The sight is our most important sense, which we receive up to 80% of information from the extermal environment through. The humam eye is the organ, which provides the vision itself. It is a very complex and perfect system consisting of many parts that have to cooperate perfectly. The particular parts of the eye are the cornea, conjuctiva, sclera, lens, vitreous body, irides, retina and many others that will be discussed briefly in the following lines.
The human eye consists of a simple objective which has two parts. These are the cornea and the lens. The amount of light which enters the eye is controlled by the iris which is located between the cornea and the lens. The pupil is located in the middle of the iris and it dilates and contracts depending on the intensity of illumination and thereby regulates the amount of light which enters the human eye.
Then the light passes through transparent vitreous body and forms inverted image on the light-sensitive retina. The retina is a light-sensitive part of the human eye and represents a sensor or a film in the camera. If we unfolded the retina into a surface it would form a circle of approximate diameter 42 mm which is exactly the diagonal of the film. The retina consists of light-sensitive cells – approximately 130 million rods and 7 million cones. In this case the eye can operate as the 137 megapixels camera. The cones are less sensitive but they are capable of distinguishing colors. The rods are much more sensitive and their task is to provide only black-and-white vision.
For the human eye to operate properly we must not forget to mention some auxiliary organs which also have a huge impact on perfect vision. This category includes ocular muscles which secure movement of the eye itself and thanks to which we can change the direction of our look. Then it is the eye socket which represents osseous shell which contains the eye and its main function is to protect the eyeball and last but not least it is important to mention the eyelids and lachrymal apparatus.
The human eye itself is a very complicated but perfectly operating "machine" thanks to which we can perceive surrounding world. But even a minor damage to the eye or to any of the individual parts of the eye can lead to deterioration of vision. Therefore we should realize that we have only one pair of eyes and it is of vital importance to look after them really carefully.
The cornea is the clear tissue without blood vessels. As a transparent, evenly cambered window it is an important part of the eye apparatus refracting light. The cornea of the adults has a center thickness of approx. 0.6 mm and 0.8 mm in the outer zone. Its diameter is approximately 11.5 mm.
The conjunctiva lines the space between eyelids, eye ball and eye cavity. On the one side it goes into the eye corner and on the other side into the cornea. Besides eyelids, the conjunctiva represents the second protective barrier against the entering of disease-carryig germs and foreign bodies. Defensive functions play a leading role. They are furnished with cells and body fluids that destroy certain germs. The conjunctiva is a mucous.
The white sclera cares about a stability of the eye ball. It consists of collagen and elastic fibers.
The front chamber is a space between the cornea and the irides, respectively lens and the rear chamber is a space between the irides and the vitreous body. The both spaces contain the eyewater.
The chamber angle is formed by the cornea and the irides. It contains a trabecular system as a filtering system and a so-called Schlemm canal as a trailing system for a mucous, which is then led into small veins of the blood system.
The lens as well as the cornea is responsible for joining of light rays and their sharp image on the retina. The lens can change its form and a refractive power that is needed for the near vision. This feature is called accommodation. Accommodation is possible because the lens is surrounded by an elastic capsule and is connected with a ciliary body by suspended apparatus. In the old age the elasticity is reduced and the accommodative abilitty diminish and people need reading eyeglasses. The lens consists of a transparent tissue. Proteins in the lens are condensated in the old age and thus it may lead to growing optical thickening ot the lens and even to a so-called cataract.
The vitreous body fills 2/3 of the eyeball and by its gell-like consistency is co-responsible for maintaining the form of the eye, in the case of eyeball injuries at least. The vitreous body is normally clear and thus enables a good optical depiction. The vitreous body consists of 98.5% of water. An uniform structure of the vitreous body may change in old age. Then there is an irregular thickening, which you may feel like "flying mosquitos" or similar shapes that move together with eye movements. This can slightly limit the vision.
The most important role of the irides is regulating of light incidence in terms of dazzlement. The irides has a central, roundish, moveable aperture, the pupil. The pupil can narrow or extend by the help of two muscles. The content of the pupil pigment determines its color: Blue eyes are less pigmented, brown eyes are highly pigmented.
On the one hand, ciliary body permits the change of form of the eye lens and on the other hand, it may affect the outfall of eyewater because of the Schlemm canal. The outer layer of the ciliary body produces eyewater and passes it into the eye. Production is about 2 ul/min, so it takes about an hour before one replacement of entire capacity of the front and the rear chamber (125 ul).
The choroid represents the middle layer of the wall of the eyeball. It consists mostly of blood vessels and it is the most bloodshot tissue of the body. The choroid is responsible for nourishment of the outer retina. Its pigmentation keeps from annoying diffused light.
There are photoreceptors in the retina that allow vision by receiving optical and color signals. Photoreceptors can be divided into cones and rods. Total 6.5 million of cones allow color vision during the day and 120 million of rods allow black and white vision in the dark. Signals of photoreceptors are led by visual nerves through different places and nerve fibers in the retina to the visual tract in the brain. Center of the retina, so-called macula or "yellow spot", is functionally the most important part of the retina. Macula has a decomposing ability and is responsible for color vision. Here is the highest density of receptors and it is considered the area of the sharpest vison.
Approximately 1.2 million of nerve fibers meet in the head of the visual nerve. They appear in bunches through short, round, sieve hole in the retina and link together into the visual nerve. The visual nerve passes 25 to 40 mm in the eye cavity and 10 to 15 mm in the skull before the two visual nerves join and cross in order to end together in the brain.
Besides the own parts of the eye, there are some other parts in the eye area that participate in providing a perfect function of the eye.
The bone eyepit is created of together seven, mutually adjacent cranial bones. The major part consists of frontal bone, wedge bone, jugal bone and upper jaw bone. Two sites are extremely fragile: the bottom of the eyepit and a part of a lateral inner wall. It plays a role especially when knocking on the eye, because the eyepit can be broke in case of an acute pressure on these sites. The eyepit has a few holes at the back and on the botom, which nerves and blood vessels go through. The eyeball, the part that is broadly called the eye, fills about 1/5 of the eyepit. The rest is filled with adipose and connective tissue, muscles, nerves and blood vessels.
There are two direct and two oblique extraocular muscles. They are placed always up, down, at the left and right as well as obliquely outside up and down on the eyeball and are responsible for its movement in all directions of view.
Upper and lower eyelids are two movable eye flexures, which most important role is to protect the eyeball. The lids close reflexively when entering of foreign bodies and when strong light dazzlement. Eyebrows and eyelashes also prevent the eye from dust and sweat entering. The regular and involutary winking of eyelids provides the even distribution of a lacrimal film on the cornea. There are sebaceous glands on the edge of eyelids that are a part of a lacrimal film. These glands can be inflammated and then developed to so-called eye barley.