Essentials of Anatomy and Physiology

impulses generated is very intense, and the brain may
interpret any intense sensation as pain. A few minutes
later the bright light seems fine because the rods are
recycling their rhodopsin slowly, and it is not breaking
down all at once.
Chemical reactions in the cones, also involving
retinal, are brought about by different wavelengths of
light. It is believed that there are three types of cones:
red-absorbing, blue-absorbing, and green-absorbing
cones. Each type absorbs wavelengths over about a
third of the visible light spectrum, so red cones,
for example, absorb light of the red, orange, and
yellow wavelengths. The chemical reactions in cones
also generate electrical impulses (see Box 9–4: Night
Blindness and Color Blindness).
The impulses from the rods and cones are trans-
mitted to ganglion neurons(see Fig. 9–6); these con-
verge at the optic disc and become the optic nerve,
which passes posteriorly through the wall of the eye-
ball. Ganglion neurons also seem to have a photo-
receptor chemical (called melanopsin) that may
contribute to the daily resetting of our biological
clocks.

The optic nerves from both eyes come together at

the optic chiasma(or chiasm), just in front of the

pituitary gland (see Fig. 8–11). Here, the medial fibers

of each optic nerve cross to the other side. This cross-

ing permits each visual area to receive impulses from

both eyes, which is important for binocular vision.

The visual areas are in the occipital lobesof the

cerebral cortex. Although each eye transmits a slightly

different picture (look straight ahead and close one

eye at a time to see the difference between the two pic-

tures), the visual areas put them together, or integrate

them, to make a single image that has depth and three

dimensions. This is called binocular vision. The

visual areas also right the image, because the image on

the retina is upside down. The image on film in a cam-

era is also upside down, but we don’t even realize that

because we look at the pictures right side up. The

brain just as automatically ensures that we see our

world right side up.

Also for near vision, the pupils constrict to block

out peripheral light rays that would otherwise blur the

image, and the eyes converge even further to keep the

images on the corresponding parts of both retinas.

The Senses 209

BOX9–4 NIGHT BLINDNESS AND COLOR BLINDNESS

color blindness on his X chromosome has no gene

at all for color vision on his Y chromosome and will

be color blind.

Night blindness, the inability to see well in dim

light or at night, is usually caused by a deficiency of

vitamin A, although some night blindness may

occur with aging. Vitamin A is necessary for the syn-

thesis of rhodopsin in the rods. Without sufficient

vitamin A, there is not enough rhodopsin present to

respond to low levels of light.

Color blindnessis a genetic disorder in which

one of the three sets of cones is lacking or non-

functional. Total color blindness, the inability to see

any colors at all, is very rare. The most common

form is red-green color blindness, which is the

inability to distinguish between these colors. If

either the red cones or green cones are nonfunc-

tional, the person will still see most colors, but will

not have the contrast that the non-working set of

cones would provide. So red and green shades will

look somewhat similar, without the definite differ-

ence most of us see. This is a sex-linked trait; the

recessive gene is on the X chromosome. A woman

with one gene for color blindness and a gene for

normal color vision on her other X chromosome will

not be color blind but may pass the gene for color

blindness to her children. A man with a gene for

Box Figure 9–B Example of color patterns used to

detect color blindness.