Long eyelashes in many cultures are a sign of beauty. Long eyelashes enhance the batting of an eye for a flirtatious woman. All mammals, which are covered with hair, have eyelashes, but none of the species (other than humans) bat an eye to attract attention. Therefore, eyelashes must have an important evolutionary function. People born without eyelashes experience a high rate of eye infections, so eyelashes must afford some protection to the eye. Filtering dust and bugs before landing in the eye is one apparent function. Also, when pebbles, chaff or clothing touch eyelashes, a blinking reflex is triggered, closing the eye automatically and preventing foreign bodies from entering the eye.

A research team headed by David Hu at the Georgia Institute of Technology, as reported in the Feb. 28, “Economist Magazine,” thinks they have discovered the mechanism by which eyelashes protect the eye. Of 22 mammals they studied, eyelash length was predominantly one-third the width of the eye. This meant, if the width of the eye from corner to corner was 30 millimeters, the eyelashes were close to 10 mm long. So, what was special about this eyelash-length-to-eye-width ratio? In their experiment, artificial eyes with eyelids were placed in a small wind tunnel. Eyelashes of varying length were attached to the eyelids for each experiment. Dust-sized, colored water droplets were sprayed into the wind tunnel. The experimental eye was bathed with artificial tears at the same time. Results revealed that the fewest dust particles landed on the eye and the least evaporation of moisture from the eye occurred when the eyelash length was one-third the width of the eye. The unique one-third eyelash length deflected or directed airflow away from the eye. Longer eyelashes funneled air into the eye, shorter lashes allowed more air to enter the eye. Hence, both lengths allowed more dust to settle on and moisture to evaporate from the eye surface. Normal tears contain enzymes that counteract microorganisms. Therefore, with less evaporation, the film of tears on the eye is thicker so more enzymes are present to eradicate potential infections.

On close inspection of my dog, I unexpectedly noticed short eyelashes 4 millimeters long with an eye width of 20 millimeters, a one-fifth eyelash length compared to eye length. I measured a similar eyelash ratio in a neighbor’s dog, so perhaps this short eyelash is the result of domestication. Since my dog’s eyelashes are short, he could expect more dust to collect in his eyes and more fluid evaporation than another mammal with longer eyelashes. I speculate this is actually the case, because each morning I have to wipe dry, brown, friable dirt and a drop of cloudy gelatinous fluid from the corner of his eyes which I tell him was left by the sandman.

Other animals such as birds, amphibians and reptiles lacking eyelashes have a semi-transparent third eyelid which can be closed to prevent foreign material from entering the eye.

The eye is a masterpiece of the human body. While eyelashes protect the eye by reducing dust, blinking forces tears from tear glands in the upper eyelid. Tears flow across the eye to the inner corner, pick up dust, drain into the nose via tear ducts and are then swallowed. Blinking occurs about once every five seconds. Also, did you know it is impossible to sneeze with your eyes open?

With aging, eyelash growth slows, creating eyelids depleted of lashes similar to a balding scalp. Prescription medications already used for glaucoma have been approved for eyelash growth, but side effects like abnormal eyelash growth occur in some people.

Hair transplants to the eyelid have been successful, but the hair continues to grow, necessitating trimming frequently. Diseases of the eyelids are uncommon, but a sty, which I personally experienced repeatedly over several years as a teenager, are a painful boil infection of the eyelash follicle and sebaceous glands.

When looking in the mirror, keep an eye on the eyelashes and be grateful they are silently protecting our precious vision.