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Cornea:
The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber, providing most of an eye's optical power. Together with the lens, the cornea refracts light, and as a result helps the eye to focus, accounting for approximately 80% of its production to 20% of the lens focusing power. The cornea contributes more to the total refraction than the lens does, but, whereas the curvature of the lens can be adjusted to "tune" the focus depending upon the object's distance, the curvature of the cornea is fixed and restored.
Iris:
The iris consists of pigmented fibrovascular tissue known as a stroma. The stroma connects a sphincter muscle which contracts the pupil, and a set of dilator muscles which open it. Just in front of the root of the iris is the region through which the aqueous humour constantly drains out of the eye, with the result that diseases of the iris often have important effects on intraocular pressure, and indirectly on vision. Depending on the amount of light, the iris makes the pupil larger or smaller.
Pupil:
The pupil is the variable-sized, black circular or slit shaped opening in the center of the iris that regulates the amount of light that enters the eye. It appears black because most of the light entering the pupil is absorbed by the tissues inside the eye.
Aqueous:
The aqueous humour is a thick watery substance that fills the space between the lens and the cornea. In a healthy eye, the aqueous humour does not mix with the firm, gel-like vitreous humour because of the lens and its Zonule of Zinn between the two.
Trabecular Meshwork:
The trabecular meshwork is an area of tissue in the eye located around the base of the cornea, near the ciliary body, and is responsible for draining the aqueous humor from the eye via the anterior chamber (the chamber on the front of the eye covered by the cornea). The tissue is spongy and lined by trabeculocytes; it allows fluid to drain into a set of tubes called Schlemm's canal flowing into the blood system.
Lens:
The lens is a transparent, biconvex (lentil-shaped) structure in the eye that, along with the cornea, helps to refract light to be focused on the retina. Its function is thus similar to a human-made optical lens. The lens is also known as the aquula (Latin, a little stream, dim. of aqua, water) or crystalline lens.
In humans, the refractive power of the lens in its natural environment is approximately 15 dioptres, roughly one-fourth of the eye's total power.
Conjunctiva:
The conjunctiva is a clear membrane that covers the sclera (white part of the eye) and lines the inside of the eyelids. It helps lubricate the eye by producing mucus and tears, although a smaller volume of tears than the lacrimal gland.It also contributes to immune surveillance and helps to prevent the entrance of microbes into the eye.
Vitreous:
The vitreous is the transparent, colorless, gelatinous mass that fills the space between the lens of the eye and the retina lining the back of the eye. It contains very few cells (mostly phagocytes which remove unwanted cellular debris in the visual field), no blood vessels, and 99% of its volume is water with salts, sugars, and a network of collagen fibers with hyaluronic acid accounting for the rest. However, the vitreous has a viscosity two to four times that of pure water, giving it a gelatinous consistency. It also has a refractive index of 1.336.
Macula:
The macula or macula lutea is an oval yellow spot near the center of the retina of the human eye. It has a diameter of about 1.5 mm and is often histologically defined as having two or more layers of ganglion cells. Near its center is the fovea, a small pit that contains the largest concentration of cone cells in the eye and is responsible for central vision. It is specialized for high acuity vision. Within the macula are the fovea and foveola which contain a high density of cones.
Optic Nerve:
The optic nerve is the second of twelve paired cranial nerves but is considered to be part of the central nervous system as it is derived from an outpouching of the diencephalon during embryonic development. The fibers from the retina run along the optic nerve to nine primary visual nuclei in the brain, from whence a major relay inputs into the primary visual cortex.
Fovea:
The fovea is responsible for sharp central vision, which is necessary in humans for reading, watching television or movies, driving, and any activity where visual detail is of primary importance. The fovea is surrounded by the parafovea belt, and the perifovea outer region the parafovea is the intermediate belt where the ganglion cell layer is composed of more than five rows of cells; the perifovea is the outermost region where the ganglion cell layer contains two to four rows of cells, and is where visual acuity is below the optimum.
Retina:
The retina is a thin layer of neural cells that lines the back of the eyeball of vertebrates and some cephalopods. It is comparable to the film in a camera. In vertebrate embryonic development, the retina and the optic nerve originate as outgrowths of the developing brain. Hence, the retina is part of the central nervous system (CNS). It is the only part of the CNS that can be imaged directly.
Choroid:
The choroid, also known as the choroidea or choroid coat, is the vascular layer of the eye lying between the retina and the sclera, with a thickness about 0.5 mm. The choroid provides oxygen and nourishment to the outer layers of the retina. The red eye effect on photos is caused by the reflection of light from choroid. It appears red because of the choroid's blood vessels.
Sclera:
The sclera, called the white or white of the eye, is the opaque (usually white, though certain animals, such as horses and lizards, can have black sclera), fibrous, protective, outer layer of the eye containing collagen and elastic fibers. In children, it is thinner and shows some of the underlying pigment, appearing slightly blue. In the elderly, however, fatty deposits on the sclera can make it appear slightly yellow.
Retinal Pigment Epithelium:
The retinal pigment epithelium is the pigmented cell layer just outside the neurosensory retina that nourishes retinal visual cells, and is firmly attached to the underlying choroid and overlying retinal visual cells. When viewed from the outer surface, these cells are smooth and hexagonal in shape. When seen in section, each cell consists of an outer non-pigmented part containing a large oval nucleus and an inner pigmented portion.
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