Chapter 15
The Special Senses
555
15
Focusing for Close Vision
Light from close objects (less than
6 m away) diverges as it approaches the eyes and comes to a
focal point farther from the lens. For this reason, close vision
demands that the eye make active adjustments. To restore focus,
three processes must occur simultaneously: accommodation of
the lenses, constriction of the pupils, and convergence of the
eyeballs. ±e signal that induces this trio of reflex responses is
blurring of the retinal image.
Figures 15.4 and 15.6). During its passage, light is bent three
times: (1) entering the cornea, (2) entering the lens, and (3)
leaving the lens.
±e cornea accounts for the majority of the refractory power
of the eye. However, the refractory power of the cornea is con-
stant. On the other hand, the lens is highly elastic, and its curva-
ture and light-bending power can actively change to allow fine
focusing.
Focusing for Distant Vision
Our eyes are best adapted (“preset
to focus”) for distant vision. To look at distant objects, we need
only aim our eyeballs so that they are both fixated on the same
spot. ±e
far point of vision
is that distance beyond which no
change in lens shape (accommodation) is needed for focusing.
For the normal or
emmetropic
(em
0
ĕ-tro
9
pik) eye, the far point
is 6 m (20 feet).
Any object being viewed can be said to consist of many small
points, with light radiating outward in all directions from each
point. However, because distant objects appear smaller, light
from an object at or beyond the far point of vision approaches
the eyes as nearly parallel rays. ±e cornea and the at-rest lens
focus the light from these distant objects precisely on the retina
(Figure 15.13a)
.
During distant vision, the sphincterlike ciliary muscles are
completely relaxed, and tension in the ciliary zonule stretches
the lens flat. Consequently, the lens is as thin as it gets and is
at its lowest refractory power when at rest. ±e ciliary muscles
relax when sympathetic input to them increases and parasym-
pathetic input decreases.
Point sources
(a) Focusing of two points of light.
(b) The image is inverted—upside down and reversed.
Focal points
Figure 15.12
Light is focused by a convex lens.
The lens
bends, or refracts, light rays so they converge on a focal point.
Lens
Inverted
image
Ciliary zonule
Ciliary muscle
Nearly parallel rays
from distant object
(a) Lens flattens for distant vision.
Sympathetic input
relaxes the ciliary muscle, tightening the ciliary zonule,
and flattening the lens.
Sympathetic activation
Divergent rays
from close object
(b) Lens bulges for close vision.
Parasympathetic input
contracts the ciliary muscle, loosening the ciliary zonule,
allowing the lens to bulge.
Inverted
image
Parasympathetic activation
Ciliary muscle
View
Lens
Ciliary zonule
(suspensory ligament)
(c) The ciliary muscle and ciliary zonule are arranged
sphincterlike around the lens.
As a result, contraction
loosens the ciliary zonule fibers and relaxation tightens
them.
Figure 15.13
Focusing for distant and close vision.
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