Anatomy & Refractive States of the Eye
Principal Ocular Structures

Convergence

DISTANCE PD	SUBTRACT FOR NEAR
Less than 62 mm	2 mm
62-66 mm	3 mm
More than 66 mm	4 mm

It was mentioned earlier that the crystalline lens is normally elastic. The more elasticity it has, the greater its ability to accommodate for near vision. In children the crystalline lens is particularly flexible. But beginning at about age 10 the crystalline lens begins to gradually lose its flexibility. This lessening of accommodative power goes largely unnoticed until about age 40 and at this point the condition has generally advanced enough to require special corrective lenses for near vision. This condition is known as presbyopia. With advancing age, the crystalline lens will sometimes become cloudy and begin to lose its transparency. This condition is known as a cataract. As the degree of cloudiness increases, visual acuity becomes gradually diminished and could eventually cause blindness in the effected eye. Cataracts are treated by removing the cloudy crystalline lens from the eye. An eye which has had the crystalline removed is called an aphakic eye. Once the lens has been removed, about 20 diopters of power has also been removed and that power must somehow be replaced. The most effective way to replace it is through the use of intraocular lens implants, also known as IOL’s. These are artificial lenses implanted in the eye to replace the one which has been removed. Many patients, after receiving this treatment find they can see better than they have in many years. Other methods of replacing the refractive power lost to the eye is through the use of contact lenses or cataract spectacle lenses. Due to improvements in intraocular lens materials and surgical techniques, and improved contact lens materials, spectacle lenses for the correction of aphakia are being used with diminishing frequency.

After passing through the crystalline lens, the light then enters the vitreous humor. The vitreous is a thick, transparent, colorless, gel-like mass which is located in a chamber between the retina and the crystalline lens. This chamber is called, appropriately enough, the vitreous chamber. The vitreous serves to maintain the proper shape and temperature of the eye. And unlike the aqueous humor it is neither regenerated nor replaced.

Cones & Rods

CONES	RODS
Day Vision	Night Vision
Detail	Peripheral Vision
Color	Black & White

Finally, the light reaches the retina. The retina is the innermost coat of the eye and is to the eye what film is to a camera. All the transparent elements of the eye we just covered are designed to work together with the goal of bringing light to a sharp focus on the retina. The retina is comprised of two kinds of photosensitive cells or receptors; these are called the rods and the cones. Refer to Table 3 above for a description of the function of each of these receptors is. As you can see the cones are responsible for day vision and perceiving detail and color. While the rods are used for night vision, peripheral vision, and perceiving black and white.

Referring again to Figure 1 notice an area on the back of the retina called the macula, and in the center of the macula is a small depression called the fovea centralis. This is the area of the retina that provides the sharpest vision. At the fovea the population of the cones is the greatest. As we radiate outwardly from the fovea, the population of cones gradually decreases and the population of rods increases. So, to view objects in sharp detail, or to perceive color, one must look directly at it, activating the high density of cones in the fovea. Again, night vision or peripheral vision is largely the function of the rods whose population gradually increases away from the fovea. Also, the retina has the ability to adapt to various lighting conditions. When going from a well-lit room to a dark room, for example, objects may not at first be clearly seen. But in about 20 minutes or so they may be seen more clearly. Although the outline of the objects can be seen in a dark room, neither detail nor color can be perceived since night vision is primarily a function of the rods.

Some of the non-transparent structures of the eye include the choroid which is a blood-rich layer underneath the retina and functions to provide nutrients to the retina as well as to other parts of the eye. The sclera is the tough leather-like outer coating of the eye also known as the “white” of the eye. It provides protection and helps maintain the eye’s shape. The sclera joins and is continuous with the cornea.

The iris is a circular membrane suspended behind the cornea and found immediately in front of the lens. It is the color part of the eye. The iris functions to regulate the amount of light entering the eye by changing the size of the pupil. The pupil is the central opening of the iris through which light is permitted to enter the eye.