Final answer:
The iris is the muscle and tissue behind the cornea that controls the pupil dilation and constriction in response to light intensity, regulated by the sympathetic and parasympathetic nervous systems.
Step-by-step explanation:
The muscle and surrounding tissue that dilate and constrict the pupil behind the cornea are part of the eye's vascular tunic. Specifically, the iris, a smooth muscle, controls the size of the pupil, constituting the colored part of the eye. Light enters through the pupil, and the iris adjusts its size in response to light intensity: it constricts in bright light and dilates in dim light.
This mechanism is supported by the sympathetic and parasympathetic nervous systems. Sympathetic stimulation causes pupillary dilation (mydriasis) through the release of norepinephrine on adrenergic receptors of the radial fibers of the iris. Conversely, the parasympathetic system induces pupillary constriction through acetylcholine release stimulating the circular iris muscles.