Dear Ashlynn,
I was the cutest kitten. I bet you were an adorable baby, too. Like me, you probably had a big, round head with chubby cheeks and huge eyes.
The fact babies have big eyes made some people think babies are born with adult-sized eyeballs. I talked about this with my friend Edward Johnson. He teaches classes about the human body in the School of Biological Sciences at Washington State University.
“It’s a very good question because there’s a lot of misinformation about it,” Johnson said. “Eyeballs do grow—but not very much compared with other parts of the body.”
He told me to think about the thigh bone—also called a femur. That bone grows a lot. The average newborn baby has a 3-inch thigh bone. The average adult has an 18-inch thigh bone. That means the thigh bone gets about six times bigger. That’s a huge change.
Eyeballs don’t grow nearly that much. Johnson told me adult eyeballs are about one-and-a-half times as big as baby eyeballs. The average newborn baby’s eyeball is 0.6 inches in size. The average adult eyeball is 0.9 inches.
Some scientists think babies have big peepers because traits like chubby cheeks and big eyes make babies cute. Looking at that cuteness turns on the brain’s reward system. That makes it feel good to take care of a baby—or even just look at one.
So, does any part of the body stay the same size from birth?
“As far as I know, all organs and tissues grow from birth up until adulthood—with the possible exception of a couple middle ear ossicles,” Johnson said. “If any structure doesn’t grow, those would be candidates.”
The middle ear ossicles are three teeny bones that sit in the middle ear. Their names are the malleus, incus and stapes. As sound travels from outside the ear through the middle ear, the bones vibrate. That makes the sound louder as it moves into the inner ear. There, it changes into a signal that goes up a nerve to the brain.
It makes sense middle ear bones don’t grow—or only grow a little bit. They’re the tiniest bones in the human body. They’re also stiffer than other bones. That helps them vibrate better.
From itty-bitty ear bones to eyeballs, your body is amazing. If you think about it, asking big questions about our world is another way of growing. You’re doing that beautifully!
Sincerely,
Dr. Universe