Final answer:
The levator palpebrae superioris muscle is responsible for lifting the eyelid and is innervated by the oculomotor nerve. Ptosis occurs when this muscle fails to function properly.
Step-by-step explanation:
The muscle that lifts the eyelid when you look up is called the levator palpebrae superioris. This muscle is innervated by the oculomotor nerve, which is the third cranial nerve. When the levator palpebrae superioris does not work correctly, it can lead to a condition known as ptosis, which is characterized by drooping of the upper eyelid.