The dermis is the thick layer of living tissue beneath your skin's surface. While the outermost layer you can see and touch (called the epidermis) is mostly dead cells, the dermis underneath is alive and busy. It contains blood vessels that feed your skin, nerve endings that let you feel sensations, hair roots, sweat glands, and tough fibers that make your skin strong and stretchy.

Think of the dermis as your skin's foundation. When you touch something hot and pull your hand away, nerve endings in the dermis send that warning signal to your brain. When you get a scrape that bleeds, you've cut through the epidermis into the dermis, where the blood vessels are. The dermis also produces sweat to cool you down and contains collagen, a protein that keeps skin firm and elastic.

Doctors who specialize in skin are called dermatologists, and they study both the epidermis and dermis to understand conditions like rashes, acne, or skin diseases. The dermis is typically about two millimeters thick, though it's thicker on your back and thinner on your eyelids.