stimulus excites a plentiful secretion of the digestive juices;
conversely, the opposite conditions, to some extent, check their
flow.

The sight of attractive food, as we all know, "makes the mouth
water," that is, it calls forth the saliva which contains one of the
digestive ferments. Thus, at the beginning of a meal, favorable
conditions for digestion are established. The saliva, however, acts
only upon starch; and, moreover, its action upon this carbohydrate is
weak unless the food is thoroughly chewed and mixed in the mouth.
Most of us, perhaps, overlook the importance of mastication, which
not only crushes all the food-stuffs, preparing them for efficient
digestion, but also stimulates the flow of the digestive juices.
Furthermore, by thoroughly masticating our food, we know intuitively
when we have had enough, and thus avoid overeating.

In the stomach the digestion of starch is continued for a time, but
the chief work of gastric digestion concerns the proteins. They alone
are attacked by pepsin, a ferment secreted by the mucous membrane of
the stomach. Moreover, since pepsin is able to act only when an acid
is present, the gastric mucous membrane also secretes hydrochloric
acid.

Just as the digestive glands in the neighborhood of the mouth become
more active when we are conscious that desirable food is at hand, so
do the glands in the stomach. Mastication also stimulates the flow of
the gastric juice, and this flow is greater if we enjoy what we eat.
Furthermore, it has been shown that, after entrance into the stomach,
the food itself increases the flow of the digestive juices. All
articles of food are not, however, equally efficient in producing