Procedure B Visual Demonstrations

Perform the following demonstrations with the help of your laboratory partner.

1. Blind-spot demonstration. There are no photoreceptors in the optic disk, which is located where the nerve fibers of the retina leave the eye and enter the optic nerve. Consequently, this region of the retina is commonly called the blind spot.

To demonstrate the blind spot, follow these steps:

a. Close your left eye, hold figure 36.5 about 35 cm away from your face, and stare at the + sign in the figure with your right eye.

b. Move the figure closer to your face as you continue to stare at the + until the dot on the figure suddenly disappears. This happens when the image of the dot is focused on the optic disk. Measure the distance using a metric ruler or a meter stick.

c. Repeat the procedures with your right eye closed. This time stare at the dot, and the + will disappear when the image falls on the optic disk. Measure the distance.

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

i^^ft Critical Thinking Application ii'jiifclai Under normal visual circumstances, explain why small objects are not lost from our vision.

2. Photopupillary reflex. The smooth muscles of the iris function to control the size of the pupil. For example, when the intensity of light entering the eye increases, a photopupillary reflex is triggered, and the circular muscles of the iris are stimulated to contract. As a result, the size of the pupil decreases, and less light enters the eye.

To demonstrate this reflex, follow these steps:

a. Ask your partner to sit with his or her hands thoroughly covering his or her eyes for 2 minutes.

b. Position a pen flashlight close to one eye with the light shining on the hand that covers the eye.

c. Ask your partner to remove the hand quickly.

d. Observe the pupil and note any change in its size.

e. Have your partner remove the other hand, but keep that uncovered eye shielded from extra light.

f. Observe both pupils and note any difference in their sizes.

3. Accommodation pupillary reflex. The pupil constricts as a normal accommodation reflex response to focusing on close objects. To demonstrate the accommodation reflex, follow these steps:

a. Have your partner stare for several seconds at some dimly illuminated object in the room that is more than 20 feet away.

b. Observe the size of the pupil of one eye. Then hold a pencil about 25 cm in front of your partner's face and have your partner stare at it.

c. Note any change in the size of the pupil.

4. Convergence reflex. The eyes converge as a normal convergence response to focusing on close objects. To demonstrate the convergence reflex, follow these steps:

a. Repeat the procedure outlined for the accommodation pupillary reflex.

b. Note any change in the position of the eyeballs as your partner changes focus from the distant object to the pencil.

5. Complete Part B of the laboratory report.

Essentials of Human Physiology

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  • fikru
    Why are small objects lost from our vision under normal circumstances?
    4 years ago

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