Construction of the eye

Introduction

It all started with a cell that was sensitive to light and could distinguish between brightness and darkness . The adventure of sight in the process of evolution began 800 million years ago. According to the Swedish biologists Dan Nilson and Susanne Pelger ( Zoologiske Institutionen , Helgonavägen 3 , S -223 62 Lund , Sweden) if we go back for some 400,000 generations back , it turns out that the only this  light-sensitive cell is the source of all the organs of sight, with which all living creatures are equipped with  , including the human. Various creatures according to their own needs have developed different methods of vision . For example, herbivorous creatures have  eyeballs  on rear sides of the head, which  increases the chances of survival and defense , because it gives the animals a wide field of view , allowing them to spot predators early enough to escape. For the same reasons in carnivores , including a human , the eyes are set at the front so as to provide a three-dimensional or spatial vision . An important element in determining the survival of the species was their  ability to judge the distance to the victim. In the case of humans , evolution  followed  the same path as in other land mammals,  whose anatomy and physiology are quite similar to ours.

Cornea

Cornea – the transparent, non-vascular structure , rich in nerve endings – is, from the  functional point of view, the first and most important optical component of the visual system. Seen at a very high magnification, the cornea consists of five layers, where only the outer layer, the epithelium, has  5-6 cell layers of different thicknesses. The outer cells are continually being replaced by those lying deeper. Therefore , a scratch on  the surface of the cornea, although usually quite painful, can heal in a few hours.

Lens

It is a small, fibrous , a biconvex lens , with  thickness of approximately 3.6 mm. Its front surface is about 3 mm from the rear surface of the cornea; radius of curvature at the center is 10 mm and the closer edge, the curvature  flattens more . The radius of curvature of the posterior surface is only 6 mm , so that the rear surface is much more rounded than the front . Apart from  the surface curvature of the lens, refractive properties of lens are due to its structure , consisting of concentric rings , whose surfaces have  progressively larger radius of curvature and  degree of refraction, the  closer to the  nucleus they are  The purpose of the lens is to change the power -breaking when looking at objects at different distances from the eye. When a person looks at something that is close,  the lens must change its curvature in order to precisely focus on the object, which is a projection on the retina . When the muscles in ciliary body (part of the uveal tract ) contract, the lens zonular tension is reduced , the lens due to the fact  that it is flexible,changes its curvature, and as a result  its focusing ability.

Aqueous /humor/

The name “watery fluid” means the fluid present in the anterior chamber of the eye between the cornea, the iris and the lens. Fluid is continuously produced in the process of secretion  and filtration of the ciliary body, its main function is to maintain the internal pressure of the eye at the appropriate power level, and  support nutrition of the cornea and lens.

Iris

The iris is the coloured membrane  placed behind the cornea and the front of lens. It is a round shape with a hole in the center – pupil – which acts as a shutter. The main task of the iris is  dosing the  amount of light that enters the eye. The tension of  muscle fiber makes the pupil narrow or expand. The colour of iris  depends on the amount of melanin pigment on the front and rear  surfaces. The  task of brown pigment  is to absorb light reflected in the eye, and if the amount of the pigment on the rear surface is very small, the iris is blue, while increasing the amount of the pigment makes the colour beer or brown . If the pigment is located on the front surface, the iris becomes green.

Uvea

The uvea is a  vascularized membrane having a structure ,which is formed by choroid of the eye in the rear and  ciliary body together with  iris in the front section. By reducing the pupil diameter , the amount of light entering the eye is decreased at the same time. When the pupil narrows, the vision is clearer and distortion on the outskirts is  minimized. Conversely, when the pupil expands, visual acuity decreases.

Vitreous

itreous  consists of a transparent, jelly-like substance that fills the whole, a fairly large cavity between the rear of the lens and the retina. The main  function it plays is   maintaining adherence of  the retina to  pigment epithelium; the optical function is not very significant

Retina

Retina is the sensitive membrane, similar to   photographic film and consists of three parts: a colour layer,  vascular layer and a layer consisting of nerve fibers together with retinal photoreceptors (rods and suppositories). The retina  suppositories (6 million in the human eye) are located mostly in the central part of the retina, called the macula , in the center of: which there  is a small hole, called fovea. Approximately 120 million rods are  located mainly in the peripheral retina. When the focused image falls on the retina, there is a range of physical, chemical and electrical processes going on , during which   through the optic nerve, the information reaches the part of the visual cortex, where it is corrected and processed.

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