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More than 2,500 barbiturates have been synthesized. At the height of their popularity, about 50 were marketed for human use.
Ether was used widely for surgeries but became less popular because of its flammability and its tendency to cause vomiting. In England, it was quickly replaced by chloroform, but this agent caused many deaths and lost popularity.
The familiar sounds of your heart are made by the heart's valves as they open and close.
The liver is the only organ that has the ability to regenerate itself after certain types of damage. As much as 25% of the liver can be removed, and it will still regenerate back to its original shape and size. However, the liver cannot regenerate after severe damage caused by alcohol.
Most childhood vaccines are 90–99% effective in preventing disease. Side effects are rarely serious.