Human Physiology, 14th edition (2016)

Sensory Physiology 297

Astigmatism

Because the curvature of the cornea and lens is not perfectly
symmetrical, light passing through some parts of these struc-
tures may be refracted to a different degree than light passing
through other parts. When the asymmetry of the cornea and/
or lens is significant, the person is said to have astigmatism
( fig.  10.35 d ). If a person with astigmatism views a circle of
lines radiating from the center, like the spokes of a wheel, the
image of these lines will not appear clear in all 360 degrees.
The parts of the circle that appear blurred can thus be used to
map the astigmatism. This condition is corrected by cylindrical
lenses that compensate for the asymmetry in the cornea or lens
of the eye.

Figure 10.35 Problems of refraction and how they are corrected. In a normal eye ( a ) , parallel rays of light are brought to
a focus on the retina by refraction in the cornea and lens. If the eye is too long, as in myopia ( b ) , the focus is in front of the retina. This
can be corrected by a concave lens. If the eye is too short, as in hyperopia ( c ) , the focus is behind the retina. This is corrected by a
convex lens. In astigmatism ( d ) , light refraction is uneven because of irregularities in the shape of the cornea or lens.

Emmetropia (normal vision)

Rays focus on retina

Myopia (nearsightedness)

Rays focus in front of retina

Astigmatism

Rays do not focus

Hyperopia (farsightedness)

Rays focus behind retina

No correction necessary

Concave lens corrects

nearsightedness

Uneven lens corrects

astigmatism

Convex lens corrects

farsightedness

(a)

(b)

(c)

(d)

CLINICAL APPLICATIONS

LASIK ( laser-assisted in situ keratomileusis ) is a surgical

procedure to correct for myopia, hyperopia, and astigma-

tism. The surgeon first cuts a flap in the cornea, which is

folded backward. Then a computer-guided laser burns cor-

neal tissue as it reshapes the cornea. For myopia, the laser

reduces the curve of the cornea to reduce refraction and

allow focus on the retina; for hyperopia, the laser increases

the curvature and refractive power; and for astigmatism, it

reshapes a more spherically correct cornea. LASIK can-

not correct for presbyopia, so reading glasses may still be

required. Alternatively, one eye of a person with myopia

might be deliberately undercorrected to allow closer vision

while the other is corrected to near 20/20, a technique

called monovision.

Clinical Investigation CLUES

Susan had myopia and was told to either get new glasses

or have LASIK surgery.

• What kind of lenses does she need to correct for
  myopia?


• What is LASIK, and how would it correct her
  myopia?

| CHECKPOINT

11. Using a line diagram, explain why an inverse image is
    produced on the retina. Also explain how the image
    in one eye corresponds to the image in the other eye.


12. Using a line diagram, show how parallel rays of light
    are brought to a focus on the retina. Explain how this
    focus is maintained as the distance from the object
    to the eye is increased or decreased (that is, explain
    accommodation).


13. Explain why a blurred image is produced in each of
    these conditions: presbyopia, myopia, hyperopia,
    and astigmatism.

10.7 Retina

There are two types of photoreceptor neurons: rods and

cones. Both contain pigment molecules that undergo