Hair is a tough, fibrous protein called keratin. It protects us from the elements, gives our skin a natural glow, and helps make our eyebrows and eyelashes dense and full. It also plays a role in our sense of style; if you pinch a long hair between your fingers, it might feel slick and smooth. This smoothness is due to the fact that the overlapping scale-like cells of the cuticle are oriented in the same direction, and it’s this property that contributes to the sheen that makes our hair shiny.
Hair grows from a hole in your epidermis called a hair follicle, and it’s mostly made of dead keratinized cells. As the strand of hair grows, it pushes these old cells upward and out of the follicle through your skin. As they move towards the surface of your skin, these cells become keratinized and fill with pigment, and this is what gives hair its color.
As hairs grow, they become elongated and more and more tightly packed together until they are a bundle of fibers that is a bit thicker than a human finger. This bundle of fibers is the hair shaft, and it’s made up of three distinct layers. The cuticle is the outer layer, and it’s a slick, glossy surface that contributes to hair’s shine and strength. The cortex is the middle layer, and it’s densely packed with keratin. This is where most of your hair’s color and texture come from, and it’s what makes your hair feel so durable.
The medulla, the innermost layer of the hair shaft, is responsible for giving your hair its structure. This layer is filled with tightly coiled fibers that give it its density and elasticity.
Each individual strand of hair is connected to a smooth muscle known as the arrector pili, which when it contracts receives nerve signals from the autonomic nervous system. This innervation causes the external hair shaft to “stand on end.” While this function may seem vestigial, it’s likely that erecting hair served to trap a layer of air to add insulation and to cut a larger silhouette to help intimidate rivals or would-be predators.
The follicles that produce the hairs on our head are roughly oval in shape, but those that give rise to other hair types, such as eyebrows and eyelashes, are curved in cross-section. Curved hair is the result of a combination of the shape of the follicle and genetic factors that influence its growth rate. Regardless of its shape, hair can be quite beautiful. Whether you’re going for a blunt bang or chic, face-framing layers, you can find a style that suits your personality and frame.