian coordinates perpendicular to the z-axis of rotational symmetry; r₀ is the radius of curvature at the corneal apex (essentially the center); and Q is an asphericity parameter. Q basically specifies the form of the concoid: Q > 0 is an ellipsoid wit the major axis in the x, y plane; Q = 0 is a sphere; -1 < Q < 0 is an ellipsoid with its major axis in the z direction; Q = -1 is a paraboloid with its axis along the z-axis; and Q < -1 is a hyperboloid. Q can also be further related to the parameters p (shape factor) and e (eccentricity) by:

P.M. Kiely (1982) pinpointed the means and standard deviations for r₀ and Q to be 7.72± 0.27mm and –0.26± 0.18 respectively, implying that a typical corneal surface corresponds to an ellipsoid with its major axis along the z direction (or optical axis), which has been confirmed in other findings (5).

The posterior surface of the cornea (endothelium) has a central radius of curvature of 6.5 mm, giving the total corneal shape a meniscus look with a thinner center and thicker edge. The total refractive index of the cornea (1.377) means that the anterior surface of the cornea has a paraxial power of about +49, and posterior surface –6D (5).

The area between the cornea and the lens, containing the iris, is known as the anterior chamber. The anterior chamber is filled with a fluid known as the aqueous humor. The aqueous humor transports oxygen and nutrients to the parts of the anterior eye it bathes, and in turn also carries away their waste products. Normally, blood vessels would maintain this job, but blood is not exactly transparent, and would seriously interfere with light trying to reach the retina. The ciliary bodies, or ciliary epithelia, constantly supply the anterior chamber with the fluid, as well as assisting in the fluid’s drainage (2). The aqueous humor flow is in and out is to such an extent that the fluid is completely replaced every four hours (6).

All of the three basic mechanisms for material transfer across an epithelial barrier are likely in use for the secretion of the aqueous humor – diffusion, ultrafiltration, and active transport. Diffusion transports solutes across cell membranes from areas of greater concentration to those of lesser concentration. Ultrafiltration happens when a hydrostatic driving force (i.e. hydrostatic pressure) increases the diffusion of a substance across a membrane. Active transport involves utilizing cellular energy to transport solutes against the concentration gradient (i.e. from lesser to greater concentration). Active transport probably contributes the most to the aqueous humor delivery, transporting a solute followed by the osmotic flow of water into the posterior chamber, while ultrafiltration apparently serves a secondary role (3).

The concentration of certain key solutes in the aqueous humor compared to those same concentrations in the plasma was the key clue to identifying active transport as the primary means of aqueous humor secretion. This means that the rate of aqueous humor formation is directly related to the rate of transfer of solutes across the ciliary epithelia. The primary portion of the ciliary epithelium concerned with the secretion is the nonpigmented epithelium, or NPE. The membrane-bound enzyme complex, sodium-potassium adenosine triphosphatase (Na⁺ /K⁺ ATPase), is an energy-dependent active transport system present in the NPE. It appears that Na⁺ is the actively transported ion, and Cl^- or HCO₃^- follow to maintain electroneutrality. In the ciliary bodies, below the layer of NPE, is the pigmented epithelium. The greater development of intracellular organelles and the higher metabolic rate of NPE when compared to PE indicates the NPE is the primary layer involved in aqueous humor formation. In fact, it is still not entirely understood what role the PE plays. Although the exact procedure involved in the regulation of the amount of aqueous humor is not entirely mapped out, the ciliary epithelia contain an enzyme-receptor complex, known as adenylate cyclase, has been found to trigger a reaction that reduces the flow of aqueous humor into the anterior chamber (3).

The composition of the aqueous humor is difficult to determine, as the composition largely depends on the nature of the freshly secreted fluid, the succeeding passive and active solute exchanges across local tissue, and the rate of drainage from the eye. Basically, this means that diffusional and metabolic alterations of the aqueous humor occur constantly. Of course the aqueous humor is largely water, but at times contains a number of components: glucose, which it feeds into the cornea; lactic acid, which the cornea pumps out to the aqueous humor; oxygen; amino acids, including alanine, valine, lysine, leucine, and arginine in humans; a small amount of proteins; immunoglobulins, part of the immune system; and traces of coagulation systems, fibrinolyitc systems, and cell growth inhibitors. Some of the other inorganic substances found in the human iris include bicarbonate, chloride, and a trace of phosphate. Other organic substances found in the human iris (all in relatively small concentrations) include ascorbate (ascorbic acid), citrate, and hyaluronate (3).

The refractive index of the aqueous humor is 1.337. The size of the aqueous humor is usually around 3.8 to 3.0 mm, although this size reduces in adults, due to the increase in thickness of the lens (5).

The ciliary epithelia and iris are often associated together as a single body, often referred to as the vascular uveal coat (which also includes the choroid) of the eye (3) or as the iris-ciliary body (4). The ciliary epithelia extend posteriorly from he iris, lying basically adjacent to the lens. The flow of aqueous humor extends from these epithelia, between the lens and the iris, and out into the anterior chamber (3).

The word iris is most likely derived from the Latin word for rainbow. It appears the term was first applied in the sixteenth century, making reference to this multicolored portion of the eye (8). The iris itself extends out over in front of the lens, forming a circular array, with a variable opening in the center, otherwise known as the pupil (2). Actually, the pupil is not located exactly in the center of the iris, but rather slightly nasally and inferiorly (below the center)(3). The iris, which is made up of two bands of muscles, controls the pupil: the dilator, who contracts to enlarge the pupil, and the sphincter, who contracts to reduce the size of the pupil. A neurotransmitter, acetylcholine, is primarily responsible for initiating sphincter response (2). Norepinephrine has also been identified as a neurotransmitter present in the iris (4). The contractions of these muscles are controlled by responses of vision detection mechanisms on the retina, and are an integral part of the vision system. The pupillary light reflex can than be ascertained to result from stimulus of receptors in the retina (3).

The iris is made of five basic sections: the anterior border layer, the iris stromal layer, the sphincter, the dilator, and the posterior pigment epithelium (8)(7). The anterior border layer lies, as it implies, on the anterior surface of the iris. It is similar to the stromal layer in structure, but more densely packed. The individual pigment cells that make up both the anterior border layer and the iris stroma are called chromataphores. The stromal layer consists of the pigment cells and a collagenous connective tissue that is arranged in arch-like processes. Throughout the iris stroma are radially arranged corkscrew-like blood vessels (8). Both of the iris muscles lie in the iris stroma. The first set of muscles is the sphincter. The sphincter pupillae is a typically smooth muscle, lying directly in front of the neuroectodermal pigment epithelium. The dilator muscles are part of the pigmented myoepithelium, which lies directly in front of the posterior iris epithelium. The myoid elongations of the dilator cells extend in front of the pigmented cell bodies. All of the first three parts, stroma, sphincter, and dilator, and intimately connected both physiologically, and in movement. The final layer of the iris is the posterior iris epithelium, a heavily pigmented layer, containing the pigment melanin (3). The main purpose of this heavy layer is to make the iris "light tight", or, in other words, impenetrable to light (much like the sclera)(8).

The visual appearance of the iris is a directly related to its multi-layered construction. The anterior layer is divided into two basic regions, the central pupillary zone and the surrounding cilliary zone. The border between these areas is known as the collarette. The collarette appears as a zigzag circumferential ridge, where the anterior border layer begins to drop into the pupil. The ciliary zone is characterized by interlacing ridges resulting from stromal support. The ridges tend to vary with the state of the pupil (contracted or dilated). Other striations can be seen as an effect of the blood vessels beneath the surface. Crypts, nevi, and freckles make up the other main source of variation on the iris. A crypt is an irregular atrophy of the border layer. Nevi are small elevations in the border layer. Freckles are local collections of chromataphores. The pupillary zone, on the other hand, tends to be relatively flat. It will occasionally feature radiating spoke-like processes and a pigment frill where the posterior layer’s heavily pigmented tissue shows at the pupil boundary (8).

The color of the iris, a readily identifiable trait, is the result of a process known as the "scattering-selective absorption combination" (6). The very back of the iris contains a very heavily pigmented epithelial layer (the pigment is melanin), designed to prevent light from entering the eye outside the pupil, light that could potentially cause flare problems in the lens. Although this pigment should ideally absorb all the light, it does not, and some scattering occurs. The color of the iris comes from the amount of pigment in the layers lying in front of this pigmented epithelial layer, due to the effect of these layers on the reflected light. For example, if these anterior layers have little pigment, the iris appears blue in color. This effect arises from Rayleigh scattering from the fine ultra-thin stromal fibers in the anterior layers (much like a similar Rayleigh scattering on a clear sunny day). However, if these layers have a greater amount of pigment, the pigment tends to contribute a yellow component. Combine this yellow component with the scattered blue, and the result is a green coloration to the iris. If the anterior layers have even further levels of pigment, the scattered light is darkened to a brown color. In persons with no pigment, the pinkish iris color emerges from the reddish light reflected from the iris capillaries, combined with the bluish Rayleigh scattering (6). The pigment, melanin, is the same pigment found in skin, and it is often thought that the concentrations in the eye are roughly similar to those in the skin; thus people with darker skin tend to have brown eye, and people with lighter skin tend to have bluish eyes.

The iris has been found to be incredibly unique from person to person, in both color and structure. In fact, it has been discovered by both opthalmologists and anatomists, examining large numbers of eyes, that even the left and right eye of and individual exhibit differences in their iris pattern. Also, the patterns appear to vary little after childhood. Developmental biology further suggests that, while the general structure of the iris is genetically determined, the particular aspects of its details are dependent upon circumstance, like the conditions in the embryonic precursor to the iris. Developmental biology also supports the lack of variance through life idea, noting that the iris is most fully developed and grows little have childhood. The only marked exceptions are the pigmentation, which does not fully mature until adolescence, and the size of the pupil, which is also not fully determined until puberty. However, once out of the teenage years, it is likely a persons iris variations will likely remain the same for the rest of their life (thus the enormous interest in utilizing iris variation in a biometric system).

The actual physical functionality of the iris is quite remarkable. It is often compared to the diaphragm of a camera, as it shares some characteristics. A typical iris has an f-number around f/2 or f/3, ideal for maximum exposure to light. The iris can change the amount of light coming into the eye in about a fifth of a second, but the reduction amount is miniscule – less than a factor of 20 (about another f-stop). This obviously points out that the iris is not responsible for control light intensity, for this is primarily the job of the rods and cones in the back of the retina. However, much like changing the f-stop on a camera, the iris can seriously reduce aberrations, especially in bright conditions, and increase depth of field. In bright light situations, much one would do on a manual camera, the iris stops down, or decreases in size. This smaller aperture/pupil allows less light in, and makes it easier to identify incoming images. When focusing in on close objects, such as a book, sewing needle, or watch, the iris stops down to increase depth of field. This is also the case in photography, as the smaller the pinhole letting in the light, the sharper the resulting image. During low light levels, however, the iris serves the opposite purpose. Much like one would do for night photography, increasing the f-stop to let in more light, the iris opens as much as possible to allow the faintest light to enter the eye. Once the rods have adjusted to current light levels, a small amount of light is all they need to perform basic image detection (1).

1. Falk, David S.; Brill, Dieter R.; Stork, David G. Seeing the Light- Optics in Nature, Photography, Color, Vision, and Holography. John Wiley & Sons, New York: 1986.

2. Tovee, Martin J. An Introduction to the Visual System. Cambridge University Press, Cambridge: 1996.

3. Moses, Robert A., M.D.; Hart, William M., Jr., Ph.D., M.D. (editors). Adler’s Physiology of the Eye- Clinical Application. The C.V. Mosby Company, St. Louis: 1987.

4. Berman, Elaine R. Biochemistry of the Eye. Plenum Press, New York: 1991.

5. Charman, Neil W. (editor). Visual Optics and Instrumentation. CRC Press, Boca Raton: 1991.

6. Keating, Michael P., Ph.D. Geometric, Physical, and Visual Optics. Butterworths, Boston: 1988.

7. Weale, R.A. Focus on Vision. Harvard University Press, Cambridge: 1982.

8. Wildes, Richard P. "Iris Recognition: An Emerging Biometric Technology". Proceedings of the IEEE. Vol. 85, No. 9: pgs.1348-1363 (September, 1999).

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