The eye is a highly specialized organ and some consider it to be an extension of the brain. Vision occurs through the conversion of light waves into nerve impulses that are transmitted to the brain. The brain then processes this information and creates an image.
Extensions of the nerve cells in the retina use special receptors for this conversion. These are called photoreceptors and are made up of two types; cones and rods. The cones produce color vision while the rods allow vision in shades of gray.
There are three basic layers of the eye; the sclera, the choroid and the retina.
The sclera is a hard fibroelastic layer that supports the eye and forms the outermost layer of the eye. The sclera is the white part of the eye. External muscles of the eye are inserted into the sclera. These external muscles are opaque except in the anterior section of the eye. At the front of the eye the muscles are transparent and form the cornea. The cornea is the eyes main means of refraction and is responsible for focusing images on the retina with the aid of the lens.
The middle layer of the eye is the choroid and it is highly vascularized. The choroid is pigmented to absorb light that reaches the retina. The anterior section of the choroid connects to the ciliary body. The ciliary body is a thickened area that is continuous with the iris of the eye. A contraction of the ciliary body regulates the shape of the crystalline lens by smooth muscle that is found in the ciliary body. The lens is a true lens with the function of focusing images on the retina. If the lens becomes opaque this causes formation of cataracts.
The eye contains two segments; the anterior segment and the posterior segment. The crystalline lens and the ciliary body separate these segments. The anterior segment contains a liquid called the aqueous humor and the posterior segment contains a gel like material called the vitreous humor.
The iris of the eye is pigmented and gives us our “eye color”. The central part of the iris is known as the pupil where light enters into the eye. The pupil acts like a diaphragm and opens and closes to allow certain amounts of light in.
The retina is composed of nerve fibers and photoreceptors and forms the innermost layer of the eyeball. The optic nerve is a convergence of all the nerve fibers into a single point that is connected directly to the brain. The visual axis of the eye is known as the macula lutea or yellow spot. The fovea is located within the macula lutea and is the area that causes our sharpest vision to occur. Medial to the fovea is the optic disk. Since the optic disk has no receptor cells it is known as the “blind spot”.
Sight is one of the most important senses. Humans and other animals depend a great deal on the sense of sight for survival and to interact with the environment. Many people who have lost their sight tend to develop acuity in the other senses such as smell and hearing.