Sense organs contain receptors which are sensitive to a particular type of environmental stimuli. After receptors receive stimuli, they generate nerve impulses. Eventually these nerve impulses reach the brain, where the phenomenon sensation occurs. The brain interprets the world around us and depends on the receptors for the proper input. Because the sense organs have a limited ability to respond to the environment, we can be fooled sometimes.
The human eye is a special sense organ because it is a large, complex organ containing many receptors. The receptors in the eye are either rods or cones, both of which are photoreceptors. The parts of the eye are depicted in Figure 18.6 on page 345 of your textbook.
With the help of figure 18.6, identify the following structures and give a function for each structure in the space provided.
Which of the structures listed aid in refracting and focusing light rays?
Which of these structures contain( s) the receptors for sight?
What are the receptors for sight?
Which of the structures listed transmit(s) nerve impulses to the brain?
We see images because the rods and cones are generating nerve impulses for transmission to the brain. Occasionally the brain retains an image even after the impulses have stopped. These images are called afterimages. A positive afterimage is one in which the bright parts of the object remain bright and the dark parts remain dark. In a negative afterimage the bright parts of the object appear dark and the dark parts appear bright.
Center a red card on a black sheet of paper and stare intently at the card. After about thirty seconds or longer and without shifting the eyes, place a white sheet of paper over the red card. Describe in the following table the color of the image that “appears” on the white paper.
Repeat by placing green, blue, and yellow cards on the black paper and later covering with a white sheet.
Card Color Color of Afterimage
The blind spot occurs where the optic nerves exit the retina. No vision is possible because of the absence of rods and cones at this location.
This procedure requires a laboratory partner. Enter the data for your own eyes in your laboratory report.
1. Determine the blind spot of the left eye.In figure A, you will see a small cross and a circle a few inches apart from one another. Hold the figure 30 cm away from your eyes. Tip the paper so both the cross and the circle are on the same level. If you wear glasses, keep them on.
Close your right eye only. Stare at the cross only with your left eye. You also should be able to see the circle in the same field of vision.
Slowly move the paper toward you until the circle disappears. At this point, the circle is being focused on the blind spot of your left eye.
Measure the distance from your eye to the paper with a meterstick when the circle first disappears.What is the distance in cm?____ (left eye).
Keep moving the paper close to your eyes until the circle reappears. (If you use a yardstick, measure in inches and multiply by 2.54 to convert to cm.)
FigureA. To test for the blind spot, use these symbols and the directions provided below. You might want to redraw these marks on a blank piece of paper to make this easier. Make the marks about the same distance apart as your eyes are.
2. Determine the blind spot of the righteye.To determine the blind spot of your right eye, repeat the procedure just describedbut with the following modifications.
Close only your left eye and look at the circle with your right eye. Move the paper close to your eyes until the cross disappears. At this point the cross is being focused on the blind spot of your right eye.Measure the distance from the paper to your right eye when the cross first disappears.
What is the distance in cm? (right eye).
Is the measured distance the same for both eyes?
Accommodation of the Eye
The eye accommodates for focus at different distances by changing the shape of the crystalline lens. The lens is controlled by the ciliary muscles (within ciliary body) attached to the lens. When you look at a distant object, the lens is in a flattened state.
When you look at a closer object, the lens becomes more rounded. The eye is said to accommodate for distance. The elasticity of the lens determines how well the eye can accommodate.
This procedure requires a laboratory partner. Enter the data for your own eyes and in your laboratory report.
1. Accommodation of your right eye.
Hold a pencil by the eraser in your right hand. Close your left eye.
Move the pencil from arm’s length toward your right eye. Focus on the point of the pencil.
Move the pencil until the point is out of focus.
Measure the distance between the pencil and your right eye. At what distance can your right eye no longer accommodate for distance? (report in cm.)
If you wear glasses, repeat this experiment without your glasses. (Do not take out contact lens if you have them)
What is the accommodation distance of the right eye without glasses? (report in cm.)
2. Accommodation of your left eye.
Repeat the procedure described. Close your right eye and look at the pencil point with your left eye. What is the accommodation distance of your left eye? cm.
If you wear glasses, repeat the same experiment without your glasses. What is the accommodation distance of the left eye without glasses? cm.
3. The “younger” lens can easily accommodate for closer distances. The more elastic the lens, the “younger” the eye.The nearest point at which the pencil point can be seen clearly is called the near point.
Age (years) 10 20 30 40 50 60
Near point (cm) 9 10 13 18 50 83
How “old” is your right eye? How “old” is your left eye?
How “old” is your right eye without glasses ?How “old” is your left eye without glasses?
The pupil is really a hole in the iris, a donut-shaped muscle. When certain fibers of the iris contract, the pupil dilates; when other fibers contract, the pupil constricts. Normally, the pupils dilate in dim light and constrict in bright light.
This test must be conducted in a dimly lit area and requires a laboratory partner
1. Pupillary reflex in both eyes. The experimenter faces the subject and looks at the pupils. The subject closes his or her eyes and covers them with the hands for approximately two minutes. The subject opens his or her eyes, and the experimenter shines a penlight into one eye, then the other, and back again. What happens to the pupils?
2. Pupillary reflex in one eye. The subject holds the edge of an index card along the nose to divide the field of vision. The experimenter tests thepupillary reflex of one eye using the penlight as just described.
Does a comparable reflex occur in the non-illuminated eye?
Do pupils adjust independently or together?
Both eyes view the same field, but because of the optic chiasma (fig. 18.10 page 348 in textbook), both eyes send information about the right field to the left side of the brain, and vice versa. If each eye by chance views a different field, the information is mixed in the brain.
Make a paper tube about 1.5 feet long (or use the cardboard tube from a roll of paper towels). Keeping both eyes open, hold the tube up to one eye and look at some distant object through the tube, then bring the free hand up in front of the other eye and move this hand back and forth along the tube. A hole will appear in your hand.
The Human Ear
The human ear is a special sense organ because it is a large, complex organ containing many receptors. In the ear, the organ of Corti, in the cochlea, contains hair cells which allow us to hear. The semicircular canals and the utricle and saccule help maintain balance. The parts of the ear are listed in Table 18.3 (page 352 in your textbook) and depicted in Figure 18.11 and Fig 18.12 (pages 349+350 in your textbook).
With the help of figure 18.11, study the ear and identify the outer, middle, and inner ear and their parts listed next. With the help of table 18.3, give a function for each part named.
1. Outer ear
2. Middle ear
Tympanic membrane (eardrum)
Ossicles (hammer, anvil, stirrup)
3. Inner ear
Which of the preceding structures transmits sound waves?
Which contains the receptors for hearing?
What are the receptors of hearing?
Which of the preceding transmits a nerve impulse to the brain?
The eustachian tube runs between the middle ear and the throat. It helps equalize the external air pressure with that in the middle ear. Is theeustachian tube indicated in the figure?
Direction of Sound
Humans locate the direction of sound according to how fast it is detected by either or both ears.
You will need a laboratory partner to perform this procedure. Enter the data for your ears in your laboratory report.
The seated subject closes both eyes. Strike two spoons together or strike a tuning fork and ask the subject to give the exact location of the sound in relation to his or her head.
What does the subject report when the sound is:
directly below and behind the head?
directlybehind the head?
directlyabove the head?
directlyin front of the face?
tothe sides of the head?
Explain why the subject incorrectly identified the location of some of these.
Skin Receptors in Humans
The skin receptors in humans concern sensations such as touch, pain, temperature, and pressure. There are individual receptors for each of these various sensations.
With the help of figure 11.8 (page 199 in your textbook), study the image of the skin and identify these areas or structures. State the location of each.
1. Subcutaneous layer
2. Adipose tissue
5. Hair follicle and hair
6. Sebaceous gland
7. Sweat gland
You will need a laboratory partner to perform the following procedures.
The subject is seated with his or her eyes closed. In the following manner, test the subject’s ability to discriminate two pointsof a pair of scissors in the skin areas listed next. Hold the points of a pair of fine scissors on the given skin area, with eitherone or both of the scissor points gently touching the subject. The subject should respond whether he or she feels one or twotouch sensations.
Record the shortest distance for a two-point discrimination.
2. Back of the neck
3. Index Finger
4. Back of the Hand
Which of these areas contains the largest number of receptors for touch?
How do you know?
Heat and Cold
Fill three medium size bowls as follows: (1) ice water; (2) tap water at room temperature; and (3) warm water
Place your left hand in the first beaker and your right hand in the third beaker. Keep both hands immersed for at least thirty seconds.
Place both hands in the second beaker. Record the sensation in the right and left hands.
Explain these results:
The taste receptors located in the mouth and the smell receptors located in the nasal cavities are the chemoreceptors that respond to molecules in the air and water.
You will need a laboratory partner to perform the following procedures.
The experimenter should be sure to use a clean cotton swab each time. The subject should be sure to rinse the mouth between applications.
The experimenter applies each of the following solutions to the tip, sides, and back of the tongue and records in the table whether or not the subject can taste them. See the Materials section at the top to find the recipes for these solutions.
Solutions: 5% sucrose (sweet)
10% NaCI (salty)
Household vinegar (sour)
Taste Tongue tip Tongue sides Tongue back
Figure18.4 (page 344 in your textbook) illustrates the human tongue. It is believed we have four taste sensations -sweet, sour, salty, and bitter – and each is located on the tongue in a different position.
See if you can identify where these taste sensations appear on your tongue. Where did you find them?
Smell and Taste
This experiment requires that you have a partner. One person is the timekeeper and the experimenter, and the other is the subject.The subject eats a piece of carrot and a piece of apple before the experiment begins. Use the hardest apple and the softest carrot available. You will need two small pieces of peeled apple and two small pieces of peeled carrot for the experiment.The four pieces should be about the same size and shape. During the experiment the subject should not allow the piece of carrot or apple to touch the roof of the mouth.
1. The subject shuts his or her eyes and holds his or her nostrils closed. When ready the experimenter places a piece of either carrot or apple on the subject’s tongue. The subject reports whether the piece is carrot or and the time taken for response is noted. Do not tell the subject whether he or she is right or wrong.
After the response, the experimenter removes the pieceofcarrot or apple and disposes of it. Repeat twice with each material.
Fill in the table, recording the actual substance, the subject’s response, and the length of time taken for the response.
Taste and Smell Experiment
Substance Subject’s response Time taken
2. The subject shuts his or her eyes and keeps his or her nostrils open. Repeat the preceding procedure but now, the subject can smell the piece of food before it is placed on the tongue. Record the results as before in the next table.
Substance Subject’s response Time taken
Did the subject respond more accurately and/or more quickly during the second test?
If so, how do you account for this?
1. Name the portion of the eye that is sensitive to light.
2. In the eye, focusing is done primarily by the ____________?
3. Which part of the eye is responsible for rounding the lens?
4. Are negative afterimages the same as or different from the original color?
5. Do the pupils become larger or smaller in the dark?
6. What part of the ear contains the sense receptors for hearing?
7. What are the three little bones in the middle ear collectively called?
8. Subjects had the most difficulty detecting the location of a sound made where in relation to their heads?
9. What is the inner layer of skin called?
10. Both taste and smell are produced by _________________________?