Definition of Cranial nerve III

Cranial nerve III: The third cranial nerve is the oculomotor nerve. The cranial nerves emerge from or enter the skull (the cranium), as opposed to the spinal nerves which emerge from the vertebral column. There are twelve cranial nerves.

The oculomotor nerve is responsible for the nerve supply to muscles about the eye:

• The upper eyelid muscle which raises the eyelid;
• The extraocular muscle which moves the eye inward; and
• The pupillary muscle which constricts the pupil.

Paralysis of the oculomotor nerve results in drooping eyelid (ptosis), deviation of the eyeball outward (and therefore double vision) and a dilated (wide-open) pupil.