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Information about the external world is conveyed to the central nervous system from sensory receptors. Chemoreceptor units for the senses of taste and smell were discussed in Chapters 15 and 17, respectively, and the range of mechanoreceptors that mediate the sense of touch in its various components was presented in Chapter 18. This chapter will examine the systems responsible for vision via photoreceptors of the eye and for the senses of equilibrium and hearing that involve mechanoreceptors in the vestibulocochlear apparatus of the ear.

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The eye (Figure 23–1) is a complex and highly developed photosensitive organ that analyses the form, intensity, and color of light reflected from objects, providing the sense of sight. The eyes are located in protective areas of the skull, the orbits, which also contain cushions of adipose tissue. Each eyeball includes a tough, fibrous globe to maintain its shape, a system of transparent tissues that refract light to focus the image, a layer of photosensitive cells, and a system of neurons whose function it is to collect, process, and transmit visual information to the brain. Each eye is composed of three concentric tunics or layers (Figure 23–2): a tough external layer consisting of the sclera and the cornea; a more vascular middle layer consisting of the choroid, ciliary body, and iris; and an inner sensory layer, the retina, which consists of an outer pigmented epithelium and an inner retina proper. The photosensitive inner layer of the retina communicates with the cerebrum through the optic nerve on the eye's posterior side; its anterior edge is called the ora serrata (Figure 23–1).

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Figure 23–1.
Graphic Jump Location

Internal anatomy of the eye.

The sagittal section of an eye shows the inter-relationships among the major ocular structures, the three major layers or tunics of the wall, important regions within those layers and the refractive elements (cornea, lens, and vitreous).

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Figure 23–2.
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Layers of the eye.

The sagittal view of an eye shows its three major layers or tunics, with the lens in the anterior opening of the vascular layer and retina.

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The lens of the eye is a biconvex transparent structure held in place by a circular system of zonular fibers, which extend from the lens into a thickening of the middle layer, the ciliary body, and by close apposition to the vitreous body on its posterior side (Figure 23–1). Partly covering the anterior surface of the lens is an opaque pigmented expansion of the middle layer called the iris. The round hole in the middle of the iris is the pupil (Figure 23–1).

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The eye contains two fluid-filled cavities: the anterior chamber, which occupies the space between the cornea and the iris and the posterior chamber,...

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