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The 2 - 3rd cranial nerves are located on either side of the top of the brainstem (highlighted by red arrow). Each 3rd cranial nerve controls eye muscle movement on the same side (ipsilateral). When there has been an insult to one 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, eye muscle and pupillary paralysis will generally begin on the same side as the brain lesion.