Which muscle is primarily responsible for elevating the upper eyelid?

The muscle primarily responsible for elevating the upper eyelid is the levator palpebrae superioris. This muscle has a direct role in lifting the upper eyelid, allowing for the opening of the eye. It originates in the lesser wing of the sphenoid bone and inserts into the skin of the upper eyelid, facilitating this action through muscle contraction.

While Muller's muscle, also known as the superior tarsal muscle, has a role in maintaining eyelid elevation and contributes to the fine-tuning of eyelid position, it is not the primary muscle responsible for the initial elevation of the eyelid. This muscle works in conjunction with the levator palpebrae superioris, particularly in situations like sympathetic stimulation.

The orbicularis oculi muscle encircles the eye and is primarily involved in closing the eyelid, hence its role is more related to eyelid movement in the opposite direction.

The rectus superioris muscle is primarily a muscle of the eye that is responsible for elevating the eyeball and does not play a direct role in lifting the eyelid itself.

Understanding the distinct functions of these muscles can give better insight into ocular anatomy and the mechanics of eyelid movement.