Anatomy & Refractive States of the Eye
Principal Ocular Structures

The eye is very much like a camera in that it contains structures which resemble the camera’s lens and photographic film. But no camera has been invented yet that, like the eye, can develop color and motion pictures in a microsecond. Perhaps the term “miracle of sight” will take on new meaning as we now go on to study the most sophisticated optical instrument of all.

Let’s begin by taking a look at some of the major parts of the eye along with the mechanisms that allow us to see. In doing so we’ll follow parallel rays of light through the eye and examine the sequence of events that actually produces sight. The following discussion refers to Figure 1 on page 4.

The first structure through which light passes upon reaching the eye is the cornea. The cornea is, in effect, like the window of the eye. It’s the eye’s major refracting body containing approximately 40 diopters of focusing power. When we say light is refracted, it simply means it is bent or deviated from its original direction. So when a patient visits an eyecare professional for an examination or a “refraction,” the doctor is, in effect, bending light in an effort to improve the patient’s vision. If you now refer to Figure 2 on page four, you will see that the cornea consists of five distinct layers. The outermost layer is the epithelium, followed by Bowman’s Membrane, the Stroma, Descemet’s Membrane, and finally, the Endothelium. The corneal epithelium is particularly interesting since the cells which comprise it regenerate themselves more quickly than those of any other part of the body. This means that a superficial scratch or corneal abrasion can heal itself relatively quickly.

Referring to Figure 1 again, we can see that the next medium light encounters after passing through the cornea is a fluid called the aqueous humor. The aqueous humor is located in the areas which are known as the anterior aqueous chamber and the posterior aqueous chamber. Looking at the diagram, the anterior chamber is bordered by the cornea, iris and pupil, while the posterior chamber is bordered by the iris and the crystalline lens. The aqueous humor is a watery substance which functions to aid in keeping the cornea moist, is a source of nutrients to the cornea, and it is also responsible for maintaining the pressure inside the eye known as the intraocular pressure. The aqueous humor is continually being manufactured by the ciliary body and it leaves the eye through a drainage canal known as the Canal of Schlemm. If the aqueous becomes inhibited from draining at a normal rate, it can cause an increase in the intraocular pressure. Some pressure inside the eye is desirable since it allows the eye to maintain its normal shape. However, if that pressure is elevated and sustained at an unacceptably high level it could cause a condition known as glaucoma. Glaucoma is usually caused by an impaired outflow of the aqueous humor which results in increased intraocular pressure. We’ll discuss the consequences of glaucoma later on when we talk about the optic nerve.