Fig. 6. Diagram of the pupil light reflex pathway. Note that the axons of ganglion cells leaving the right nasal retina cross at the chiasm to join the uncrossed, temporal retinal fibers coming from the left eye at the optic tract. From here, the afferent axons in the right and left optic tract mediating the pupillary light reflex enter the brachium of the superior colliculus on each side, before reaching the lateral geniculate body, and synapse with cells in the right and left pretectal olivary nuclei. The neurons in each pretectal nucleus send an uncrossed branch to the Edinger-Westphal nucleus on the same side and a crossed branch to the Edinger-Westphal nucleus on the opposite side. The exact site where the axons branch is still not well understood in primates, but the arrangement causes both pupils to contract regardless of which eye is stimulated. The small number of neurons in the Edinger-Westphal nucleus are preganglionic parasympathetic neurons that travel within the oculomotor nerve with axons that innervate the extraocular muscles.