Near Point Meter

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Near Point Accomedation Test
Figure 36.3 To determine the near point of accommodation, slide the 3"x 5" card along the meter stick toward your partner's open eye until the closest location where your partner can still see the word sharply focused.
Average Human Anatomy

Table 36.1

Near Point of Accommodation

Age (years)

Average Near Point (cm)

10

7

20

10

30

13

40

20

50

45

60

90

a. Hang the astigmatism chart on a well-illuminated wall at eye level.

b. Have your partner stand 20 feet in front of the chart, gently cover the left eye with a 3" x 5" card, focus on the spot in the center of the radiating lines, and report which lines, if any, appear more sharply focused and darker.

c. Repeat the procedure using the left eye.

d. Record the results in Part A of the laboratory report.

3. Accommodation test. Accommodation is the changing of the shape of the lens that occurs when the normal eye is focused for close vision. It involves a reflex in which muscles of the ciliary body are stimulated to contract, releasing tension on the suspensory ligaments that are fastened to the lens capsule. This allows the capsule to rebound elastically, causing the surface of the lens to become more convex. The ability to accommodate is likely to decrease with age because the tissues involved tend to lose their elasticity.

To evaluate the ability to accommodate, follow these steps:

a. Hold the end of a meter stick against your partner's chin so that the stick extends outward at a right angle to the plane of the face (fig. 36.3).

b. Have your partner close the left eye.

c. Hold a 3" x 5" card with a word typed in the center at the distal end of the meter stick.

d. Slide the card along the stick toward your partner's open eye, and locate the point closest to the eye where your partner can still see the letters of the word sharply focused. This distance is called the near point of accommodation, and it tends to increase with age (table 36.1).

e. Repeat the procedure with the right eye closed.

f. Record the results in Part A of the laboratory report.

4. Color vision test. Some individuals exhibit defective color vision because they lack certain cones, usually those sensitive to the reds or greens. Because this trait is an X-linked (sex-linked) inheritance, the condition is more prevalent in males (7%) than in females (0.4%). Individuals who lack or possess decreased sensitivity to the red-sensitive cones possess protanopia color blindness; those who lack or possess decreased sensitivity to green-sensitive cones possess deuteranopia color blindness. The color-blindness condition is often more of a deficiency or a weakness than one of blindness.

To conduct the color vision test, follow these steps:

a. Examine the color test plates in Ichikawa's or Ishihara's book to test for any red-green color vision deficiency. Also examine figure 36.4.

b. Hold the test plates approximately 30 inches from the subject in bright light. All responses should occur within 3 seconds.

Figure 36.4 Samples of Ishihara's color plates. These plates are reproduced from Ishihara's Tests for Colour Blindness published by KANEHARA & CO., LTD., Tokyo, Japan, but tests for color blindness cannot be conducted with this material. For accurate testing, the original plates should be used.

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4 This color plate is nonselective. All individuals, normal or color blind, will read this plate as 12.

Normal individuals read this plate as 8. Red/green colorblind individuals see a 3 here.

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J A normal person sees a 16 here Color-deficient individuals ^M read this plate incorrectly or not at all.

^^ Normal individuals read a 96 here. Protanopes see only fc^k a 6. Deuteranopes see only a 9.

Figure 36.5 Blind-spot demonstration.

c. Compare your responses with the correct answers in Ichikawa's or Ishihara's book. Determine the percentage of males and females in your class who exhibit any color-deficient vision. If an individual exhibits color-deficient vision, determine if the condition is protanopia or deuteranopia.

d. Record the class results in Part A of the laboratory report.

5. Complete Part A of the laboratory report.

Essentials of Human Physiology

Essentials of Human Physiology

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