Occulomotor nerve is the third nerve which controls the eye movements and pupillary direct and indirect reflexes. Injury to the third nerve can occur due to a surgical reason or due to a medical reason like an inflammation. They can be distinguished by pupillary response. If the pupil is spared the cause is medical and if the pupil is fixed and dilated the cause of trauma is surgical. The reason being that the parasympathetic fibers of the nerve are on the outside and during surgery they are prone to get damaged leading to no pupillary reflexes while in medical causes center of the nerve is involved leaving parasympathetic uninterrupted. Hence the pupil is unaffected.