208
Chapter 7
Vision
The eyes are composed of an optical portion, which focuses
the visual image on the receptor cells, and a neural component,
which transforms the visual image into a pattern of graded and
action potentials.
Right
hemisphere
Toes
Genitalia
Foot
Leg
Hip
Trunk
Neck
Head
Shoulder
Arm
Elbow
Forearm
Wrist
Hand
Little
Ring
Middle
Index
Thumb
Eye
Nose
Face
Upper lip
Lips
Lower lip
Gum and jaw
Tongue
Pharynx
Intraabdominal
Front
Top view
Back
Occipital
lobe
Parietal
lobe
Frontal lobe
Right
hemisphere
Left
hemisphere
Primary
motor
cortex
Somatosensory
cortex
Central
sulcus
Figure 7–20
The location of pathway terminations for different parts of the body in somatosensory cortex, although there is actually much overlap between
the cortical regions. The left half of the body is represented on the right hemisphere of the brain, and the right half of the body is represented
on the left hemisphere, which is not shown here.
12 3
One
wavelength
Time (s)
Intensity
(b)
Figure 7–21
The electromagnetic spectrum. (a) Visible light ranges in wavelength
from 400 to 750 nm (1nm = 1 billionth of a meter). (b) Wavelength
is the inverse of frequency. The frequency of this wave is 2 Hz
(cycles/s).
(a)
Energy
Wavelength
Light
The receptors of the eye are sensitive only to that tiny portion
of the vast spectrum of electromagnetic radiation that we call
visible light (
Figure 7–21a
). Radiant energy is described in
terms of wavelengths and frequencies. The
wavelength
is the
distance between two successive wave peaks of the electromag-
netic radiation (
Figure 7–21b
). Wavelengths vary from several
kilometers at the long-wave radio end of the spectrum to tril-
lionths of a meter at the gamma-ray end. The
frequency
(in
hertz, the number of cycles per second) of the radiation wave
varies inversely with wavelength. Those wavelengths capable of
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