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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.
The immune system needs 9.5 hours of sleep in total darkness to recharge completely.
Patients who have been on total parenteral nutrition for more than a few days may need to have foods gradually reintroduced to give the digestive tract time to start working again.
The first successful kidney transplant was performed in 1954 and occurred in Boston. A kidney from an identical twin was transplanted into his dying brother's body and was not rejected because it did not appear foreign to his body.
Cancer has been around as long as humankind, but only in the second half of the twentieth century did the number of cancer cases explode.