Biology 12

Near-sightedness

Far-sightedness

Astigmatism

normal

eyeball

normal

eyeball

Long eyeball; rays focus in

front of retina when viewing

distant object

Concave lens allows subject

to see distant objects.

Short eyeball; rays focus

behind retina when viewing

close objects

Convex lens allows subject

to see close objects.

Uneven cornea; rays do not

focus evenly

Uneven lens allows subject

to see objects clearly.

Light from

far object

Light from

far object

Light from

near object

A B

Chapter 5 The Nervous System • MHR 157

The Human Ear

The ear contains mechanoreceptorsthat translate
the movement of air into a series of nerve impulses
that the brain is able to interpret as sound. To
investigate how distance affects ability to hear
sounds, conduct the investigation on page 158.

The Skin and Homeostasis

The skin has two characteristics that make it

important in homeostasis — it is exposed to the

outside world, and it is the largest sense organ in

the body. Receptors in the skin provide information

about changes in the environment. You have

already studied the role of sensory receptors in the

skin that help control thermoregulation. The skin

also has sweat glands that aid in cooling, and

adipose (fatty) tissue that provides insulation. As

well, the skin has a rich supply of blood vessels

that are instrumental in cooling and maintaining

heat within the body.

Many of the sensory receptors in the skin are

free nerve endings. These are dendrites that are not

attached to specialized structures; they respond to

heat gained, heat lost, or pain. Some of these, called

http://www.mcgrawhill.ca/links/biology12
People who are colour-blind are unable to distinguish some or
all colours of the visible light spectrum. This condition, a
sex-linked genetic trait, affects about 8 percent of males and
0.4 percent of females. To access a set of “Ishihara charts”
(a test for colourblindness) that can be used to test your own
colour vision, go to the web site above, and click on Web Links.

WEB LINK

Figure 5.18(A) The cornea and the lens bring light rays
(shown here as lines) into focus, but sometimes they are
unable to compensate for the shape of the eyeball or for an

irregular curvature of the cornea. (B) In these instances,

corrective lenses can allow the individual to see normally.