Figure 922

The human eye. The blood vessels depicted run along the back of the eye between the retina and vitreous humor, not through the vitreous humor. %

directing the light rays back to a point after emerging from the lens. The image is focused on a specialized area known as the fovea centralis (Figure 9-22), the area of the retina that gives rise to the greatest visual clarity. The image on the retina is upside down relative to the original light source (Figure 9-23), and it is also reversed right to left.

Light rays from objects close to the eye strike the cornea at greater angles and must be bent more in order to reconverge on the retina. Although, as noted above, the cornea performs the greater part quantitatively of focusing the visual image on the retina, all adjustments for distance are made by changes in lens shape. Such changes are part of the process known as accommodation.

The shape of the lens is controlled by the ciliary muscle and the tension it applies to the zonular fibers, which attach this smooth muscle to the lens (Figure 9-24). To focus on distant objects, the zonular fibers pull the lens into a flattened, oval shape. When their pull is removed for near vision, the natural elasticity of the lens causes it to become more spherical. This more spherical shape provides additional bending of the light rays, which is important to focus near objects on the retina. The ciliary muscle, which is stimulated by parasympathetic nerves, is circular, like a sphincter, so that it draws nearer to the lens as it contracts and therefore removes tension on the zonular fibers, resulting in accommodation for viewing near objects (Figure 9-25). Accommodation also includes other mechanisms that move the lens slightly toward the back of the eye, turn the eyes inward toward the nose

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

The Sensory Systems CHAPTER NINE

The Sensory Systems CHAPTER NINE

FIGURE 9-23

Refraction (bending) of light by the lens system of the eye. For simplicity, we show light refraction only at the surface of the cornea where the greatest refraction occurs. Refraction also occurs in the lens and at other sites in the eye. %

FIGURE 9-23

Refraction (bending) of light by the lens system of the eye. For simplicity, we show light refraction only at the surface of the cornea where the greatest refraction occurs. Refraction also occurs in the lens and at other sites in the eye. %

Near vision

Near vision

FIGURE 9-25

Accommodation of the lens for near vision. Although refraction is shown only at the surface of the cornea, the change in refraction during accommodation is a function of the lens, not the cornea. % KQ]

(convergence), and constrict the pupil. The sequence of events for accommodation is reversed when distant objects are viewed.

The cells that make up most of the lens lose their internal membranous organelles early in life and are thus transparent, but they lack the ability to replicate. The only lens cells that retain the capacity to divide are on the surface of the lens, and as new cells are formed, older cells come to lie deeper within the lens. With increasing age, the central part of the lens becomes denser and stiffer and acquires a coloration that progresses from yellow to black.

FIGURE 9-24

Ciliary muscle, zonular fibers, and lens of the eye. %

FIGURE 9-24

Ciliary muscle, zonular fibers, and lens of the eye. %

FIGURE 9-25

Accommodation of the lens for near vision. Although refraction is shown only at the surface of the cornea, the change in refraction during accommodation is a function of the lens, not the cornea. % KQ]

PART TWO Biological Control Systems

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART TWO Biological Control Systems

Since the lens must be elastic to assume a more spherical shape during accommodation for near vision, the increasing stiffness of the lens that occurs with aging makes accommodation for near vision increasingly difficult. This condition, known as presbyopia, is a normal part of the aging process and is the reason that people around 45 years of age may have to begin wearing reading glasses or bifocals for close work.

The changes in lens color that occur with aging are responsible for cataract, which is an opacity of the lens and one of the most common eye disorders. Early changes in lens color do not interfere with vision, but vision is impaired as the process slowly continues. The opaque lens can be removed surgically. With the aid of an implanted artificial lens or compensating eyeglasses, effective vision can be restored, although the ability to accommodate is lost.

Cornea and lens shape and eyeball length determine the point where light rays reconverge. Defects in vision occur if the eyeball is too long in relation to the focusing power of the lens (Figure 9-26). In this case, the images of near objects fall on the retina, but the

Normal

Nearsighted (eyeball too long)

Nearsighted corrected

Farsighted (eyeball too short)

Farsighted corrected

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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