468
UNIT THREE
lens, providing nutrients and maintaining the shape of the
front of the eye. It subsequently leaves the anterior cham-
ber through veins and a special drainage canal, the scleral
venous sinus (canal of Schlemm), in its wall at the junction
of the cornea and the sclera.
The smooth muscle F bers of the iris form two groups, a
circular set
and a
radial set.
These muscles control the size
of the pupil, through which light passes. The circular set of
muscle F
bers acts as a sphincter, and when it contracts, the
pupil gets smaller (constricts) and the intensity of the light
entering decreases. When the radial muscle F
bers contract,
the diameter of the pupil increases (dilates) and the intensity
of the light entering increases.
The sizes of the pupils change constantly in response to
pupillary refl
exes triggered by such factors as light intensity,
gaze, accommodation, and variations in emotional state.
±or example, bright light elicits a refl ex, and impulses travel
along parasympathetic nerve F
bers to the
circular muscles
of
the irises. The pupils constrict in response. Conversely, in
dim light, impulses travel on sympathetic nerve F
bers to the
radial muscles
of the irises, and the pupils dilate
(f g. 12.31)
.
RECONNECT
To Chapter 11, Autonomic Nervous System,
pages 424–428.
The amount and distribution of melanin in the irises and
the density of the tissue in the body of the iris determine eye
color. If melanin is present only in the epithelial cells on the
iris’s posterior surface, the iris refl ects more wavelengths of
light, and appears blue or green. When the same distribution
of melanin is denser in the body of the iris, eye color is gray.
When melanin is within the body of the iris as well as in the
posterior epithelial covering, the iris appears brown.
Sclera
Scleral venous sinus
(canal of Schlemm)
Iris
Lens
Posterior
chamber
Aqueous humor
Anterior
chamber
Cornea
Vitreous humor
Ciliary process
Ciliary muscles
Ciliary
body
FIGURE 12.30
Aqueous humor (arrows), secreted into the posterior chamber, circulates into the anterior chamber and leaves it through the
scleral venous sinus (canal of Schlemm).
FIGURE 12.29
In accommodation, (
a
) the lens thickens as the ciliary
muscle F
bers contract. (
b
) The lens thins as ciliary muscle F
bers relax.
Ciliary muscle
fibers contracted
(a)
Lens thick
Suspensory
ligaments relaxed
Ciliary muscle
fibers relaxed
Lens thin
Suspensory
ligaments taut
(b)
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