The food that is swallowed passes down the esophagus and enters the stomach. Before speaking of this it will be necessary to recall what was said in Chapter VII about the behavior of the stomach walls. Between meals these are in a relaxed and flabby state, with the opposite walls lying more or less in contact. There is usually a little liquid and some swallowed air in the stomach, so that it is not actually empty, even when we speak of it as being so. Just before mealtime the walls of the stomach draw up so that instead of a flabby bag we have a fairly tense tubular organ. It is at this time that the contractions of the stomach wall, which we recognize as hunger, begin to come on. There is a sphincter muscle between the esophagus and stomach which closes the opening tightly and so prevents the pressure within the stomach from forcing gas or liquid back up into the esophagus. This sphincter opens automatically only in connection with the act of swallowing. Every time we make a swallowing movement a sort of wave passes down the esophagus, and when this wave arrives at the stomach the sphincter relaxes, allowing whatever was moving down the esophagus to enter. If one watches a horse or cow in the act of swallowing, the rather deliberate progress of this wave down the neck can be followed. What happens as we eat a meal is that every mouthful, as it is swallowed, is passed through the sphincter into the stomach and room is made for it by a gradual relaxation of the stomach wall, so that, as we saw in Chapter VII, the pressure of the stomach against its contents stays fairly steady, in spite of the fact that more and more material is being taken into the stomach from the esophagus. One result of this behavior of the stomach is that the first food that is swallowed is nearest to the stomach walls, and that which is swallowed afterward is nearer the middle, being inclosed on all sides by the previously swallowed food. This is fairly important because the gastric juice is secreted by glands in the wall of the stomach and so will get in contact first with the food that is swallowed first, and only afterward will reach the food that was swallowed later. Occasionally the sphincter between the stomach and the esophagus relaxes unexpectedly; this is said to happen more often in smokers than in nonsmokers. The result is that some of the sharply acid stomach contents are forced up into the esophagus and vigorous swallowing is necessary to crowd them back down into the stomach. The burning sensation which accompanies this is known as heartburn, although, as we have just seen, it is really entirely a matter of the esophagus and has nothing whatever to do with the heart.