What are the functions of the pupil and iris, and how do they change in bright and dim light?

The pupil is the central opening, and the iris is the colored part surrounding it, containing circular and radial muscles.

The pupil is the aperture through which light enters the eye. The iris controls the size of the pupil, thereby regulating the amount of light that reaches the retina.

The function of the pupil is to $\boxed{\text{allow light to enter the eye}}$. The iris $\boxed{\text{controls the size of the pupil}}$.

In bright light, the circular muscles of the iris contract.

Simultaneously, the radial muscles of the iris relax.

This combined action causes the pupil to constrict (become smaller), reducing the amount of light entering the eye.

In bright light, the $\boxed{\text{circular muscles contract and radial muscles relax, causing the pupil to constrict}}$.

In dim light, the radial muscles of the iris contract.

Simultaneously, the circular muscles of the iris relax.

This combined action causes the pupil to dilate (become larger), allowing more light to enter the eye.

In dim light, the $\boxed{\text{radial muscles contract and circular muscles relax, causing the pupil to dilate}}$.