The digestive system may be conceived in its simplest form as a muscular tube into which glands all along its course pour secretions. Some of these glands are embedded in the wall of the tube ; some, such as the pancreas and the liver, are so large that they lie outside and discharge their secretion through a duct which empties into the digestive canal. The muscular action of the walls of the canal pushes the food ever onward.

After the revival of learning men began to become interested in certain apparently spontaneous changes which occurred in substances left alone in nature. For instance, why does meat get corrupted? Why do maggots swarm in it? Why does milk sour? Why does grape-juice turn into wine? Why does apple-juice turn into vinegar? Finally, why does food undergo a change in the stomach? All of these changes seemed to them to be of the same nature. They were all thought to be due to “spontaneous generation.” Meat changed, maggots swarmed, milk soured, food digested - by the act of spontaneous generation or spontaneous metamorphosis.