Structure of the human eye > What is the retina?

The retina is the individual organ of sight and forms the internal layer of the human eye. It covers whole ventricular side of the eyeball right to the pupillary edge of the iris. It attaches to the middle layer of the eye wall through its external surface and the vitreous body attaches to its internal surface. The retina functions as the film in a camera. It catches the image and sends it to the visual centers of the brain through the optic nerve. We divide the retina into two structurally and functionally different sections. Posterior section, which is situated in the posterior part of the eyeball behind the level of the equator, is formed by the optic part of the retina, its part in front of the equator.

The optic part of the retina

The optic part of the retina is functionally the most important layer of the eyeball wall. It is a thin and fragile membrane which is thick around 0, 1 – 0, 4 mm and is adjacent to the choroid. It has orange or even red color which is a result of blood vessels of the choroid being more visible.

We distinguish a couple of formations on the background of the human eye. The yellow spot lies slightly laterally opposing the posterior pole of the eye and it is of circular or even elliptic shape with the diameter around 3 mm. There is a depression in its middle which is called fovea centralis. This place lies on the top of the optic axis of the eye and it represents the place of the sharpest vision because the central ray projects itself right there. In the area of the yellow spot there are accumulated only the cones which have the highest density there and secure the colour vision.

In the distance of around 4mm medially from the yellow spot there is a whitish field which overreaches and through which the optic nerve protrudes from the human eyeball. It has a circular shape with the diameter of around 1,5 mm and its middle is a bit deepened. It does not contain any light sensitive elements and from the physiological point of view it represents the so-called blind spot.

Structure of the retina

Microscopic structure of the retina is very complex because it is formed of 11 layers. Therefore we will mention only its functional parts:

The external layer

The external layer is adjacent to the choroid a together with the pigment layer of the choroid it functions as a light insulating layer which absorbs the incident light rays and prevents them from reflecting inside the human eye. Light sensitive protuberances of the sensual cells of the retina are integrated into the pigment layer.

The internal layer

The internal layer contains its own primary sensual cells (photoreceptors) and neurons which collect the information from the photoreceptors and transmit them from the eye to the brain through the optic nerve. The sensual cells in the retina are of two types: rods and cones. Rods register the amount of falling light and represent the most of the sensual cells of the retina. Their total amount is around 130 millions. Cons serve for the perception of colours and they are much lesser in number, around 7 millions, and they are placed in the area of the yellow spot where they represent the only type of the sensual cells. Their conductive protuberances attach to the layer of tiny bipolar neurons. Consequently, the layer of large multipolar neurons attaches to the conductive protuberances of bipolar neurons. Their neurits form the most internal layer of the retina.

There are plenty of supportive cells and tiny associative neurons which are placed between the aforementioned nerve cells. The layers of the retina are organized in such a way that the ray of light has to pass through the whole retina to reach the light sensitive protuberances of cones and rods. Therefore the human retina is called the inversive type of the retina.

The blind part of the retina

The blind part of the retina lies in front of the orra serrata and covers the internal surface of the ciliary body and the iris. The structure of this part of the retina is very simple. It is formed only of the pigment layer and does not contain any sensual cells.

The examination of the human eye background is of great importance not only in ophthalmology, where the state of the retina is assessed, but also in other clinical specializations. This examination is very important because it can reveal various states of damage of the vascular system or for example in the diagnostics of the diabetes.