In a patient with ipsilateral expanding intracranial supratentorial mass, there is displacement of the hippocampal gyrus and uncal herniation across the tentorial edge, resulting in entrapment of the third nerve. This can result in hutchinson's pupil. Hutchinson's pupil occur in comatose patients with unilaterally dilated poorly reactive pupils. In the third nerve the pupillomotor fibers run peripherally and are subjected to early damage from compression. Ref: Textbook of Ophthalmology edited by Sunita Agarwal, page 305.