How does the pupillary response prevent injury? what would happen without it?

Part I

In extension to measuring the amount of light that enters the eye, the pupillary light reflex presents a helpful diagnostic tool. It provides for examining the uprightness of the sensory and motor functions of the eye. if your pupils didn't shut suddenly when abruptly disclosed to light, you'll get a tremendous shot of light (simultaneously with UV rays and other possibly dangerous rays) to your retina, which can severely mess up your vision.

Part II

Without it, we would go blind. If there is not sufficient light and the pupils do not dilate, a small number of light will pass to the retina and the image will be damaged.

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