During the 19th century, scientists found that when certain parts of the brain of a person were hurt, he would lose the power of doing certain things. And so people thought that each part of the brain does a different job. But modern research has discovered that this is not so, for it is not easy to show exactly what each part of the brain does. In the past fifty years there has been a great increase in the amount or research done on the brain. Scientists have found out that the way the brain works is not so simple as people in general may think. Chemists tell us that about 100,000 chemical changes take place in the brain every second. Some recent researches also suggest that we can remember everything that happened to us. We may not be able to think of the things we have heard and seen but it is kept there in the store-house of the human mind. Earlier scientists thought that the power of one's brain got weaker and weaker as one grows older. But it is plenty of exercise, it keeps its power. It is now thought that is not true. As long as the brain is given plenty of exercise, it keeps its power. It has proved that an old person who has always been active in the mind has a quicker mind than a young person who has done physical work without using much of his brain. In the last century, scientists found that a person would lose the power to do certain things   _  .
when he got weaker in health if certain parts of his brain were hurt after he did a quite different job when he grew older