Cranial nerve III, also known as the oculomotor nerve, is responsible for controlling most of the eye's movements. It innervates several muscles, including the superior rectus, medial rectus, inferior rectus, and inferior oblique, which help in moving the eye up, down, and inward. This nerve also plays a crucial role in controlling the size of the pupil and the shape of the lens for focusing. Damage to cranial nerve III can lead to symptoms such as ptosis (drooping of the eyelid), diplopia (double vision), and difficulty in moving the eye. It is essential for proper eye function and coordination, making it vital for activities like reading and driving.