Chapter 15
The Special Senses
553
15
3.
Sam’s optometrist tells him that his intraocular pressure is
high. What is this condition called and which fluid does it
involve?
For answers, see Appendix H.
Optics and the Eye
Trace the pathway of light through the eye to the retina,
and explain how light is focused for distant and close
vision.
Outline the causes and consequences of astigmatism,
myopia, hyperopia, and presbyopia.
Overview: Light and Optics
To comprehend the function of the eye as a photoreceptor or-
gan, we need to understand the properties of light.
Wavelength and Color
Electromagnetic radiation
includes
all energy waves, from long radio waves (with wavelengths
measured in meters) to very short gamma (γ) waves and X rays
with wavelengths of 1 nm and less. Our eyes respond to the part
of the spectrum called
visible light
, which has a wavelength
range of approximately 400–700 nm
(Figure 15.10a)
. (1 nm
5
10
2
9
m, or one-billionth of a meter.)
Visible light travels in the form of waves, and its wavelengths
can be measured very accurately. However, light can also be en-
visioned as small particles or packets of energy called
photons
or
quanta
. Attempts to reconcile these two findings have led to the
present concept of light as packets of energy (photons) traveling
in a wavelike fashion at very high speeds (300,000 km/s or about
186,000 mi/s). We can think of light as a vibration of pure energy
(“a bright wiggle”) rather than a material substance.
When visible light passes through a prism, each of its compo-
nent waves bends to a different degree, dispersing the beam of
Homeostatic Imbalance
15.6
If the drainage of aqueous humor is blocked, fluid backs up as in
a clogged sink. Pressure within the eye may increase to danger-
ous levels and compress the retina and optic nerve—a condition
called
glaucoma
(glaw-ko
9
mah). ±e eventual result is blind-
ness (
glaucoma
5
vision growing gray) unless the condition
is detected early. Unfortunately, many forms of glaucoma steal
sight so slowly and painlessly that people do not realize they
have a problem until the damage is done. Late signs include see-
ing halos around lights and blurred vision.
±e glaucoma examination is simple. ±e intraocular pres-
sure is determined by directing a puff of air at the cornea and
measuring the amount of corneal deformation it causes. ±is
exam should be done yearly aFer age 40. ±e most common
treatment is eye drops that increase the rate of aqueous humor
drainage or decrease its production. Laser therapy or surgery
can also help.
Lens
±e
lens
is a biconvex, transparent, flexible structure that can
change shape to precisely focus light on the retina. It is enclosed
in a thin, elastic capsule and held in place just posterior to the
iris by the ciliary zonule (²igure 15.8). Like the cornea, the lens
is avascular; blood vessels interfere with transparency.
±e lens has two regions: the
lens epithelium
and the lens
fibers. ±e lens epithelium, confined to the anterior lens surface,
consists of cuboidal cells that eventually differentiate into the
lens fibers
that form the bulk of the lens. ±e lens fibers, which
are packed tightly together like the layers in an onion, contain
no nuclei and few organelles. ±ey do, however, contain trans-
parent, precisely folded proteins called
crystallins
that form the
body of the lens. Since new lens fibers are continually added, the
lens enlarges throughout life, becoming denser, more convex,
and less elastic, all of which gradually impair its ability to focus
light properly.
Homeostatic Imbalance
15.7
A
cataract
(“waterfall”) is a clouding of the lens that causes
the world to appear distorted, as if seen through frosted glass
(Figure 15.9)
. Some cataracts are congenital, but most result
from age-related hardening and thickening of the lens or are
a secondary consequence of diabetes mellitus. Heavy smoking
and frequent exposure to intense sunlight increase the risk for
cataracts.
Oxidative stress and metabolic changes in the deeper lens
fibers promote clumping of the crystallin proteins. A recent
study unexpectedly showed that supplementation with the anti-
oxidant vitamin C actually increases cataract formation. ²or-
tunately, the offending lens can be surgically removed and an
artificial lens implanted to save the patient’s sight.
Check Your Understanding
1.
What are tears and what structure secretes them?
2.
What is the blind spot and why is it blind?
Figure 15.9
Photograph of a cataract.
The lens, not the cornea,
is milky and opaque.
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