Anatomic Features

The eye is a special sense organ made up of three coats, or tunics, as follows:

Within this globe are three spaces: the anterior chamber, posterior chamber, and vitreous chamber. The crystalline lens is located in the region of the posterior chamber (Figure 1-1).

Figure 1-1 The visual system.

(From Kronfeld PC: The human eye, Rochester, NY, 1943, Bausch & Lomb Press.)

The outer dense connective tissue of the eye provides protection for the structures within and maintains the shape of the globe, providing resistance to the pressure of the fluids inside. The sclera is the opaque white of the eye and is covered by the transparent conjunctiva. The transparent cornea allows light rays to enter the globe and, by refraction, helps bring these light rays into focus on the retina. The region in which the transition from cornea to sclera and conjunctiva occurs is the limbus.

The vascular layer of the eye is the uvea, which is made up of three structures, each having a separate function but all are interconnected. Some of the histologic layers are continuous throughout all three structures and are derived from the same embryonic germ cell layer. The iris is the most anterior structure, acting as a diaphragm to regulate the amount of light entering the pupil. The two iris muscles control the shape and diameter of the pupil and are supplied by the autonomic nervous system. Continuous with the iris at its root is the ciliary body, which produces the components of the aqueous humor and contains the muscle that controls the shape of the lens. The posterior part of the uvea, the choroid, is an anastomosing network of blood vessels with a dense capillary network; it surrounds the retina and supplies nutrients to the outer retinal layers.

The neural tissue of the retina, by complex biochemical processes, changes light energy into a signal that can be transmitted along a neural pathway. The signal passes through the retina, exits the eye through the optic nerve, and is transmitted to various parts of the brain for processing.

The interior of the eye is made up of three chambers. The anterior chamber is bounded in front by the cornea and posteriorly by the iris and anterior surface of the lens. The posterior chamber lies behind the iris and surrounds the equator of the lens, separating it from the ciliary body. The anterior and posterior chambers are continuous with one another through the pupil, and both contain aqueous humor that is produced by the ciliary body. The aqueous humor provides nourishment for the surrounding structures, particularly the cornea and lens. The vitreous chamber, which is the largest space, lies adjacent to the inner retinal layer and is bounded in front by the lens. This chamber contains a gel-like substance, the vitreous humor.

The crystalline lens is located in the area of the posterior chamber and provides additional refractive power for accurately focusing images onto the retina. The lens must change shape to view an object that is close to the eye, through the mechanism of accommodation.

Anatomic Directions and Planes

Anatomy is an exacting science, and specific terminology is basic to its discussion. The following anatomic directions should be familiar (Figure 1-2):

Figure 1-2 Anatomic directions.

(From Palastanga N, Field D, Soames R: Anatomy and human movement, Oxford, UK, 1989, Butterworth-Heinemann.)

The following planes are used in describing anatomic structures (Figure 1-3):

Figure 1-3 Anatomic planes.

(From Palastanga N, Field D, Soames R: Anatomy and human movement, Oxford, UK, 1989, Butterworth-Heinemann.)

Because the globe is a spherical structure, references to locations can sometimes be confusing. In references to anterior and posterior locations of the globe, the anterior pole (i.e., center of the cornea) is the reference point. For example, the pupil is anterior to the ciliary body (see Figure 1-1). When layers or structures are referred to as inner or outer, the reference is to the entire globe unless specified otherwise. The point of reference is the center of the globe, which would lie within the vitreous. For example, the retina is inner to the sclera (see Figure 1-1). In addition, the term sclerad is used to mean “toward the sclera,” and vitread is used to mean “toward the vitreous.”

Refractive Conditions

If the refractive power of the optical components of the eye, primarily the cornea and lens, correlate with the distances between the cornea, lens, and retina so that incoming parallel light rays come into focus on the retina, a clear image will be seen. This condition is called emmetropia (Figure 1-4, A). No correction is necessary for clear distance vision. In hyperopia (farsightedness), the distance from the cornea to the retina is too short for the refractive power of the cornea and lens, thereby causing images that would come into focus behind the retina (Figure 1-4, B). Hyperopia can be corrected by placing a convex lens in front of the eye to increase the convergence of the incoming light rays. In myopia (nearsightedness), because the lens and cornea are too strong or, more likely, the eyeball is too long, parallel light rays are brought into focus in front of the retina (Figure 1-4, C). Myopia can be corrected by placing a concave lens in front of the eye, causing the incoming light rays to diverge.

Figure 1-4 Refractive conditions.

A, Emmetropia, in which parallel light comes to a focus on the retina. B, Hyperopia, in which parallel light comes to a focus behind the retina (dotted lines). A convex lens is used to correct the condition and bring the light rays into focus on the retina. C, Myopia, in which parallel light comes to a focus in front of retina (dotted lines). A concave lens is used to correct the condition and bring the light rays into focus on the retina.

(Courtesy Dr Karl Citek, OD, Pacific University College of Optometry, Forest Grove, Ore.)