Fig. 1 Pupillary light reflex. Light in left eye (dotted arrow) stimulates retina (RET), whose afferent axons (fine dashed lines) ascend optic nerve (ON), decussate at chiasm (CHI), and terminate in pretectal nuclear complex (PTN). Lateral geniculate nucleus (LGN) is bypassed by these pupillomotor fibers. The PTN is connected by crossed and uncrossed intercalated neurons to both Edinger–Westphal parasympathetic motor nuclei (E–W), which comprise the dorsal aspect of the oculomotor nuclear complex.3 Preganglionic parasympathetic fibers (heavy dashed lines) leave ventral aspect of midbrain in the substance of the third cranial nerves. After synapsing in the ciliary ganglia (CG), the postganglionic fibers innervate the pupillary sphincter muscles. Note that uniocular light stimulus evokes bilateral and symmetric pupillary constriction. Brain stem diagram represents section through level rostral to superior colliculi (SC).