Final answer:
The round opening in the eye whose size is regulated by the dilator and sphincter muscles is called the pupil, which is a component of the iris.
Step-by-step explanation:
The size of the round opening in the eye, which is controlled by the dilator and sphincter muscles, is the pupil. The pupil is part of the iris, which is the colored portion of the eye. The iris adjusts the amount of light entering the eye by constricting or dilating the pupil. In bright light, the pupil becomes smaller to reduce light entry; in dim light, the pupil enlarges to allow more light in.
This autonomic response is regulated by the sympathetic and parasympathetic nervous systems, with neurotransmitters such as norepinephrine causing dilation and acetylcholine (ACh) causing constriction.