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 that 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, 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.
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 consisting of the choroid, ciliary body, and iris
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. ||Download (.pdf) TABLE 23–1 Tunics of the eye.
| ||Structures ||Components ||Function |
| ||Fibrous Tunic (External Layer) |
|Sclera ||Dense irregular connective tissue || |
Supports eye shape
Protects delicate internal structures
Extrinsic eye muscle attachment site
|Cornea ||Two layers of epithelium with organized connective tissue in between || |
Protects anterior surface of the eye
Refracts (bends) incoming light
|Vascular Tunic or Uvea (Middle Layer) |
|Choroid ||Areolar connective tissue; highly vascularized || |
Supplies nourishment to retina
Pigment absorbs extraneous light
|Ciliary body ||Ciliary smooth muscle and ciliary processes; covered with a secretory epithelium || |
Holds suspensory ligaments that attach to the lens and change lens shape for far and near vision
Epithelium secretes aqueous humor
|Iris ||Two layers of smooth muscle (sphincter pupillae and dilator pupillae) and connective tissue, with a central pupil ||Controls pupil diameter and thus the amount of light entering the eye |
|Retina (Internal Layer) |
|Pigmented layer ||Pigmented epithelial cells || |
Absorbs extraneous light
Provides vitamin A for photoreceptor cells
|Neural layer ||Photoreceptors, bipolar neurons, ganglion cells, ...|