Human Physiology, 14th edition (2016)

292 Chapter 10

within the iris; dilation is produced by contraction of radial

muscles. Constriction of the pupils results from parasympa-

thetic stimulation through the occulomotor (III) nerve, whereas

dilation results from sympathetic stimulation ( fig. 10.28 ).

The posterior part of the iris contains a pigmented epithe-

lium that gives the eye its color. The color of the iris of the eye

is determined by the amount of pigment—blue eyes have the

least pigment, brown eyes have more, and black eyes have the

greatest amount of pigment. In the condition of albinism —a

congenital absence of normal pigmentation caused by an inabil-

ity to produce melanin pigment—the eyes appear pink because

the absence of pigment allows blood vessels to be seen.

The lens is composed of living cells but is avascular (lack-

ing in blood vessels), requiring its own microcirculatory sys-

tem to sustain its cells. Even so, its metabolism is anaerobic

and the cells near its center have low metabolic rates. The lens

is transparent, composed predominately of cells called “mature

fibers” that lack organelles. These cells have a flattened hexag-

onal shape in cross-section and are interconnected by numer-

ous gap junctions. Interestingly, mutations in the genes that

code for the connexin proteins of gap junctions (chapter 7; see

fig. 7.22) are known to cause hereditary cataracts (described in

the nearby Clinical Applications box).

The iris is like the diaphragm of a camera; it can increase or
decrease the diameter of its aperture (the pupil) to admit more
or less light. Variations in the diameter of the pupil are similar
in effect to variations in the f-stop of a camera. Constriction
of the pupils is produced by contraction of circular muscles

CLINICAL APPLICATION

A cataract is an area of cloudiness in the lens, which is

normally transparent because (1) it is avascular, (2) its

organelles have been eliminated, and (3) the cell cytoplasm

is filled with proteins called crystalline. Damage from ultra-

violet light, dehydration, or oxidation may cause the crys-

talline proteins to change shape and aggregate, producing

the cataract. Cataracts interfere with vision in over half of

people over the age of 65, and are often treated by surgical

removal of the lens. This involves incisions in the cornea

and lens capsule and insertion of a probe that emits ultra-

sound to soften and help fragment the lens, so that it can be

removed by aspiration. The lens is replaced by an artificial,

flexible intraocular lens, which is inserted into the lens cap-

sule and greatly improves vision in almost all cases.

Figure 10.28 Dilation and constriction of the pupil. In dim light, the radially arranged smooth muscle fibers are stimulated
to contract by sympathetic neurons, dilating the pupil. In bright light, the circularly arranged smooth muscle fibers are stimulated to
contract by parasympathetic neurons, constricting the pupil.

From superior

cervical

ganglion

Radially arranged smooth

muscle fibers of the iris

Circularly arranged

smooth muscle fibers of

the iris

Pupil

Ciliary

ganglion

From

oculomotor

nerve

Postganglionic

parasympathetic

axon

Postganglionic

sympathetic

axon

In dim light

In normal light

In bright light