3rd Cranial Nerve
The side of the brain (e.g. swelling, bleeding or tumour), the mass can expand until pressure is exerted on the 3rd cranial nerve.  A cerebral hemisphere lesion produces motor and/or sensory abnormalities on the opposite side of the body.  Because the 3rd cranial nerve on the same side as the expanding mass is generally compressed first, pupillary paralysis will generally begin on the same side as the brain lesion.

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