6. Which part of the eye uses muscle tissue to open and close? How does this work?
The part of the eye that uses muscle tissue to open and close is the iris. The iris is a thin circular structure in the eye that can expand or contract to control the size of the pupil, the opening in the center of the iris.
The iris contains two sets of muscles, known as the dilator and sphincter muscles. The dilator muscles, when stimulated, cause the iris to expand and the pupil to dilate, allowing more light to enter the eye. The sphincter muscles, when stimulated, cause the iris to contract and the pupil to constrict, reducing the amount of light entering the eye.
This process is controlled by the autonomic nervous system, which responds to changes in light levels in the environment. In bright light, the sphincter muscles contract and the pupil constricts to reduce the amount of light entering the eye. In dim light, the dilator muscles expand and the pupil dilates to allow more light to enter the eye.