Information about the external world is conveyed to the central nervous system (CNS) from sensory receptors. Chemoreceptive cells for the senses of taste and smell were discussed with the digestive and respiratory system (see Chapters 15 and 17, respectively) and the various mechanoreceptors that mediate the sense of touch were presented with the skin (see Chapter 18). This chapter describes the eye, both its photoreceptors and auxiliary structures, and the ear which mediates the senses of equilibrium and hearing via mechanoreceptors in the vestibulocochlear apparatus.
EYES: THE PHOTORECEPTOR SYSTEM
Eyes (Figure 23–1) are highly developed photosensitive organs for analyzing the form, intensity, and color of light reflected from objects and providing the sense of sight. Protected within the orbits of the skull which also contain adipose cushions, each eyeball consists externally of a tough, fibrous globe that maintains its 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.
The sagittal section of an eye shows the interrelationships 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).
Each eye is composed of three concentric tunics or layers (Table 23–1):
A tough external fibrous layer consisting of the sclera and the transparent cornea;
A middle vascular layer that includes 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.
Not part of these layers, the lens is a perfectly transparent biconvex structure held in place by a circular system of zonular fibers that attach it to the ciliary body and by close apposition to the posterior vitreous body (Figure 23–1). Partly covering the anterior surface of the lens is an opaque pigmented extension of the middle layer called the iris, which surrounds a central opening, the pupil (Figure 23–1).
Located in the anterior portion of the eye, the iris and lens are bathed in clear aqueous humor that fills both the anterior chamber between the cornea and iris and the posterior chamber between the iris and lens (Figure 23–1). Aqueous humor flows through the pupil that connects these two chambers.
The posterior vitreous chamber, surrounded by the retina, lies behind the lens and its zonular fibers ...