Whether on your face, your head or your body, a single strand of human hair consists of three layers—the medulla, the cortex and the cuticle. The cortex is the middle layer, and it's responsible for determining the hair's pigment, texture and elasticity. It's a spiral shape, like a telephone cord, which is why hair stretches before it breaks if pulled. When people have split ends, you can see the cortex frayed on the ends, due to damage of the protective outside layer of the hair shaft (known as a cuticle). The most common cause of split ends is not trimming your hair regularly, and because of this split ends are typically associated with hair on top of your head.