Along with the different sweat glands and oil glands, there’s one more type of body equipment rooted in the dermis—your hair.
Hair consists of long, flexible strands of dead cells. These cells are filled with keratin, the same wonder substance that strengthens the outer layers of your skin. As you can see, the surface of the hair shaft consists of overlapping scales, like shingles on a roof.
This close-up holds the secret to hair frizz. On a humid day, tiny water droplets work their way in between the scales of the hair shaft, making the hair thicker and rougher. Conditioners try to prevent the problem by leaving an oily, water-repelling coating on the hair. Some anti-frizz products accomplish the same thing using silicone, which simultaneously seals the hair and weighs it down, straightening it.
Your body creates each hair in a hair follicle— a tiny pouch deep in your dermis, where your hair is woven together out of living cells. As each new layer of cells is tacked onto the bottom of the hair, the cells die, and the hair becomes just a little bit longer. In other words, your body treats your hair the same way it treats your skin—it keeps the living cells on the inside and puts the dead stuff on the outside. Which is good in a way, because a head full of living hair would make for an agonizing day at the barbershop.
Your hair stores a permanent record of the toxins you ingest, including illegal drugs like cocaine, amphetamines, heroin, and marijuana. A standard hair drug test searches for traces of drugs consumed over the last 90 days. But take longer strands of hair or some slow-growing body hair and you can easily put the last year of your life under the microscope.
Compared to other animals, humans appear relatively hairless. But the surprising truth is that you have more hair follicles crammed onto each square inch of your skin than the hairiest chimpanzee, monkey, or gorilla. The difference is that most of your hair (whether you’re a man or a woman) is nearly invisible. It consists of a fine, slow-growing, almost colorless covering of downy hair called vellus hair. Vellus hair blankets your body, insulating your skin and heightening your sensitivity to touch. It’s the reason you can sometimes “feel” a person moving past you in a darkened room—the passing air currents disturb your fine hairs and trigger the sensitive nerves attached to them. However, vellus hair is easily overlooked and nearly invisible without a magnifying glass. It’s sometimes known as “peach fuzz.”
Terminal hair is the more obvious hair found on your body, including the hair on your head, your eyebrows, and your eyelashes. After puberty, terminal hair appears in many more places on your body—some where it’s wanted, and some where it’s decidedly inconvenient.
Terminal hair is thicker, longer, and darker than vellus hair, although some individuals can have light-colored and fine terminal hair. Terminal hair also boasts a range of textures and colors. The difference between wavy, curly, and straight hair is all in the shape of the follicle that produces it. For example, a perfectly round follicle constructs straight hair, while an oval follicle produces wavy hair. All the hair-care products in the world can’t alter the shape of your follicles.
Like most of your body’s equipment, terminal hair has good reasons for existing:
• The terminal hair on your eyelashes keeps dirt and insects out of your eyes. Your ear hairs and nose hairs play a similar role. The terminal hair on your eyebrows • prevents sweat and rain from dripping onto your face.
• On your head, terminal hair helps prevent sunburns on sunny days and heat loss on cold ones.
• Pubic hair is a type of terminal hair that serves as a secondary sexual characteristic. That means it’s there to advertise that you’re a fully functioning adult with the appropriate baby-making abilities.
And, of course, humans have picked up the habit of using hair for something entirely different—as a powerful expression of self-identity that can announce everything from your gender to your political affiliation.
Source of Information : Oreilly - Your Body Missing Manual (08-2009)