Can you please help me understand the part in bold?

The fragment below is taken from the book "The Fire Witness" by Lars Kepler.

The detective is at a crime scene - a girl has been found dead and placed in bed in a strange position: with her ankles crossed and hands covering her face.

"He leans cautiously over her. Her bare skin is emitting very faint heat now. He looks at the hands covering her face and sees that she didn't manage to scratch the perpetrator: there's no skin under her fingernails."

I am struggling to picture fingernails with no skin underneath and how they are related to scratching.

Can someone help me? That would be greatly appreciated.