Fig. 3. Schematic representation of neural pathways that control pupil size in humans. The efferent sympathetic pathway begins in the thalamus and is shown by the solid line . General anesthesia is thought to produce blockade at the site between the thalamus and the hypothalamus (1). Note that the afferent sympathetic pathway bypasses the ciliospinal center (cs ) and continues cephalad to the thalamus. The efferent parasympathetic pathway begins in the nucleus of the third cranial nerve and is shown by the dotted–dashed line . Inhibitory pathway (dotted pathways ) into the parasympathetic nucleus arises within either the hypothalamus or via the spinotectal pathway. Local anesthetics block the afferent pathways from the spinal cord (2). as = aqueduct of Sylvius; cc = corpus callosum; cg = ciliary ganglion; cs = ciliospinal center of Budge, mostly T1 and T2 segments of the interomediolateral cell column; ht = hypothalamus; iml = interomediolateral cell column; lcn = long ciliary nerves; ns = neuromuscular synapse of postganglionic sympathetic nerves; n5 = ophthalmic division of the fifth nerve; p = pons; pc = posterior commissure; scg = superior cervical ganglion; t = thalamus; III = oculomotor nucleus. Adapted with permission from Loewenfeld. 2