Although the liver can detoxify some chemicals, it can turn some substances into mutation and cancer-causing chemicals. This is a test to see whether a certain chemical increases the mutation rate in bacteria. The chemical is mixed with ground rat liver to simulate the chemical processes that occur in the human liver, before being tested on the bacteria. The major portion of the dry weight of feces is actually dead bacteria, mostly of the type known as E. coli , which are much used in microbiology and genetics. Gases are produced by the bacteria in the large intestine, and may include nitrogen, methane, and hydrogen sulfide.
People normally produce several liters of gas each day.
The end of the large intestine is called the rectum, where feces are stored until expelled. Folds in the walls of the rectum help support the weight of the feces so that gravity doesn't produce unpleasant results. The anus is the sphincter at the end of the rectum. In older people, digestion tends to be more inefficient, so there is more material on which bacteria can work. This is one of the reasons older people tend to produce more gas.