the structure that prevents perspiration from running down the forehead into the eyes and shades the eyes from sunlight is the

The Eyebrows

Explanation:

The eyebrows are not just those short hairs that humans can use to make signals for non-verbal communication with other humans, they, as one of the accessory structures of the eye, protect the eyes from perspiration (sweats) dripping down the forehead, or water running down from the head from entering the eyes, causing irritation. They also help shade the eyes from the high impact of the sunlight and help keep dirt away too.

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