Now we encounter the second part of the action which is just as interesting as the first part. The small intestine is a rather narrow tube; its cavity is not much more than three-quarters of an inch or so in diameter. If the outlet sphincter from the stomach were to stay relaxed after it once let go, the rapid pressing of the food from the stomach would fill the small intestine quickly to a distance of several feet. It happens, however, that the next of the digestive juices to act upon the food are the pancreatic juice and bile, which are secreted respectively by the pancreas and liver, and are poured into the small intestine by ducts from these organs just a few inches beyond the outlet sphincter from the stomach. In order for these juices to mix well with the food it is important that only a little food come into the small intestine at a time. Otherwise a large part of it would pass the opening of these ducts so quickly that there would be no chance for it to become mixed with the juice from them. What actually happens is that the sphincter relaxes under the stimulus of surplus hydrochloric acid and then as soon as a little food has passed through closes again. The closing as well as the opening is operated by hydrochloric acid, but there is this difference: the opening is the result of the stimulation of acid upon the stomach side of the sphincter; when the same acid comes in contact with the intestinal side of the sphincter, its presence there causes the sphincter to contract. Of course, this means that whenever any food passes out from the stomach into the small intestine the sphincter is stimulated by acid simultaneously from both sides, but under that condition the stimulus which causes the sphincter to contract is more potent than that which causes it to relax, so that whenever there is acid on both sides the sphincter will be shut. The only way in which it can be opened to allow more food to pass out is to get rid of the acid on the intestinal side. This is fairly quickly accomplished, because both the pancreatic juice and bile are strongly alkaline liquids, so that as fast as they mix with the food they tend to neutralize the acid; the result is, of course, that by the time these digestive juices have impregnated the food mass thoroughly and so have accomplished their purpose, the stimulus which keeps the sphincter closed has disappeared and there is left only the stimulus to open it, due to the presence of acid in the stomach. As soon as this condition is reached, the sphincter will open again, another mass of food will pass through bringing its acid with it, which, of course, closes the sphincter promptly, and so the whole story goes on again. This action has been described in detail because it is one of the very best examples we have of the remarkably ingenious arrangements by which the complex bodily functions are carried on automatically.