The eye works like a camera.
The bulb of the eye is the darkened chamber. The rays of light collected by the cornea and the crystalline lenses that function like the camera zoom, and pass through the opening of the pupil, that is the camera diaphragm, until they reach the retina, the film they are focused onto. The image then created is processed by the brain.
The bulb is surrounded by an external, white, firm and transparent membrane called the sclera.
The cornea is the spherical, transparent and clear anterior part of the eye. It is allows light to go through it.
The iris, the colored part of the eye lies behind the cornea.
It has an aperture in the middle, the pupil, to allow light to enter. Iris is provided with muscles causing constriction (miosis) or dilation (mydriasis) of the pupil, depending on the prevailing light conditions. This is a reflex response of the autonomous nervous system. The presence of too much light causes miosis while pupils dilate in the dark to allow more light to enter.
The retina and the iris form an angle, called anterior chamber corner. The anterior chamber is filled with a transparent fluid called the aqueous humor and preserves the shape and structure of the anterior segment of the eye. The angle plays a significant role in glaucoma, depending on its being open or closed because this is where the aqueous humor drains.
The crystalline lens is located behind the iris and is the size of a lentil. The cornea with the lens refracts and gathers the rays of light on the retina, where they are focused onto the macula. The lens has the ability to change its shape using a muscle in order to increase or decrease its refractive power by means of an adjustment mechanism. Thus, it can “zoom”and focus on the picture, depending on the distance between us andthe object we are looking at.
Behind the lens, the posterior segment of the balbular cavity is formed by the vitreous humor, a gel-like transparent material. The sclera is internally lined by the choroid, rich in blood vessels.
In conclusion, the retina is located on the inner part of the eye. The light is received by the cells that are sensitive to light, the cones and the rods and is converted to a neural stimulus by complicated mechanisms. Neurons receive that stimulus and transmit it via the optic nerve.
The optic nerve is like a cord transmitting the stimuli to the initial part of the visual tract of the brain. Subsequently, the stimuli end up, via the visual tract to the back of the head, the visual cortex where they are processes.
There are six muscles involved in ocular mobility. Four rectus muscles: the internal and external rectus muscles, the superior and inferior rectus muscles and two oblique muscles, the superior and inferior oblique muscle. The muscles are contracting depending on the direction we want to look at, turning the bulb in that direction.
HOW WE ARE ABLE TO SEE
When the eyes are perfectly normal, commonly called emmetropic, that is eyes free of refractive errors such as myopia, hyperopia, or astigmatism, light passes throught the cornea, the pupil, the lens, the vitraeous humor and focuses on, that is forms the reflections of external objects in the back of the eye on the retina and specifically at a location called macula lutea. The macula is responsible for our central clear vision. The remaining part of the retina is responsible for the peripheral vision. Reflections are projected upside-down on the macula and are converted into neural signals and, via the optic nerve they are transmitted to the back of the brain, the visual cortex via the visual tract. It is there that the image is perceived.
In myopic people, the rays of light focus before reaching the retina. They can see objects that are near them but distant objects appear to be blurry.
People with hyperopia can see distant objects better but find it difficult to focus on close objects because rays of light focus behind the retina.
In that case, both distant and close objects look distorted. This is due to the fact that the cornea curvature is uneven throughout its surface and as a result, the rays of light do not focus on the same level.