The pupil of the eye is the black circle in the center of the iris. This light is then focused on the retina, which is the layer of light sensitive cells at the back of the eye. When it is dark, our pupils dilate or open wider to allow more light in. Doing this maximizes our vision.

Muscles in the colored part of your eye, called the iris, control your pupil size. Your pupils get bigger or smaller, depending on the amount of light around you. In low light, your pupils open up, or dilate, to let in more light. When it’s bright, they get smaller, or constrict, to let in less light.

These muscles control the size of the pupil and thus determine how much light reaches the sensory tissue of the retina. The sphincter muscle of the iris is a circular muscle that constricts the pupil in bright light, whereas the dilator muscle of the iris expands the opening when it contracts.

For starters, oxytocin and dopamine — the “love hormones” — have an effect on pupil size. Your brain gets a boost of these chemicals when you’re sexually or romantically attracted to someone. This surge in hormones appears to make your pupils dilate. Dilation may also be related to the biological need to reproduce.

The iris
The iris controls the amount of light that enters the eye by opening and closing the pupil. The iris uses muscles to change the size of the pupil.

Pupil dilation is controlled by the iris dilator muscle. The dilator muscle consists of fibers that are oriented radially, and connect the exterior of the iris with the interior. When the dilator muscle contracts, it pulls the interior of the iris outward, thus increasing the size of the pupil (Figure ​

Pupil dilation occurs when the opening in the center of your iris grows bigger to let in more light. Under normal circumstances, pupils can dilate to let in more light or in response to a variety of stimuli. During an eye exam, a doctor will administer eye drops to increase the size of a patient’s pupils.

Fluid fills most of the inside of the eye. The chambers in front of the lens (both the anterior and posterior chambers) are filled with a clear, watery fluid called aqueous humor. The large space behind the lens (the vitreous chamber) contains a thick, gel-like fluid called vitreous humor or vitreous gel.

When your pupil dilates, the radial muscles constrict, and the sphincter muscles relax. This makes the iris pigmented surface that we see sort of fold like an accordion, the inner part of the iris moves outward towards the outer part, and the pupil gets larger as a result.

The pupil is under competing autonomic control in response to light levels hitting the retina. The sympathetic system will dilate the pupil when the retina is not receiving enough light, and the parasympathetic system will constrict the pupil when too much light hits the retina.

Pupil, in the anatomy of the eye, the opening within the iris through which light passes before reaching the lens and being focused onto the retina. The size of the opening is governed by the muscles of the iris, which rapidly constrict the pupil when exposed to bright light and expand (dilate) the pupil in dim light.

The pupil is an opening that lets light into your eye. Since most of the light entering your eye does not escape, your pupil appears black. In dim light, your pupil expands to allow more light to enter your eye. In bright light, it contracts.

In bright light, it contracts. Your pupil can range in diameter from 1/16 inch (1.5 mm) to more than 1/3 inch (8 mm). Light detected by the retina of your eye is converted to nerve impulses that travel down the optic nerve. Some of these nerve impulses go from the optic nerve to the muscles that control the size of the pupil.

Parasympathetic nerve fibres from the third (oculomotor) cranial nerve innervate the muscle that causes constriction of the pupil, whereas sympathetic nerve fibres control dilation. The pupillary aperture also narrows when focusing on close objects and dilates for more distant viewing. At its maximum contraction,…