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OBJECTIVES

OBJECTIVES

After studying this chapter, you should be able to:

  • Describe the various parts of the eye and list the functions of each.

  • Describe the organization of the retina.

  • Explain how light rays in the environment are brought to a focus on the retina and the role of accommodation in this process.

  • Define hyperopia, myopia, astigmatism, presbyopia, and strabismus.

  • Describe the electrical responses produced by rods and cones, and explain how these responses are produced.

  • Describe the electrical responses and function of bipolar, horizontal, amacrine, and ganglion cells.

  • Trace the neural pathways that transmit visual information from the rods and cones to the visual cortex.

  • Define and explain dark adaptation and visual acuity.

  • Describe the neural pathways involved in color vision.

  • Identify the muscles involved in eye movements.

INTRODUCTION

The eye is often compared to a camera, with the cornea acting as the lens, the pupillary diameter functioning like the aperture of the camera, and the retina serving as the film. However, the eye, especially the retina, is far more sophisticated than even the most expensive camera. Within its protective casing, each eye has a layer of photoreceptors that respond to light, a lens system that focuses the light on these receptors, and a system of nerves that conducts impulses from the receptors to the brain. This chapter reviews the way the components of the visual system operate to set up conscious visual images.

ANATOMY OF THE EYE

The major parts of the eye are the sclera (protective covering), cornea (transfer light rays), choroid (nourishment), retina (photoreceptor cells), crystalline lens, and a pigmented iris containing circular and radial muscle fibers that constrict and dilate the pupil, respectively, to control the amount of light reaching the retina (Figure 9–1). The iris, ciliary body, and choroid are collectively called the uvea. The lens is held in place by the suspensory ligament (zonule) attached to the ciliary body. The eye is well protected from injury by the bony walls of the orbit. The cornea is moistened and kept clear by tears from the lacrimal gland that empty via the lacrimal duct into the nose. Blinking helps keep the cornea moist.

FIGURE 9–1

A schematic of the anatomy of the eye. (Reproduced with permission from Fox SI. Human Physiology, 10th ed. New York, NY: McGraw-Hill; 2008.)

The aqueous humor, which flows through the pupil and fills the anterior chamber, is produced in the ciliary body by diffusion and active transport from plasma. It is reabsorbed through a network of trabeculae into the canal of Schlemm (filtration angle). Obstruction of this outlet leads to increased intraocular pressure, a risk factor for glaucoma (Clinical Box 9–1). The posterior chamber is a narrow aqueous-containing space between the iris, zonule, and the lens. ...

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