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system employs autonomic nerve pathways.

© The McGraw-H Companies, 2001

m special senses understanding ^Vo rds aud-, to hear: auditory—

pertaining to hearing. choroid, skinlike: choroid coat— middle, vascular layer of the eye.

cochlea, snail: cochlea—coiled tube within the inner ear. corn-, horn: cornea—transparent outer layer in the anterior portion of the eye. iris, rainbow: iris—colored, muscular part of the eye. labyrinth, maze: labyrinth— complex system of connecting chambers and tubes of the inner ear. lacri-, tears: lacrimal gland—tear gland.

lut-, yellow: macula lutea— yellowish spot on the retina.

macula, spot: macula lutea— yellowish spot on the retina.

malle-, hammer: malleus—one of the three bones in the middle ear. oculi-, eye: orbicularis oculi—

muscle associated with the eyelid.

olfact-, to smell: olfactory—

pertaining to the sense of smell.

palpebra, eyelid: levetor palpebrae superioris— muscle associated with the eyelid.

photo-, light: photoreceptor— specialized structures in the eye responsive to light. scler-, hard: sclera—tough, outer protective layer of the eye. therm-, heat: thermoreceptor— receptor sensitive to changes in temperature. tympan-, drum: tympanic membrane—eardrum. vitre-, glass: vitreous humor— clear, jellylike substance within the eye.

chapter objectivi

After you have studied this chapter, you should be able to

Name five kinds of receptors and explain the function of each. Explain how receptors stimulate sensory impulses. Explain how a sensation is produced. Distinguish between somatic and special senses.

Describe the receptors associated with the senses of touch and pressure, temperature, and pain.

6. Describe how the sense of pain is produced.

7. Explain the importance of stretch receptors in muscles and tendons.

8. Explain the relationship between the senses of smell and taste.

9. Name the parts of the ear and explain the function of each part.

10. Distinguish between static and dynamic equilibrium.

11. Name the parts of the eye and explain the function of each part.

12. Explain how the eye refracts light.

13. Explain how the brain perceives depth and distance.

14. Describe the visual nerve pathway.

ohn Dalton, a famous English chemist, saw things differ-

Jently than most people. In a 1794 lecture, he described his visual world. Sealing wax that appeared red to other people was as green as a leaf to Dalton and his brother. Pink wildflowers were blue, and Dalton perceived the cranesbill plant as "sky blue" in daylight, but "very near yellow, but with a tincture of red" in candlelight. He concluded, "... that part of the image which others call red, appears to me little more than a shade, or defect of light." The Dalton brothers, like 7% of males and 0.4% of females today, had the inherited trait of color blindness.

Dalton was very curious about the cause of his color blindness, so he made arrangements with his personal physician, Joseph Ran-some, to dissect his eyes after he died. Ransome snipped off the back of one eye, removing the retina, where the cone cells that provide color vision are nestled among the more abundant rod cells that impart black-and-white vision. Because Ransome could see red and green normally when he peered through the back of his friend's eyeball, he concluded that it was not an abnormal filter in front of the eye that altered color vision.

Fortunately, Ransome stored the eyes in dry air, where they remained relatively undamaged. In 1994, Dalton's eyes underwent DNA analysis at London's Institute of Ophthalmology. The research showed that Dalton's remaining retina lacked one of three types of pigments, called photopigments, that enable cone cells to capture certain incoming wavelengths of light.

Although people have studied color blindness for centuries, we are still learning more about it. Recently, researchers investigated why color blind men lacking cones that capture green light are affected to different degrees. They discovered that color blind men who can discern a few shades of green have red cone cells that can detect some wavelengths of light that fall within the green region of the spectrum. Color vision may be more complex than we had thought.

People who are color blind must function in a multicolored world. To help them overcome the disadvantage of not seeing important color differences, researchers have developed computer algorithms that convert colored video pictures into shades they can see. This circle of dots is a test to determine whether someone is color blind. Affected individuals cannot see a different color in certain of the circles in such a drawing. As a result, their brains cannot perceive the embedded pattern that forms the number 16 that others can see.

People who are color blind must function in a multicolored world. To help them overcome the disadvantage of not seeing important color differences, researchers have developed computer algorithms that convert colored video pictures into shades they can see. This circle of dots is a test to determine whether someone is color blind. Affected individuals cannot see a different color in certain of the circles in such a drawing. As a result, their brains cannot perceive the embedded pattern that forms the number 16 that others can see.

The above has been reproduced from Ishihara's Tests for Colour Blindness published by Kanehara & Co. Ltd. Tokyo, Japan, but tests for colour blindness cannot be conducted with this material. For accurate testing, the original plates should be used.

Recall from chapter 11 (p. 400) that the terms "axon" and nerve fiber are used synonymously. Also recall that unipolar neurons, which include most sensory neurons, have an unusual structure in which the portion of the neuron associated with the dendrites, called a peripheral process, is considered to behave like an axon. Because of this, and for simplicity, the neuron processes which bring sensory information into the CNS will be called sensory fibers or afferent fibers, no matter what type of neuron is involved.

All senses work in basically the same way. Sensory receptors are specialized cells or multicellular structures that collect information from the environment and stimulate neurons to send impulses along sensory fibers to the brain. There the cerebral cortex forms a perception, a person's particular view of the stimulus. Table 12.1 outlines the pathways from sensation to perception that describe an apple.

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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