Major Anatomic Features of the Eye

Figure 2.1 illustrates schematically the major components of the human eye, which resembles that of most other primates. The sclera is a tough outer coat that is fibrous in humans but contains bone or cartilage in some other vertebrate species. The cornea is continuous with the sclera and provides the first element of the refracting media that bend the light to form an image on the retina. The lens lies behind the iris and in front of the vitreous humor, which fills the greater part of the globe. Aqueous humor fills the posterior chamber (the space between the lens and iris) and the anterior chamber (the space between the iris and the cornea). The posterior and anterior chambers are continuous through the pupil, the aperture formed by the iris.

The general features of the retina, the multilayered neural structure lining the back of the eyeball, can be visualized in the living eye with an ophthalmoscope or special camera (Figure 2.2). Axons leave the retina through the optic disc or optic papilla and enter the optic nerve to reach the brain. At the posterior pole of the eye, the retina thins to form the fovea, an area specialized for high-acuity vision. The visual axis is an imaginary line from the fovea through the center of the pupil (Figure 2.1). Behind the retina is the pigment epithelium, which is separated from the sclera by the vascular choroid.