To Mati | Νικος Βακαλης

THE EYE

CORNEA

EYE

CRYSTAL LENS

s iris, is called inanatomyofeyethe discoid perforated septum in the anterior part of the eye lying between itcorneaand hislensand in the middle of which is its openingdaughter. Its role is to regulate the amount of light that enters the eye and reaches the eyeretina, contracting when the light is abundant and expanding when there is little, thus helping hervisionandsense of depth. This is possible because the iris is made up of muscle tissue. It is part of itstratum corneumof the eye.

The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Along with the anterior chamber and lens, the cornea refracts light, accounting for approximately two-thirds of the eye's total optical power.[1][2] In humans, the refractive power of the cornea is approximately 43 dioptres.[3] The cornea can be reshaped by surgical procedures such as LASIK.[4]

While the cornea contributes most of the eye's focusing power, its focus is fixed. Accommodation (the refocusing of light to better view near objects) is accomplished by changing the geometry of the lens. Medical terms related to the cornea often start with the prefix "kerat-" from the Greek word κέρας, horn.

HYALOID

The vitreous body is a clear, colorless, gelatinous material located in the cavity of the eyeball between the lens and the retina. It contains few cells, has no vessels and is 99% water with salts, glucose and few collagen fibers.

With age the vitreous liquefies and loses its gelatinous texture causing cells or other organic particles to float freely giving the impression of flying flies (myopsia).

Although the vitreous is in close contact with the retina, it is not attached to its entire surface, but only in the periphery, along the vessels and mainly in the area of the optic nerve. If the vitreous liquefies it can detach from the retina and then the point of fixed attachment remains on the posterior surface of the vitreous, which now floats and gives the impression of a round or elongated black spot described as a spider or fly.

IRIS

n humans and most mammals and birds, the iris (plural: irides or irises) is a thin, annular structure in the eye, responsible for controlling the diameter and size of the pupil, thus the amount of light reaching the retina. Eye color is defined by that of the iris. In optical terms, the pupil is the eye's aperture, while the iris is the diaphragm.

The iris is usually strongly pigmented, with the color typically ranging between brown, hazel, green, gray, and blue. Occasionally, the color of the iris is due to a lack of pigmentation, as in the pinkish-white of oculocutaneous albinism,[1] or to obscuration of its pigment by blood vessels, as in the red of an abnormally vascularised iris. Despite the wide range of colors, the only pigment that contributes substantially to normal human iris color is the dark pigment melanin. The quantity of melanin pigment in the iris is one factor in determining the phenotypic eye color of a person. Structurally, this huge molecule is only slightly different from its equivalent found in skin and hair. Iris color is due to variable amounts of eumelanin (brown/black melanins) and pheomelanin (red/yellow melanins) produced by melanocytes. More of the former is found in brown-eyed people and of the latter in blue- and green-eyed people.

RETINA

The retina (from Latin: rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which then processes that image within the retina and sends nerve impulses along the optic nerve to the visual cortex to create visual perception. The retina serves a function which is in many ways analogous to that of the film or image sensor in a camera.

The neural retina consists of several layers of neurons interconnected by synapses and is supported by an outer layer of pigmented epithelial cells. The primary light-sensing cells in the retina are the photoreceptor cells, which are of two types: rods and cones. Rods function mainly in dim light and provide monochromatic vision. Cones function in well-lit conditions and are responsible for the perception of colour through the use of a range of opsins, as well as high-acuity vision used for tasks such as reading. A third type of light-sensing cell, the photosensitive ganglion cell, is important for entrainment of circadian rhythms and reflexive responses such as the pupillary light reflex.

Light striking the retina initiates a cascade of chemical and electrical events that ultimately trigger nerve impulses that are sent to various visual centres of the brain through the fibres of the optic nerve. Neural signals from the rods and cones undergo processing by other neurons, whose output takes the form of action potentials in retinal ganglion cells whose axons form the optic nerve.[1] Several important features of visual perception can be traced to the retinal encoding and processing of light.

In vertebrate embryonic development, the retina and the optic nerve originate as outgrowths of the developing brain, specifically the embryonic diencephalon; thus, the retina is considered part of the central nervous system (CNS) and is actually brain tissue.[2][3] It is the only part of the CNS that can be visualized non-invasively. Much like the rest of the brain is isolated from the vasular system via the Blood-brain barrier, the retina is similarly protected by the Blood-retinal barrier.

MACULA

Structures in the macula are specialized for high-acuity vision. Within the macula are the fovea and foveola that both contain a high density of cones, which are nerve cells that are photoreceptors with high acuity.

In detail, the normal human eye contains three different types of cones, with different ranges of spectral sensitivity. The brain combines the signals from neighboring cones to distinguish different colors. There is only one type of rod, but the rods are more sensitive than the cones, so in dim light, they are the dominant photoreceptors active, and without information provided by the separate spectral sensitivity of the cones it is impossible to discriminate colors. In the fovea centralis, cones predominate and are present at high density. The macula is thus responsible for the central, high-resolution, color vision that is possible in good light; and this kind of vision is impaired if the macula is damaged, for example in macular degeneration.[14]

OPTIC NERVE

he optic nerve transmits all visual information including brightness perception, color perception and contrast (visual acuity). It also conducts the visual impulses that are responsible for two important neurological reflexes: the light reflex and the accommodation reflex. The light reflex refers to the constriction of both pupils that occurs when light is shone into either eye. The accommodation reflex refers to the swelling of the lens of eye that occurs when one looks at a near object (for example: when reading, the lens adjusts to near vision).[1]

The eye's blind spot is a result of the absence of photoreceptors in the area of the retina where the optic nerve leaves the eye.[1]