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The immune system needs 9.5 hours of sleep in total darkness to recharge completely.
Although the Roman numeral for the number 4 has always been taught to have been "IV," according to historians, the ancient Romans probably used "IIII" most of the time. This is partially backed up by the fact that early grandfather clocks displayed IIII for the number 4 instead of IV. Early clockmakers apparently thought that the IIII balanced out the VIII (used for the number 8) on the clock face and that it just looked better.
Human neurons are so small that they require a microscope in order to be seen. However, some neurons can be up to 3 feet long, such as those that extend from the spinal cord to the toes.
There are 20 feet of blood vessels in each square inch of human skin.
More than 34,000 trademarked medication names and more than 10,000 generic medication names are in use in the United States.