Raw vegetables and fruits remain about the same length of time as potatoes.
Sugar is usually absorbed within an hour.
The cereals, if well cooked, take but two hours.
Coarse or badly masticated food, tough meats, unripe fruits, and much fat hinder digestion.
Undigested food passing into the intestine may fail to be acted on there and will sometimes produce diarrhea.
Fluids leave the stomach more rapidly than solids. Seven ounces of water entirely leaves the stomach in one and one-half hours, seven ounces of boiled milk in about two hours. Water and buttermilk almost immediately begin to pass out of the stomach; milk begins to pass out in about fifteen minutes.
The flow of gastric juice, as the flow of saliva, is governed by the nerves; the sight, taste, and smell of food, and the attitude of mind toward it, to a certain extent, regulate its flow.
After the food has accumulated, during the progress of a meal, the stomach begins a series of wave-like movements called peristaltic waves.[7] These waves propel the food through the length of the stomach towards its lower opening, known as the pyloric orifice. During this process the food is thoroughly mixed with the gastric juice.
During the early stages of digestion of solids, the sphincter muscles of the pylorus keep the lower opening of the stomach closed, but, as digestion progresses, the pylorus gradually relaxes to let the digested, soluble portion of the food pass into the intestine. If the food still remains in a solid form, by reason of being improperly cooked or poorly masticated, as it touches the pylorus, these sphincter muscles, almost as if they were endowed with reasoning faculties, close, forcing the undigested mass back to be further acted on by the gastric juice—the solid mass is not allowed to pass until dissolved.
If the individual abuses the stomach and causes it to work overtime, it becomes exhausted and demands rest; it refuses to discharge the gastric juice in proper proportion; the peristaltic movements are weak; and food is not promptly or forcefully moved along the stomach and mixed with the gastric juice. This condition is termed indigestion.