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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 syndrome.


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 29–2).

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