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Noise of knuckle crack is created by gas cavity; study of marine animals indicates bigger is better

Noise of knuckle crack is created by gas cavity

The unmistakable sound of a knuckle cracking is caused by a gas cavity forming between finger joints, according to a new study. The cavity, or bubble, forms in the synovial fluid, the lubricant between joints. The discovery was reported in the current issue of the journal PLOS One.

One of the paper’s authors, Jerome Fryer, a chiropractor in British Columbia, lay inside a cine MRI machine, which stitches together video from a series of rapid scans. Researchers at the University of Alberta cracked each of his knuckles by pulling on a cable attached to his fingers.

“You’ll see the black cavity that occurs just as the cracking occurs” on the video, said Dr. Greg Kawchuk, a professor of rehabilitation medicine at the university and the lead author.

In 1947, researchers in England first theorized that the characteristic popping sound occurs when a gas cavity forms. Then, a group of researchers in the 1970s suggested it was the collapse of the cavity, not the formation, that caused the cracking.

Kawchuk added that knuckle cracking did not cause arthritis. “Those are just tales,” he said.

On the contrary, he said, understanding exactly why knuckle-cracking doesn’t seem to harm the joints could help researchers develop better therapeutic materials for patients with osteoarthritis and other degenerative joint diseases.

Study of marine animals indicates bigger is better

Cope’s rule states that evolution favors larger bodies, suggesting that the average size of animals on the planet should always trend upward. The hypothesis has its proponents, but there has been scant evidence to support it.

Now, an expansive study of marine animals suggests that Cope’s rule may hold true, at least for life underwater.

Researchers at Stanford University spent four years combing through fossil records dating to the Cambrian Period 542 million years ago, ultimately compiling data on body sizes for more than 17,000 genera of marine animals.

The scientists found that the minimum body size for marine animals has decreased by only 10 percent over that time, while maximum body size has increased by a factor of 100,000. Overall, mean body size has increased 150 percent, the researchers reported.

“For a long time, people have had this hypothesis that there’s not much directional trend in evolution, that random events and random drift can produce increasing size or complexity,” said Noel Heim, a paleontologist at Stanford and lead author of the study. “That model just isn’t compatible with the trends we see.”

– New York Times