How Many Human Body Parts Remain Undiscovered?

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In recent months, scientists have described a new layer in the cornea of the human eye and a long-overlooked ligament in the knee.

In our modern age of imaging and other advanced medical technologies, how is it possible that we still don’t know everything there is to know about our anatomy?

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Despite a long history of fascination with the human body, experts said, holes continue to exist in our knowledge because we are enormously complex creatures. What’s more, there's a lot of variation from one person to the next. Reality is a far cry from the clear and colorful pictures in anatomy textbooks.

As medical students begin the reverent work of cadaver dissection, the complexities can be overwhelming.

“When students open up the body for the first time, it’s really hard. It’s really confusing,” said Daniel Schmitt, an evolutionary anthropologist and course director for human gross anatomy at Duke University in Durham, N.C.

“It’s like getting up close to a pointillist painting where you just see dots,” he said. “It’s like going to a new city, a new country, a new world. They just haven’t anticipated what it’s going to be like. It’s beautiful, but the first reaction of many students is: ‘It’s too hard. I can’t do this.’”

Gradually, Schmitt said, rules and patterns emerge. As doctors become experts, they learn where to look for structures that tend to appear in the same general areas from one person to the next.

But even when surgeons develop intricate knowledge about specific parts of the body, they come to expect the unexpected. There is a muscle in the arm called the palmaris longus, for example, that is absent in about 15 percent of people. Some people have the muscle in one arm but not the other.

Surgeons routinely have to make decisions on the fly when they find that someone’s veins or nerves take unexpected paths.

“Human variation is remarkable,” Schmitt said. “Everyone is different.”