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The Science of Eyebrows


It’s easy to take the eyebrows for granted. They sit over our eyes like a hairy little roof and typically don’t require much attention.

But there’s a lot going on behind the scenes. Here we take a look at eyebrows’ anatomy, purpose, evolution, and modern medical interventions.

Anatomy of Eyebrows

Terminology

The anatomical term for eyebrows is supercilia (singular: supercilium) from Latin. Supercilia also refer to plumage around the eyes of certain birds!

Muscles

A few different muscles are responsible for the many expressions human beings make with our eyebrows.

The corrugator supercilii muscle is located on the side of the eyebrow closest to the nose. There is one over each eye. They pull the eyebrows downward and towards the middle of the face. This movement is part of frowning and is also a response to intense light (squinting). Repeated use of this muscle over time can result in vertical wrinkles on the forehead above the nose.

Next to the corrugator supercilii muscles and a little farther away from the nose are the depressor supercilii muscles. The depressor supercilii muscles also contribute to the downward turn of the brows near the nose, causing frowning and squinting.
The procerus is one muscle positioned between the eyes and along the upper bridge of the nose. When contracted, the procerus pulls down the inner corners of the eyebrows (closest to the nose), resulting in a frown.

The orbicularis oculi is a larger muscle that surrounds the eye and has a few purposes. The part of the orbicularis oculi muscle towards the nose assists with frowning. The part of the orbicularis oculi muscle near the outer (“tail”) end of the brow is a lateral brow depressor. This means that this section of the muscle pulls the brows down a bit and counterbalances the action of the frontalis muscles, which pull the eyebrows up.
The occipitofrontalis (“frontalis”) muscles are arranged vertically under our forehead, one above each eyebrow. These muscles allow us to raise our brows by drawing back our forehead. In humans, the only function of these muscles is to show facial expressions, such as raising the eyebrows to indicate surprise. Over the years, using this pair of muscles can create horizontal expression lines (wrinkles) in the mid-forehead.

Hair

Eyebrow hair growth happens in a cycle of stages. There’s the anagen stage, when hair is actively growing; the catagen stage, when hair is transitioning from growth to rest, and the telogen stage, when hair is resting or shedding. Some experts include a fourth stage, the exogen stage, which is characterized specifically by shedding. After an eyebrow hair goes through all stages in the cycle, the process begins again.

For eyebrows the anagen (growth) stage lasts between 4 to 7 months. (Compare this to the 2 to 7+ years for scalp hair!)

For eyebrows, the catagen (transitional) stage usually lasts for 3 to 4 weeks.

The telogen (rest/shedding) stage lasts for about 2 to 4 months for scalp hair, but for eyebrows, the telogen phase is a lot longer: 9 months.

For more in-depth information, read my post about eyebrow growth.

Purpose of Eyebrows

Physical Purpose

The physical reason we have eyebrows is to keep sweat, water, and debris out of eyes.

Eyebrows form a web of hair that can catch dirt, dust, and dandruff before it falls into the eyes. Brows are also slanted on the side, causing liquid to flow away from the eyes. In many people eyebrows, along with the slight brow ridge in humans, help shade the eyes from the bright light. Another possible purpose for brows is to trigger physical sensations that help us detect small foreign objects, such as insects or plant particles, that threaten to get into our eyes.

Social and Psychological Purpose

Expressing emotions through eyebrow movements is part of being human. In fact, it’s even more basic than that. It’s part of being a primate, as all primates send social messages through eyebrow activity.

But unlike many of our primate cousins, humans have a high level of contrast between our brows and the hairless skin that encircles them, thus emphasizing the brow region during communication.

Eyebrows’ importance is also passed on through culture. For centuries, in cultures worldwide, abundant hair has been linked to fertility, strength, and power. This association has often extended to eyebrows. Ancient people beautified their brows with makeup and shaping, and people do so even now.

Eyebrows can display facial symmetry. And they can be altered cosmetically to disguise asymmetry. Facial symmetry is often mentioned as a key factor in human attractiveness. People may be programmed to associate symmetrical features with health of potential mates.

For more details, take a look at my post, Social and Psychological Effects of Eyebrow Loss.

Evolution of Eyebrows

Ancestors of modern humans had a distinct bony brow ridge, a feature which is thought to have restricted their eyebrow movements. The purpose of the brow ridge isn’t entirely clear, but modern humans no longer have it. Did the loss of the brow ridge enable homo sapiens to better communicate and collaborate nonverbally? Collaboration is key for the survival and advancement of a highly social species. Right now, this idea is still a theory.

Another interesting theory about how we got our eyebrows is suggested by Joseph Jordania, who believes that sleeping early humans’ upward arching eyebrows, along with our downward arching eyelashes, created oval eyespots on our face. These eyespots made it look as if our eyes were open when we were sleeping, thus fending off stealth predators, such as large cats, that don’t like to be noticed as they’re sneaking up on their prey.

Modern Medical Interventions

Although people in prehistoric times are known to have resorted to surgery at times, modern science has made surgery a fairly safe, routine procedure. Sterilization, antibiotics, anesthesia, and other more recent developments make it possible for people nowadays to have elective procedures like eyebrow lifts and transplants. These surgeries can improve patients’ quality of life or be life altering in some cases.

Eyebrow Lifts

Eyebrow lifts create more space between the eyes and the eyebrows. By opening up this area, the procedure can create a more harmonious or youthful appearance. Variations on the eyebrow lift can also reverse birth defects and help reconstruct the faces of accident victims. A surgeon makes incisions at or just behind the natural hairline, lifts the tissue beneath the skin, and closes the incision. The result can last many years if not indefinitely.

Eyebrow Transplants

Another surgical innovation is eyebrow transplants, which work much like scalp hair transplants. Individual hairs or groups of hairs growing from the same follicle are harvested from the patient, usually from the back of the head where hair is more plentiful and resistant to balding. These harvested hairs are then implanted into the eyebrow area.

Hair may shed at first due to shock, but the odds are very good that the living follicle under the skin will survive, and that hair will grow back. Once transplanted, follicles will continue living and producing hair for as long as they would have done so in their original location.

Eyebrow transplants are becoming more common. They can be helpful in restoring partial hair loss due to aging or thyroid disease as well as complete eyebrow loss due to alopecia or trichotillomania.

Injectables

Quite a few injectable substances now exist to make the area around the eyes and eyebrows appear more beautiful, symmetrical, or youthful.

There are dermal fillers that can be injected to plump up areas that have lost volume due to aging, accidents, or congenital causes. Some brand names of dermal fillers are Radiesse, Juvederm, Restylane, Sculptra, and Bellafill. These last from a few months to a few years.

Even more useful for the eyebrow area are botulinum toxin injections. These use small, weak, localized doses of the bacterium Clostridium botulinum to block signals from the nerves to the muscles. The injection can immobilize an area and thus relax the creases that develop from repeatedly making facial expressions.

The four types of botulinum toxin injections include Botox, Dysport, Xeomin, and Myobloc. These are often used to soften the vertical and horizontal lines on the forehead that come from use of the eyebrow muscles.

Any Ideas or Questions?

Eyebrows might be an afterthought for a lot of people, but in fact, there’s a lot going on in those expressive little patches of hair on our forehead! Have I left anything out? Is there something you’ve been wondering about?

Here’s something I’ve been mulling over:

Eyebrows assist us in communicating, collaborating, and expressing empathy with others, but quite a few people use Botox to diminish their facial expressions. This is a trade-off some folks are happy to make. Do you think the trade-off is worth it?

Share your thoughts in the Comments section!

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Wendy

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