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Sensory receptors convey information about the external world to the brain. Chemoreceptive cells for the senses of taste and smell were discussed with the digestive and respiratory system (Chapters 15 and 17, respectively) and various mechanoreceptors mediating the sense of touch were presented with the skin (Chapter 18). This chapter describes the eye, both its photoreceptors and auxiliary structures, and the ear, which mediates both the sense of equilibrium and hearing via mechanoreceptors in the vestibulocochlear apparatus.


Eyes (Figure 23–1) provide the sense of sight, having developed as highly sensitive organs for analyzing the form, intensity, and color of light reflected from objects. Protected by adipose cushions within the orbits of the skull, each eyeball consists externally of a tough, fibrous globe, which maintains an eye’s overall shape. Internally the eye contains transparent tissues that refract light to focus the image, a layer of photosensitive cells, and a system of neurons that collect, process, and transmit visual information to the brain.


Internal anatomy of the eye.

A schematic hemisected eye shows the interrelationships among the major ocular structures, the three major layers or tunics of the posterior wall (sclera, choroid, and retina), important regions within those layers, and the refractive elements (cornea, lens, and vitreous). Note that the art depicts most components in three dimensions to better explain their interrelationships. For example, the large, scalloped structure at the anterior portion of the vitreous chamber is the ciliary body, whose parts are labeled in section at the upper right. The ora serrata represents the anterior boundary of the retina.

As indicated in Table 23–1, an eye has three concentric tunics or layers:

  • A tough external fibrous layer consisting of the sclera and the transparent cornea;

  • A middle vascular layer consisting of the choroid, ciliary body, and iris; and

  • An inner sensory layer, the retina, which communicates with the cerebrum through the posterior optic nerve (Figure 23–1).

TABLE 23–1Tunics of the eye.

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