Most head injuries only damage the scalp, such as cuts, scrapes, bruises or swellings. Even with a small injury, you can get a big lump or bruise, because there are a lot of blood-vessels beneath your scalp. For the same reason, small cuts on the head may bleed a lot. Bruises on the forehead sometimes cause black eyes 1 to 3 days later. This is caused by blood spreading downward by gravity.