The iris is a structure composed of connective tissue and blood
vessels that lies just anterior to the lens. The central opening
in the iris forms the pupil. The color of the iris is determined
by pigmented cells within the stroma. Pigment may accumulate in
these cells during the first year of life, and the color of the
iris often changes during this time. The posterior layer of the
iris is deeply pigmented. It extends slightly onto the anterior
surface at the edge of the pupil.
The size of the pupil is variable. It changes in response to
neural input to the smooth muscles within the iris. The dilator
muscle of the iris is stimulated by sympathetic pathways. The chain
of neurons responsible for dilation begin in the hypothalamus and
synapses in the thoracic vertebra (first-order neuron), then passes
out of the spinal column, across the pulmonary apex to synapse in
the superior cervical ganglion (second-order neuron), then along
the internal carotid plexus and through the cavernous sinus to join
with the ophthalmic division of cranial nerve V and travel to the
dilator muscle (third-order neuron) (Figure 29–1). The
iris sphincter muscle is innervated by the parasympathetic system.
These neurons originate in the Edinger-Westphal subnucleus of the third
cranial nerve and travel along the inferior division of the nerve
to the ciliary ganglion (preganglionic fibers), then to the iris
sphincter through the short ciliary nerves (postganglionic fibers).
These pathways mediate pupil constriction to light and near.
The sympathetic pathway for iris dilator muscles (pupil
dilation). The first-order neuron begins in the hypothalamus, travels
through the spinal cord, and synapses in the thoracic vertebrae.
The second-order neuron travels over the pulmonary apex and synapses
in the superior cervical ganglion. The third-order neuron travels
adjacent to the internal carotid artery, then through the cavernous sinus
to reach the eye. Damage anywhere along this pathway may cause Horner
At 6 weeks of gestation the iris begins to form in association
with the tunica vasuculosa lentis.
This is a network of blood vessels that extends through mesenchymal
tissue on the anterior surface of the developing lens. The muscles
of the iris sphincter begin to develop at around 3 months’ gestation,
followed at 6 months by formation of the dilator muscles. The iris
stroma forms from neural crest cells. The posterior pigmented epithelium
and muscles form from neuroectoderm. Much of the iris remains incompletely
formed at birth, including pigmentation of the anterior layer and
formation of the dilator muscles. This is why the iris often appears
somewhat hypoplastic, even in normal newborns. During normal embryogenesis
the tunica vasulosa lentis resorbs in the pupillary opening by birth.
Remnants of the membrane may be seen in premature infants (Figure