Major Ocular Structures
The eye is made up of three layers: the outer layer called the fibrous tunic, which consists of the sclera and the cornea; the middle layer responsible for nourishment, called the vascular tunic, which consists of the iris, the choroid, and the ciliary body; and the inner layer of photoreceptors and neurons called the nervous tunic, which consists of the retina.
The eye also contains three fluid-filled chambers. The volume between the cornea and the iris is known as the anterior chamber, while the volume between the iris and the lens is know as the posterior chamber, both chambers contain a fluid called aqueous humor. Aqueous humor is watery fluid produced by the ciliary body. It maintains pressure (called intraocular pressure or IOC) and provides nutrients to the lens and cornea. Aqueous humor is continually drained from the eye through the Canal of Schlemm. The greatest volume, forming about four-fifths of the eye, is found between the retina and the lens called the vitreous chamber. The vitreous chamber is filled with a thicker gel-like substance called vitreous humor which maintains the shape of the eye.
Light enters the eye through the transparent, dome shaped cornea. The cornea consists of five distinct layers. The outermost layer is called the epithelium which rests on Bowman's Membrane. The epithelium has the ability to quickly regenerate while Bowman's Membrane provides a tough, difficult to penetrate barrier. Together the epithelium and Bowmanâ€™s Membrane serve to protect the cornea from injury. The innermost layer of the cornea is called the endothelium which rests on Descemet's Membrane. The endothelium removes water from cornea, helping to keep the cornea clear. The middle layer of the cornea, between the two membranes is called the stroma and makes up 90% of the thickness of the cornea.
From the cornea, light passes through the pupil. The amount of light allowed through the pupil is controlled by the iris, the colored part of the eye. The iris has two muscles: the dilator muscle and the sphincter muscle. The dilator muscle opens the pupil allowing more light into the eye and the sphincter muscle closes the pupil, restricting light into the eye. The iris has the ability to change the pupil size from 2 millimeters to 8 millimeters.
Just behind the pupil is the crystalline lens. The purpose of the lens is to focus light on the retina. The process of focusing on objects based on their distance is called accommodation. The closer an object is to the eye, the more power is required of the crystalline lens to focus the image on the retina. The lens achieves accommodation with the help of the ciliary body which surrounds the lens. The ciliary body is attached to lens via fibrous strands called zonules. When the ciliary body contracts, the zonules relax allowing the lens to thicken, adding power, allowing the eye to focus up close. When ciliary body relaxes, the zonules contract, drawing the lens outward, making the lens thinner, and allowing the eye to focus at distance.
Light reaches its final destination at the retina. The retina consists of photoreceptor cells called rods and cones. Rods are highly sensitive to light and are more numerous than cones. There are approximately 120 million rods contained within the retina, mostly at the periphery. Not adept at color distinction, rods are suited to night vision and peripheral vision. Cones, on the other hand, have the primary function of detail and color detection. There are only about 6 million cones contained with in the retina, largely concentrated in the center of the retina called the fovea. There are three types of cones. Each type receives only a narrow band of light corresponding largely to a single color: red, green, or blue. The signals received by the cones are sent via the optic nerve to the brain where they are interpreted as color. People who are color blind are either missing or deficient in one of these types of cones.
Index of Refraction for Transparent Components of the Eye
Crystalline Lens: 1.42
Aqueous Humor: 1.33
Vitreous Humor: 1.33