It is known to all that the left and right eyes are the peripheral organs for vision. Each eyeball functions like a camera. A lens is there which produces images of any objects and as a result we can see it. The images fall on a membrane called the retina. The retina have some special types of cells which can convert the light images into nervous impulses. the nervous impulses pass through the optic nerves and other parts of the visual path and reaches at the visual area of the cortex where the actual vision perceived.
The greeter part of the eyeball is shaped like a sphere and its diameter is about 24 mm.One sixth of its anterior part os more convex than the posterior part. It represents part of a sphere having a diameter of about 15 mm. The outer wall of the posterior five sixths of the eyeball is made by a thick white opaque membrane which is called the sclera.One sixth of the anterior wall is transparent and it is called the cornea.
The white of the eyeball is formed by the sclera and the dark central part is formed by the cornea, which is transparent. The dark appearance of the cornea is due to the presence of pigmented diaphragm, the Iris, deep to the cornea. The ad is an aperture in the Centre of the iris called the pupil. The pupil appears black as the interior of the eye is dark.
There are three main layers by which the wall of the eyeball is made up. The outer most layer is called the fibrous coat which is formed posteriorly by the sclera and anteriorly by the cornea. The next layer is the vascular coat. It has some subdivision: the part lining the inner surface of most of the sclera is thin and is called the choroid.Near the junction of the sclera with the cornea the vascular coat is thick and forms the ciliary body, which is continuous with the iris. The space between the iris and the cornea is called the anterior chamber. The space between the iris and the front of the lens is called the posterior chamber. The inner most layer of the wall is called the retina.
Light falling on the retina from any object, has to pass through a number of reflecting media before reaching the retina and forming an image on it. The reflecting media are the cornea, if fluid (aqueous humour), the lens and a jellylike vitreous bodywhich fills the eyeball posterior to the lens.
The centre of the cornea is named by the anterior pole and the opposite end is named by the posterior pole of the eyeball. From the anterior pole to the posterior pole the visual axis of the eye is passes through. The optic nerve is attached to the back of the back of the eyeball a short distance medial to the posterior pole. The equator of the eyeball is called to an imaginary line passing round the eyeball midway between the anterior and posterior pole. Another imaginary line passing through the both pole is called the meridian.