To prevent changes in an animal's pupil size, which part must be paralyzed?

The iris is the part of the eye that controls pupil size by adjusting how much light enters the eye. It accomplishes this through the contraction and relaxation of its muscle fibers. When an animal's iris is functioning normally, it responds to varying levels of light and focuses on different objects by changing the size of the pupil. If the iris is paralyzed, it loses its ability to control the pupil size, leading to a fixed pupil that does not respond to changes in light conditions. This paralysis prevents any changes in pupil size, making the iris the essential part for this function.

In contrast, the cornea is primarily responsible for focusing light but does not adjust pupil size. The lens works with the cornea to focus light onto the retina, but it too does not influence pupil diameter. The retina is involved in processing visual information and sending it to the brain, rather than controlling pupil size directly. Thus, paralysis of the iris is necessary to prevent changes in the size of the pupil.