Pupillary fibers travel on the outside or inside of CN III?

• A. Outside
• B. Inside
• C. Both
• D. Neither

Answer: (A)

Pupillary fibers are the parasympathetic fibers that innervate the sphincter pupillae and ciliary muscle. In CN III, these parasympathetic fibers sit at the outside of the nerve—the peripheral layer—rather than in the central motor core. They travel along CN III briefly, then branch off to reach the ciliary ganglion, and from there the postganglionic fibers ride with short ciliary nerves to the pupil and ciliary body. This exterior position explains why compressive lesions (like a berry aneurysm near the posterior communicating artery) tend to affect the pupil early, producing dilation, while central ischemic injury to CN III often spares the pupil at first because the inner motor fibers are involved more than these peripheral parasympathetic fibers.