After the food leaves the stomach, it is acted on by several digestive enzymes. Some of these are secreted by glands located, like the stomach glands, in the wall of the intestine. But the most powerful digestive juices of all are those secreted by the pancreas and poured into the intestine in its upper part. One of the pancreatic enzymes, trypsin, is a rapid and powerful split-ter of the protein foods.
Another, lipase, splits fats into simpler absorbable compounds. The third, amylase, resembles the salivary secretion, ptyalin, in that it breaks down complex starches and sugars into simpler chemical forms. The pancreatic secretion mixes with the food, as has been said, at the upper part of the small intestine.
There is a long stretch of the small intestine, over twenty feet, through which the food passes after this admixture, becoming more fully digested.
What causes the digestive juices to be poured out? It is a very interesting and pertinent question. We know that they are not flowing all the time. We know this by observation about the saliva for instance. At times the mouth is dry – when food is taken into the mouth, the saliva begins to flow. This is also true of the stomach and intestinal juices. They appear in the presence of food.
Why? Pawlow, a Russian physiologist, supplied a very interesting part of the answer. He showed that the digestive juices flow at the sight and especially at the smell of food. The more appetizing the food, the larger the amount of secretion. We say the mouth ” waters ” at the sight or smell of something we like to eat; this is literally true: the mouth does water, and so does the stomach. Carlson confirmed Pawlow’s experiments, which were done on animals, in man.
He obtained a man whose cesophagus had been burned and who had to have an opening made in his stomach by a surgeon. Carlson could thus measure the amount of gastric juice secreted under different circumstances. When the man smelled something he liked to eat, the gastric juice was poured out in large quantities; when something disgusting or unappetizing was seen or smelled, the stomach wall became dry. Chewing food of pleasant taste also stimulated the flow of stomach juices.