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After studying this chapter, you should be able to:

  • Identify the function of the various parts of the eye.

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

  • Explain the refractive deficits responsible for hyperopia, myopia, presbyopia, and astigmatism.

  • Describe the functional organization of the retina.

  • List the sequence of events involved in phototransduction.

  • Describe the electrical responses produced by bipolar cells, horizontal cells, amacrine cells, and ganglion cells.

  • Define dark adaptation, visual acuity, and age-related macular degeneration.

  • Trace the neural pathways that transmit visual information from photoreceptors to the visual cortex.

  • Describe the neural pathways involved in color vision and the types of color blindness.

  • Predict the visual field deficits that would occur after lesions within specific parts of the visual pathway.

  • Identify the muscles involved in the four types of eye movements and the function of these movements.


The eyes are complex sense organs that have evolved from primitive light-sensitive spots on the surface of invertebrates. They gather information about the environment; the brain interprets this information to form an image of what appears within the field of vision. 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. A great deal of work has been done on the neurophysiology of vision; in fact, it may be the most studied and best understood sensory system. This chapter describes the way the components of the visual system operate to set up conscious visual images.


The principal structures of the eye are shown in Figure 10–1. The outer white protective layer of the eyeball is the sclera through which no light can pass. It is modified anteriorly to form the transparent cornea, through which light rays enter the eye. The lateral margin of the cornea is contiguous with the conjunctiva, a clear mucous membrane that covers the sclera. Just inside the sclera is the choroid, a vascular layer that provides oxygen and nutrients to the structures in the eye. The retina, the neural tissue containing the photoreceptors, lines the posterior two-thirds of the choroid.


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

The crystalline lens is a transparent structure held in place by a circular lens suspensory ligament (zonule). The zonule is attached to the ciliary body that contains circular muscle fibers and longitudinal muscle fibers that attach near the corneoscleral junction. The pigmented and opaque iris, the ...

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