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3. Medial rectus—rotates the eye toward the midline.

4. Lateral rectus—rotates the eye away from the midline.

5. Superior oblique—rotates the eye downward and away from the midline.

6. Inferior oblique—rotates the eye upward and away from the midline.

The motor units of the extrinsic eye muscles contain the fewest muscle fibers (five to ten) of any muscles in the body, so they can move the eyes with great precision. Also, the eyes move together so that they align when looking at something. Such alignment is the result of complex motor adjustments that contract certain eye muscles while relaxing their antagonists. For example, when the eyes move to the right, the lateral rectus of the right eye and the medial rectus of the left eye must contract. At the same time, the medial rectus of the right eye and the lateral rectus of the left eye must relax. A person whose eyes are not coordinated well enough to align has strabismus. Table 12.5 summarizes the muscles associated with the eyelids and eye.

D Explain how the eyelid is moved. Describe the conjunctiva.

What is the function of the lacrimal apparatus? Describe the function of each extrinsic eye muscle.

When one eye deviates from the line of vision, the person has double vision (diplopia). If this condition persists, the brain may eventually suppress the image from the deviated eye. As a result, the turning eye may become blind (suppression amblyopia). Treating the eye deviation early in life with exercises, eyeglasses, and surgery can prevent such monocular blindness. For this reason, vision screening programs for preschool children are very important.

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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