The Eye and Vision

The eye is a complex optical system - very similar to a camera. Vision begins when light enters the eye through the cornea, a powerful focusing surface. From there, it travels through clear aqueous fluid, and passes through a small aperture called the pupil. As muscles in the iris relax or constrict, the pupil changes size to adjust the amount of light entering the eye. Light rays are focused through the lens, and proceed through a clear jelly-like substance in the center of the eye called vitreous, which helps give the eye form and shape. When light rays finally land on the retina, the part of the eye similar to film in a camera, they form an upside-down image. The retina converts the image into an electrical impulse that travels along the optic nerve to the brain, where it is interpreted as an upright image.

It may be helpful, with regard to refractive surgical procedures that involve the cornea (most do), to have a better understanding of the anatomy of the cornea. The cornea is, on average, 545 microns thick. A micron (or micrometer) is one-thousandth of a millimeter. So the cornea is roughly a half a millimeter thick. There are 25.4 millimeters in an inch, so a millimeter is approximately one-25th of an inch. The cornea, then, is about one-50th of an inch thick. It has five layers. From outside to inside, they are: epithelium, Bowman’s membrane, stroma, Descemet’s membrane, and endothelium. The epithelium usually takes up about 45 to 50 microns of the thickness. Because Bowman’s membrane, Descemet’s membrane, and the endothelium are all very thin, the stroma accounts for the majority of the remainder of the thickness of the cornea.

The epithelium (outer layer) of the cornea is made up of epithelial cells that are constantly regenerated. The epithelium rests on Bowman’s membrane, which is a basement membrane for the epithelium of the cornea. The stroma of the cornea is made up of a highly organized layer of collagen fibers, the micro-structural details of which make the cornea as clear as a finely ground lens. Descemet’s membrane is the basement membrane for the endothelium, the latter of which is on the deep side of the cornea opposed to the aqueous humor. The endothelium is the layer of cells that pump the fluid from the cornea, keeping it relatively dehydrated and, therefore, clear.

The pupil of the eye is also critically important to the optical system of the eye. The pupil is actually an aperture that is made up by the iris, which is the anatomically colored part of the eye. Pupil size and function varies quite dramatically from one individual to another. Physiologic pupil size may vary from two or three millimeters up to eight millimeters or more, depending on age, eye color, lighting and many other factors. A large pupil may result in night vision aberrations following certain forms of refractive surgery. These issues will be discussed further when attention is turned to specific refractive surgical procedures.

The lens of the eye is also critically important to vision. After the cornea refracts incoming light rays, the lens of the eye further refracts the light in a condensing (convergent) fashion to focus on the retina (in the emmetropic eye, that is, the eye without refractive error or need for glasses). The young natural lens has the ability to accommodate, that is, to alter its shape in order to view objects that are closer to the eye. However, with aging, the ability to accommodate is gradually lost. This is known as presbyopia. Presbyopia usually onsets at around age forty when many people begin to have noticeable difficulty focusing at near. This difficulty is alleviated by reading glasses and may be amenable, to some degree, by certain refractive procedures, especially conductive keratoplasty (CK).

After incoming light rays are refracted in a convergent fashion by the cornea and the lens of the eye, they are focused on the central aspect of the retina (again, in the emmetropic eye) known as the macula. The macula contains photoreceptors that convert incoming light rays into electrical stimuli that are transferred to the brain via the optic nerve. Ultimately, the occipital cortex of the brain interprets vision.

This chapter is purposely succinct. I’ve included only the bare fundamentals of eye anatomy and physiology as I would expect you, the reader, to desire. Obviously, this chapter could go on for three hundred pages or more to fully explain ocular anatomy and physiology, but it probably wouldn’t help you to understand refractive surgical procedures. So, I’m sticking to the basics in our refractive errors page.